explanation blue bibcodes open ADS page with paths to full text
Author name code: kupka
ADS astronomy entries on 2022-09-14
author:"Kupka, Friedrich"
---------------------------------------------------------
Title: Stellar evolution models with overshooting based on 3-equation
non-local theories, II. Main-sequence models of A- and B-type stars
Authors: Ahlborn, Felix; Kupka, Friedrich; Weiss, Achim; Flaskamp,
Martin
2022arXiv220712512A Altcode:
Convective overshoot mixing is a critical ingredient of stellar
structure models, but is treated in most cases by ad hoc extensions of
the mixing-length theory for convection. Advanced theories which are
both more physical and numerically treatable are needed. Convective
flows in stellar interiors are highly turbulent. This poses a number
of numerical challenges for the modelling of convection in stellar
interiors. We include an effective turbulence model into a 1D stellar
evolution code in order to treat non-local effects within the same
theory. We use a turbulent convection model which relies on the
solution of second order moment equations. We implement this into a
state of the art 1D stellar evolution code. To overcome a deficit
in the original form of the model, we take the dissipation due to
buoyancy waves in the overshooting zone into account. We compute
stellar models of intermediate mass main-sequence stars between 1.5
and 8 $M_\odot$. Overshoot mixing from the convective core and modified
temperature gradients within and above it emerge naturally as a solution
of the turbulent convection model equations. For a given set of model
parameters the overshooting extent determined from the turbulent
convection model is comparable to other overshooting descriptions, the
free parameters of which had been adjusted to match observations. The
relative size of the mixed cores decreases with decreasing stellar
mass without additional adjustments. We find that the dissipation by
buoyancy waves constitutes a necessary and relevant extension of the
turbulent convection model in use.
---------------------------------------------------------
Title: Stellar evolution models with overshooting based on 3-equation
non-local theories I. Physical basis and the computation of the
dissipation rate
Authors: Kupka, F.; Ahlborn, F.; Weiss, A.
2022arXiv220712296K Altcode:
Context. Mixing by convective overshooting has long been suggested
to play an important role for the amount of hydrogen available to
nuclear burning in convective cores of stars. The best way to model
this effect is still debated. Aims. We suggest an improved model for
the computation of the dissipation rate of turbulent kinetic energy
which can be used in non-local models of stellar convection and
can readily be implemented and self-consistently used in 1D stellar
evolution calculations. Methods. We review the physics underlying
various models to compute the dissipation rate of turbulent kinetic
energy, {\epsilon}, in local and particularly in non-local models of
convection in stellar astrophysics. The different contributions to
the dissipation rate and their dependence on local stratification and
on non-local transport are analysed and a new method to account for
at least some of these physical mechanisms is suggested. Results. We
show how the new approach influences predictions of stellar models of
intermediate-mass main-sequence stars and how these changes differ
from other modifications of the non-local convection model that
focus on the ratio of horizontal to vertical (turbulent) kinetic
energy. Conclusions. The new model is shown to allow for a physically
more complete description of convective overshooting and mixing in
massive stars. Dissipation by buoyancy waves is found to be a key
ingredient which has to be accounted for in non-local models of
turbulent convection.
---------------------------------------------------------
Title: On the Potential of the Reynolds Stress Approach to Model
Convective Overshooting in Grids of Stellar Evolution Models
Authors: Kupka, Friedrich
2021plat.confE..88K Altcode:
Convection is one of the main physical processes probed by means
of asteroseismology these days and it is a key topic of several
workpackages within the PLATO mission. A lot of attention in this
field is currently given to the parameter calibration of fairly simple
models by means of 3D RHD numerical simulations or the direct use of
the latter in asteroseismological analyses. However, this approach is
not available to all situations of interest where convection plays a
role in stellar mixing and in the evolution of the thermal structure
of a star, particularly not for overshooting and mixing when they take
place deeply inside a star. For such cases Reynolds stress models
provide an interesting alternative. In this talk I will report on
the potential of this method, also for future calculations of model
grids for asteroseismology, and summarize earlier results and recent
progress made with this approach.
---------------------------------------------------------
Title: Stellar evolution models with entropy-calibrated mixing-length
parameter: application to red giants
Authors: Spada, Federico; Demarque, Pierre; Kupka, Friedrich
2021MNRAS.504.3128S Altcode: 2021arXiv210408067S; 2021MNRAS.tmp.1089S
We present evolutionary models for solar-like stars with an improved
treatment of convection that results in a more accurate estimate of
the radius and effective temperature. This is achieved by improving
the calibration of the mixing-length parameter, which sets the length
scale in the 1D convection model implemented in the stellar evolution
code. Our calibration relies on the results of 2D and 3D radiation
hydrodynamics simulations of convection to specify the value of the
adiabatic specific entropy at the bottom of the convective envelope in
stars as a function of their effective temperature, surface gravity, and
metallicity. For the first time, this calibration is fully integrated
within the flow of a stellar evolution code, with the mixing-length
parameter being continuously updated at run-time. This approach replaces
the more common, but questionable, procedure of calibrating the length
scale parameter on the Sun, and then applying the solar-calibrated
value in modelling other stars, regardless of their mass, composition,
and evolutionary status. The internal consistency of our current
implementation makes it suitable for application to evolved stars, in
particular to red giants. We show that the entropy calibrated models
yield a revised position of the red giant branch that is in better
agreement with observational constraints than that of standard models.
---------------------------------------------------------
Title: Accurate Short-Characteristics Radiative Transfer in A
Numerical Tool for Astrophysical RESearch (ANTARES)
Authors: Kostogryz, Nadiia M.; Kupka, Friedrich; Piskunov, Nikolai;
Fabbian, Damian; Krüger, Daniel; Gizon, Laurent
2021SoPh..296...46K Altcode:
We aim to improve the accuracy of radiative energy transport in
three-dimensional radiation hydrodynamical simulations in ANTARES
(A Numerical Tool for Astrophysical RESearch). We implement in the
ANTARES short-characteristics numerical schemes a modification of
the Bézier interpolant solver. This method yields a smoother surface
structure in simulations of solar convection and reduces the artifacts
appearing due to the limited number of rays along which the integration
is done. Reducing such artifacts leads to increased stability of the
code. We show that our new implementation achieves a better agreement
of the temperature structure and its gradient with a semi-empirical
model derived from observations, as well as of synthetic spectral-line
profiles with the observed solar spectrum.
---------------------------------------------------------
Title: Surface effects and turbulent pressure. Assessing the
Gas-Γ<SUB>1</SUB> and Reduced-Γ<SUB>1</SUB> empirical models
Authors: Belkacem, K.; Kupka, F.; Philidet, J.; Samadi, R.
2021A&A...646L...5B Altcode: 2021arXiv210106065B
The application of the full potential of stellar seismology is made
difficult by the improper modelling of the upper-most layers of
solar-like stars and their influence on the modelled frequencies. Our
knowledge of these so-called `surface effects' has improved thanks to
the use of 3D hydrodynamical simulations, however, the calculation
of eigenfrequencies relies on empirical models for the description
of the Lagrangian perturbation of turbulent pressure, namely: the
reduced-Γ<SUB>1</SUB> model (RGM) and the gas-Γ<SUB>1</SUB> model
(GGM). Starting from the fully compressible turbulence equations,
we derived both the GGM and RGM models by using a closure to model
the flux of turbulent kinetic energy. We find that both models
originate from two terms: the source of turbulent pressure due to
compression produced by the oscillations and the divergence of the
flux of turbulent pressure. We also demonstrate that they are both
compatible with the adiabatic approximation and, additionally, that
they imply a number of questionable assumptions, mainly with respect
to mode physics. Among other hypotheses, it is necessary to neglect
the Lagrangian perturbation of the dissipation of turbulent kinetic
energy into heat and the Lagrangian perturbation of buoyancy work.
---------------------------------------------------------
Title: The ANTARES code: recent developments and applications
Authors: Kupka, Friedrich; Zaussinger, Florian; Fabbian, Damian;
Krüger, Daniel
2020JPhCS1623a2016K Altcode:
ANTARES (A Numerical Tool for Astrophysical RESearch) is a
multi-purpose numerical tool to solve different variants of the
equations of hydrodynamics as they appear in problems of astrophysics,
geophysics, and engineering sciences and which require the construction
of detailed numerical simulation models. A presentation of the current
feature set of the code with a focus on recent add-ons is given here
in addition to a summary on several results from recent applications of
ANTARES to solar physics, the physics of planets, and basic convection
studies including the damping of pressure modes (solar oscillations)
in numerical simulations of convection at the solar surface and the
coupling of layers in numerical simulations of sheared and non-sheared
double-diffusive convection.
---------------------------------------------------------
Title: On long-duration 3D simulations of stellar convection using
ANTARES
Authors: Kupka, F.; Fabbian, D.; Krüger, D.; Kostogryz, N.; Gizon, L.
2020IAUGA..30..373K Altcode:
We present initial results from three-dimensional (3-D) radiation
hydrodynamical simulations for the Sun and targeted Sun-like stars. We
plan to extend these simulations up to several stellar days to study
p-mode excitation and damping processes. The level of variation of
irradiance on the time scales spanned by our 3-D simulations will
be studied too. Here we show results from a first analysis of the
computational data we produced so far.
---------------------------------------------------------
Title: 3D Hydrodynamical Simulations of Stellar Convection for Helio-
and Asteroseismology
Authors: Kupka, F.
2020svos.conf..209K Altcode:
Hydrodynamical simulations of stellar convection are an essential
theoretical tool for gaining insight into the physics of mixing
and heat transport by convection, and also into the interaction of
convection with pulsation. They are particularly useful for obtaining
an accurate description of the structure of the superadiabatic layer,
which is important to explain the observed frequencies of $p$-modes
in solar-like oscillating stars. The simulations can also be used to
probe analytical models of excitation and damping of modes, and thus
explain their amplitudes, and eventually the physical completeness of
such models. This presentation discussed general challenges of such 3D
hydrodynamical simulations developed for helio- and asteroseismology;
it summarized some recent results in this field for the Sun, and which
are also relevant to other lower main-sequence stars.
---------------------------------------------------------
Title: Thermal Convection in Stars and in Their Atmosphere
Authors: Kupka, Friedrich
2020mdps.conf...69K Altcode: 2020arXiv200111540K
Thermal convection is one of the main mechanisms of heat transport and
mixing in stars in general and also in the photospheric layers which
emit the radiation that we observe with astronomical instruments. The
present lecture notes first introduce the role of convection in
astrophysics and explain the basic physics of convection. This is
followed by an overview on the modelling of convection. Challenges
and pitfalls in numerical simulation based modelling are discussed
subsequently. Finally, a particular application for the previously
introduced concepts is described in more detail: the study of convective
overshooting into stably stratified layers around convection zones
in stars.
---------------------------------------------------------
Title: Shine BRITE: shedding light on stellar variability through
advanced models
Authors: Fabbian, D.; Kupka, F.; Krüger, D.; Kostogryz, N. M.;
Piskunov, N.
2020svos.conf..155F Altcode: 2020arXiv200201560F
The correct interpretation of the large amount of complex data from
next-generation (in particular, space-based) observational facilities
requires a very strong theoretical underpinning. One can predict
that, in the near future, the use of atmospheric models obtained with
three-dimensional (3-D) radiation magneto-hydrodynamics (RMHD) codes,
coupled with advanced radiative transfer treatment including non-local
thermodynamic equilibrium (non-LTE) effects and polarisation, will
become the norm. In particular, stellar brightness variability in cool
stars (i.e., spectral types F-- M) can be caused by several different
effects besides pulsation. In this review we have briefly discussed
some published results, and mentioned aspects of recent progress. It
then attempted to peek into what the future may hold for understanding
this important aspect of the lives of stars.
---------------------------------------------------------
Title: Layer formation in double-diffusive convection over resting
and moving heated plates
Authors: Zaussinger, Florian; Kupka, Friedrich
2019ThCFD..33..383Z Altcode: 2018arXiv181111800Z
We present a numerical study of double-diffusive convection
characterized by a stratification unstable to thermal convection,
while at the same time a mean molecular weight (or solute
concentration) difference between top and bottom counteracts this
instability. Convective zones can form in this case either by the
stratification being locally unstable to the combined action of both
temperature and solute gradients or by another process, the oscillatory
double-diffusive convective instability, which is triggered by the
faster molecular diffusivity of heat in comparison with that one of
the solute. We discuss successive layer formation for this problem
in the case of an instantaneously heated bottom (plate) which forms
a first layer with an interface that becomes temporarily unstable and
triggers the formation of further, secondary layers. We consider both
the case of a Prandtl number typical for water (oceanographic scenario)
and of a low Prandtl number (giant planet scenario). We discuss the
impact of a Couette like shear on the flow and in particular on layer
formation for different shear rates. Additional layers form due to the
oscillatory double-diffusive convective instability, as is observed for
some cases. We also test the physical model underlying our numerical
experiments by recovering experimental results of layer formation
obtained in laboratory setups.
---------------------------------------------------------
Title: Solar p-mode damping rates: Insight from a 3D hydrodynamical
simulation
Authors: Belkacem, K.; Kupka, F.; Samadi, R.; Grimm-Strele, H.
2019A&A...625A..20B Altcode: 2019arXiv190305479B
Space-borne missions such as CoRoT and Kepler have provided a rich
harvest of high-quality photometric data for solar-like pulsators. It is
now possible to measure damping rates for hundreds of main-sequence and
thousands of red-giant stars with an unprecedented precision. However,
among the seismic parameters, mode damping rates remain poorly
understood and thus barely used for inferring the physical properties
of stars. Previous approaches to model mode damping rates were based on
mixing-length theory or a Reynolds-stress approach to model turbulent
convection. While they can be used to grasp the main physics of the
problem, such approaches are of little help to provide quantitative
estimates as well as a definitive answer on the relative contribution
of each physical mechanism. Indeed, due to the high complexity of the
turbulent flow and its interplay with the oscillations, those theories
rely on many free parameters which inhibits an in-depth understanding of
the problem. Our aim is thus to assess the ability of 3D hydrodynamical
simulations to infer the physical mechanisms responsible for damping of
solar-like oscillations. To this end, a solar high-spatial resolution
and long-duration hydrodynamical 3D simulation computed with the
ANTARES code allows probing the coupling between turbulent convection
and the normal modes of the simulated box. Indeed, normal modes
of the simulation experience realistic driving and damping in the
super-adiabatic layers of the simulation. Therefore, investigating
the properties of the normal modes in the simulation provides a
unique insight into the mode physics. We demonstrate that such an
approach provides constraints on the solar damping rates and is able
to disentangle the relative contribution related to the perturbation
(by the oscillation) of the turbulent pressure, the gas pressure,
the radiative flux, and the convective flux contributions. Finally,
we conclude that using the normal modes of a 3D numerical simulation
is possible and is potentially able to unveil the respective role of
the different physical mechanisms responsible for mode damping provided
the time-duration of the simulation is long enough.
---------------------------------------------------------
Title: Numerical simulation of DA white dwarf surface convection
Authors: Zaussinger, F.; Kupka, F.; Montgomery, M.; Egbers, Ch.
2018JPhCS1031a2013Z Altcode:
White dwarfs are compact objects with masses comparable to our Sun,
but a radius similar to our Earth. They are the final evolutionary
stage for about 95% of all stars in the Galaxy, i.e., for all stars
that have a final mass less than the Chandrasekhar mass (about 1.4
times the solar mass), the upper mass limit for which hydrostatic
equilibrium can be maintained by the degeneracy pressure of electrons
at very high densities. The outermost shell of most white dwarfs
contains a convective layer. Even if the latter is very thin (≲
10 km), it is important for mixing properties, observed radiation,
and pulsational stability of the whole object. During a long phase
white dwarfs have effective temperatures T<SUB>eff</SUB> of about
10000K ∼ 14000K, since the time scale to reach such temperatures
by cooling is already ≈ 10<SUP>9</SUP> years. Here, we focus on DA
(hydrogen-rich) white dwarfs with T<SUB>eff</SUB> ≈ 12000K. This is
at the transition from shallow to deep convection zones. Due to very
high gravitational acceleration (∼ 10<SUP>6</SUP> g at the surface)
the material is overturned about five times per second over the distance
of a few kilometers. Numerical simulations of such objects have to
be done for a compressible flow and feature highly turbulent granules
at the surface, which are qualitatively comparable to the convection
cells observed at the surface of the Sun. For this study we compare
three white dwarf surface simulations with realistic microphysical
properties and full 3D radiative transport. The simulations differ
in effective temperature, namely, T<SUB>eff</SUB> = 11800K, 12100K,
and 12400K. A statistical analysis of the convective processes as
function of T<SUB>eff</SUB> is presented.
---------------------------------------------------------
Title: Mixing and overshooting in surface convection zones of DA
white dwarfs: first results from ANTARES
Authors: Kupka, F.; Zaussinger, F.; Montgomery, M. H.
2018MNRAS.474.4660K Altcode: 2017arXiv171200641K
We present results of a large, high-resolution 3D hydrodynamical
simulation of the surface layers of a DA white dwarf (WD) with
T<SUB>eff</SUB> = 11 800 K and log (g) = 8 using the ANTARES code,
the widest and deepest such simulation to date. Our simulations are in
good agreement with previous calculations in the Schwarzschild-unstable
region and in the overshooting region immediately beneath it. Farther
below, in the wave-dominated region, we find that the rms horizontal
velocities decay with depth more rapidly than the vertical ones. Since
mixing requires both vertical and horizontal displacements, this could
have consequences for the size of the region that is well mixed by
convection, if this trend is found to hold for deeper layers. We discuss
how the size of the mixed region affects the calculated settling times
and inferred steady-state accretion rates for WDs with metals observed
in their atmospheres.
---------------------------------------------------------
Title: Studying p-mode damping and the surface effect with
hydrodynamical simulations
Authors: Kupka, F.; Belkacem, K.; Samadi, R.; Deheuvels, S.
2017sbcs.conf..222K Altcode:
Hydrodynamical simulations can be used as a complementary tool
to observations for the study of the damping of p-modes and the
so-called surface effect in solar-like oscillators. Here, we present
the state-of-the-art in this research. Examples of applications include
our Sun and the CoRoT target stars HD 49385 and HD 49933.
---------------------------------------------------------
Title: Modelling of stellar convection
Authors: Kupka, Friedrich; Muthsam, Herbert J.
2017LRCA....3....1K Altcode:
The review considers the modelling process for stellar convection rather
than specific astrophysical results. For achieving reasonable depth and
length we deal with hydrodynamics only, omitting MHD. A historically
oriented introduction offers first glimpses on the physics of stellar
convection. Examination of its basic properties shows that two very
different kinds of modelling keep being needed: low dimensional models
(mixing length, Reynolds stress, etc.) and "full" 3D simulations. A list
of affordable and not affordable tasks for the latter is given. Various
low dimensional modelling approaches are put in a hierarchy and basic
principles which they should respect are formulated. In 3D simulations
of low Mach number convection the inclusion of then unimportant sound
waves with their rapid time variation is numerically impossible. We
describe a number of approaches where the Navier-Stokes equations are
modified for their elimination (anelastic approximation, etc.). We
then turn to working with the full Navier-Stokes equations and deal
with numerical principles for faithful and efficient numerics. Spatial
differentiation as well as time marching aspects are considered. A list
of codes allows assessing the state of the art. An important recent
development is the treatment of even the low Mach number problem without
prior modification of the basic equation (obviating side effects) by
specifically designed numerical methods. Finally, we review a number
of important trends such as how to further develop low-dimensional
models, how to use 3D models for that purpose, what effect recent
hardware developments may have on 3D modelling, and others.
---------------------------------------------------------
Title: Semi-convective layer formation
Authors: Zaussinger, F.; Kupka, F.; Egbers, Ch.; Neben, M.; Hücker,
S.; Bahr, C.; Schmitt, M.
2017JPhCS.837a2012Z Altcode:
Semi-convective mixing, as an example of double-diffusive convection,
is of general importance in multi-component fluid mixing processes. In
astrophysics it occurs when the mean molecular weight gradient caused
by a mixture of light material on top of heavier one counteracts
the convective instability caused by a temperature gradient. Direct
numerical simulations of double-diffusive fluid flows in a realistic
stellar or planetary parameter space are currently non-feasible. Hence,
a model describing incompressible semi-convection was developed,
which allows to investigate semi-convective layer formation. A
detailed parameter study with varying Rayleigh number and stability
parameter has been performed for the giant planet case. We conclude
that semi-convective layering may not play that important role as
suggested in earlier works for the planetary case.
---------------------------------------------------------
Title: Multidimensional modelling of classical pulsating stars
Authors: Muthsam, H. J.; Kupka, F.
2016CoKon.105..117M Altcode: 2016arXiv160103325M
After an overview of general aspects of modelling the pulsation-
convection interaction we present reasons why such simulations
(in multidimensions) are needed but, at the same time, pose a
considerable challenge. We then discuss, for several topics, what
insights multidimensional simulations have either already provided or
can be expected to yield in the future. We finally discuss properties
of our ANTARES code. Many of these features can be expected to be
characteristic of other codes which may possibly be applied to these
physical questions in the foreseeable future.
---------------------------------------------------------
Title: The ANTARES Code: New Developments
Authors: Blies, P. M.; Kupka, F.; Muthsam, H. J.
2015ASPC..498..191B Altcode:
We give an update on the ANTARES code. It was presented by Muthsam
et al. (2010) and has since experienced various improvements and
has also been extended by new features which we will mention in this
paper. Two new features will be presented in a bit more detail: the
parallel multigrid solver for the 2D non-linear, generalized Helmholtz
equation by Happenhofer (2014) and the capability to use curvilinear
grids by Grimm-Strele (2014).
---------------------------------------------------------
Title: Multidimensional realistic modelling of Cepheid-like variables
- II. Analysis of a Cepheid model
Authors: Mundprecht, Eva; Muthsam, Herbert J.; Kupka, Friedrich
2015MNRAS.449.2539M Altcode: 2015arXiv150200449M
Non-local, time-dependent convection models have been used to explain
the location of double-mode pulsations in Cepheids in the HR diagram as
well as the existence and location of the red edge of the instability
strip. These properties are highly sensitive to model parameters. We use
2D radiation-hydrodynamical simulations with realistic microphysics and
grey radiative transfer to model a short-period Cepheid. The simulations
show that the strength of the convection zone varies significantly
over the pulsation period and exhibits a phase shift relative to the
variations in radius. We evaluate the convective flux and the work
integral as predicted by the most common convection models. It turns out
that over one pulsation cycle the model parameter α<SUB>c</SUB>, has
to be varied by up to a factor of beyond 2 to match the convective flux
obtained from the simulations. To bring convective fluxes integrated
over the He II convection zone and the overshoot zone below into
agreement, this parameter has to be varied by a factor of up to ∼7.5
(Kuhfuß). We then present results on the energetics of the convection
and overshoot zone by radially symmetric and fluctuating quantities. To
successfully model this scenario by a static, 1D or even by a simple
time-dependent model appears extremely challenging. We conclude that
significant improvements are needed to make predictions based on 1D
models more robust and to improve the reliability of conclusions on
the convection-pulsation coupling drawn from them. Multidimensional
simulations can provide guidelines for developing descriptions of
convection then applied in traditional 1D modelling.
---------------------------------------------------------
Title: Achievable efficiency of numerical methods for simulations
of solar surface convection
Authors: Grimm-Strele, H.; Kupka, F.; Muthsam, H. J.
2015CoPhC.188....7G Altcode: 2014arXiv1406.1891G
We investigate the achievable efficiency of both the time and the space
discretisation methods used in Antares for mixed parabolic-hyperbolic
problems. We show that the fifth order variant of WENO combined with a
second order Runge-Kutta scheme is not only more accurate than standard
first and second order schemes, but also more efficient taking the
computation time into account. Then, we calculate the error decay rates
of WENO with several explicit Runge-Kutta schemes for advective and
diffusive problems with smooth and non-smooth initial conditions. With
this data, we estimate the computational costs of three-dimensional
simulations of stellar surface convection and show that SSP RK(3,2)
is the most efficient scheme considered in this comparison.
---------------------------------------------------------
Title: Realistic simulations of stellar surface convection with
ANTARES: I. Boundary conditions and model relaxation
Authors: Grimm-Strele, H.; Kupka, F.; Löw-Baselli, B.; Mundprecht,
E.; Zaussinger, F.; Schiansky, P.
2015NewA...34..278G Altcode: 2013arXiv1305.0743G
We have implemented open boundary conditions into the ANTARES
code to increase the realism of our simulations of stellar surface
convection. Even though we greatly benefit from the high accuracy
of our fifth order numerical scheme (WENO5) the broader stencils
needed for the numerical scheme complicate the implementation of
boundary conditions. We show that the effective temperature of a
numerical simulation cannot be changed by corrections at the lower
boundary since the thermal stratification does only change on the
Kelvin-Helmholtz time scale. Except for very shallow models this time
scale cannot be covered by multidimensional simulations due to the
enormous computational requirements. We demonstrate to what extent
numerical simulations of stellar surface convection are sensitive to
the initial conditions and the boundary conditions. An ill-conceived
choice of parameters for the boundary conditions can have a severe
impact. Numerical simulations of stellar surface convection will
only be (physically) meaningful and realistic if the initial model,
the extent and position of the simulation box and the parameters from
the boundary conditions are chosen adequately.
---------------------------------------------------------
Title: The PLATO 2.0 mission
Authors: Rauer, H.; Catala, C.; Aerts, C.; Appourchaux, T.; Benz,
W.; Brandeker, A.; Christensen-Dalsgaard, J.; Deleuil, M.; Gizon,
L.; Goupil, M. -J.; Güdel, M.; Janot-Pacheco, E.; Mas-Hesse,
M.; Pagano, I.; Piotto, G.; Pollacco, D.; Santos, Ċ.; Smith, A.;
Suárez, J. -C.; Szabó, R.; Udry, S.; Adibekyan, V.; Alibert, Y.;
Almenara, J. -M.; Amaro-Seoane, P.; Eiff, M. Ammler-von; Asplund, M.;
Antonello, E.; Barnes, S.; Baudin, F.; Belkacem, K.; Bergemann, M.;
Bihain, G.; Birch, A. C.; Bonfils, X.; Boisse, I.; Bonomo, A. S.;
Borsa, F.; Brandão, I. M.; Brocato, E.; Brun, S.; Burleigh, M.;
Burston, R.; Cabrera, J.; Cassisi, S.; Chaplin, W.; Charpinet, S.;
Chiappini, C.; Church, R. P.; Csizmadia, Sz.; Cunha, M.; Damasso, M.;
Davies, M. B.; Deeg, H. J.; Díaz, R. F.; Dreizler, S.; Dreyer, C.;
Eggenberger, P.; Ehrenreich, D.; Eigmüller, P.; Erikson, A.; Farmer,
R.; Feltzing, S.; de Oliveira Fialho, F.; Figueira, P.; Forveille,
T.; Fridlund, M.; García, R. A.; Giommi, P.; Giuffrida, G.; Godolt,
M.; Gomes da Silva, J.; Granzer, T.; Grenfell, J. L.; Grotsch-Noels,
A.; Günther, E.; Haswell, C. A.; Hatzes, A. P.; Hébrard, G.; Hekker,
S.; Helled, R.; Heng, K.; Jenkins, J. M.; Johansen, A.; Khodachenko,
M. L.; Kislyakova, K. G.; Kley, W.; Kolb, U.; Krivova, N.; Kupka, F.;
Lammer, H.; Lanza, A. F.; Lebreton, Y.; Magrin, D.; Marcos-Arenal,
P.; Marrese, P. M.; Marques, J. P.; Martins, J.; Mathis, S.; Mathur,
S.; Messina, S.; Miglio, A.; Montalban, J.; Montalto, M.; Monteiro,
M. J. P. F. G.; Moradi, H.; Moravveji, E.; Mordasini, C.; Morel, T.;
Mortier, A.; Nascimbeni, V.; Nelson, R. P.; Nielsen, M. B.; Noack,
L.; Norton, A. J.; Ofir, A.; Oshagh, M.; Ouazzani, R. -M.; Pápics,
P.; Parro, V. C.; Petit, P.; Plez, B.; Poretti, E.; Quirrenbach, A.;
Ragazzoni, R.; Raimondo, G.; Rainer, M.; Reese, D. R.; Redmer, R.;
Reffert, S.; Rojas-Ayala, B.; Roxburgh, I. W.; Salmon, S.; Santerne,
A.; Schneider, J.; Schou, J.; Schuh, S.; Schunker, H.; Silva-Valio,
A.; Silvotti, R.; Skillen, I.; Snellen, I.; Sohl, F.; Sousa, S. G.;
Sozzetti, A.; Stello, D.; Strassmeier, K. G.; Švanda, M.; Szabó,
Gy. M.; Tkachenko, A.; Valencia, D.; Van Grootel, V.; Vauclair,
S. D.; Ventura, P.; Wagner, F. W.; Walton, N. A.; Weingrill, J.;
Werner, S. C.; Wheatley, P. J.; Zwintz, K.
2014ExA....38..249R Altcode: 2014ExA...tmp...41R; 2013arXiv1310.0696R
PLATO 2.0 has recently been selected for ESA's M3 launch opportunity
(2022/24). Providing accurate key planet parameters (radius, mass,
density and age) in statistical numbers, it addresses fundamental
questions such as: How do planetary systems form and evolve? Are there
other systems with planets like ours, including potentially habitable
planets? The PLATO 2.0 instrument consists of 34 small aperture
telescopes (32 with 25 s readout cadence and 2 with 2.5 s candence)
providing a wide field-of-view (2232 deg <SUP>2</SUP>) and a large
photometric magnitude range (4-16 mag). It focusses on bright (4-11
mag) stars in wide fields to detect and characterize planets down to
Earth-size by photometric transits, whose masses can then be determined
by ground-based radial-velocity follow-up measurements. Asteroseismology
will be performed for these bright stars to obtain highly accurate
stellar parameters, including masses and ages. The combination of
bright targets and asteroseismology results in high accuracy for
the bulk planet parameters: 2 %, 4-10 % and 10 % for planet radii,
masses and ages, respectively. The planned baseline observing strategy
includes two long pointings (2-3 years) to detect and bulk characterize
planets reaching into the habitable zone (HZ) of solar-like stars
and an additional step-and-stare phase to cover in total about 50 %
of the sky. PLATO 2.0 will observe up to 1,000,000 stars and detect
and characterize hundreds of small planets, and thousands of planets
in the Neptune to gas giant regime out to the HZ. It will therefore
provide the first large-scale catalogue of bulk characterized planets
with accurate radii, masses, mean densities and ages. This catalogue
will include terrestrial planets at intermediate orbital distances,
where surface temperatures are moderate. Coverage of this parameter
range with statistical numbers of bulk characterized planets is unique
to PLATO 2.0. The PLATO 2.0 catalogue allows us to e.g.: - complete
our knowledge of planet diversity for low-mass objects, - correlate the
planet mean density-orbital distance distribution with predictions from
planet formation theories,- constrain the influence of planet migration
and scattering on the architecture of multiple systems, and - specify
how planet and system parameters change with host star characteristics,
such as type, metallicity and age. The catalogue will allow us to study
planets and planetary systems at different evolutionary phases. It
will further provide a census for small, low-mass planets. This will
serve to identify objects which retained their primordial hydrogen
atmosphere and in general the typical characteristics of planets
in such low-mass, low-density range. Planets detected by PLATO 2.0
will orbit bright stars and many of them will be targets for future
atmosphere spectroscopy exploring their atmosphere. Furthermore,
the mission has the potential to detect exomoons, planetary rings,
binary and Trojan planets. The planetary science possible with PLATO
2.0 is complemented by its impact on stellar and galactic science via
asteroseismology as well as light curves of all kinds of variable stars,
together with observations of stellar clusters of different ages. This
will allow us to improve stellar models and study stellar activity. A
large number of well-known ages from red giant stars will probe the
structure and evolution of our Galaxy. Asteroseismic ages of bright
stars for different phases of stellar evolution allow calibrating
stellar age-rotation relationships. Together with the results of ESA's
Gaia mission, the results of PLATO 2.0 will provide a huge legacy to
planetary, stellar and galactic science.
---------------------------------------------------------
Title: Improved Time Integration for WENO Methods in Astrophysical
Applications
Authors: Kupka, F.; Grimm–Strele, H.; Happenhofer, N.; Higueras,
I.; Koch, O.; Muthsam, H. J.
2014ASPC..488..243K Altcode:
Weighted essentially non-oscillatory methods are a powerful approach to
discretize advection and pressure gradient terms in the hydrodynamical
equations, since they yield higher effective resolution than traditional
methods. But in some astrophysical problems, low Mach number flows have
to be tackled or the flow may change from a low to a high Mach number
flow, spatially or in time, and diffusion or radiative transfer can
impose severe limitations on time steps allowed in explicit time
integration methods. We provide a summary of new developments on
semi-implicit time integration methods useful for astrophysical problems
such as numerical simulations of stellar surfaces and envelopes, and
the basic physical question of how to improve models of double-diffusive
convection. We discuss several applications of these new methods.
---------------------------------------------------------
Title: The effects of rotation on a double-diffusive layer in a
rotating spherical shell
Authors: Blies, Patrick; Kupka, Friedrich; Zaussinger, Florian;
Hollerbach, Rainer
2014arXiv1404.6086B Altcode:
So far, numerical studies of double-diffusive layering in turbulent
convective flows have neglected the effects of rotation. We undertake
a first step into that direction by investigating how Coriolis
forces affect a double-diffusive layer inside a rotating spherical
shell. For this purpose we have run simulations in a parameter regime
where these layers are expected to form and successively increased
the rate of rotation with the result that fast rotation is found to
have a similar stabilising effect on the overall convective flux as
an increase of the stability ratio $R_{\rho}$ has in a non-rotating
setup. We have also studied to what extent the regimes of rotational
constraints suggested by King, Stellmach, and Buffett (2013) for
rotation in the case of Rayleigh-Bénard convection are influenced
by double-diffusive convection: their classification could also be
applicable to the case of double-diffusive convection in a spherical
shell if it is extended to be also a function of the stability ratio
$R_{\rho}$. Furthermore, we examined the ratio of saline and thermal
Nusselt numbers and compared our results with models of Spruit (2013),
Rosenblum et al. (2011) and Wood, Garaud, and Stellmach (2013). We
find our data to be fitted best by Spruit's model. Our result that
fast rotation further decreases the convective transport, which is
already lowered by double-diffusive convection, could play a major
role for e.g. the modeling of the interior of some rapidly rotating
giant planets, as gaseous giant planets have recently been proposed
to be influenced by double-diffusive convection.
---------------------------------------------------------
Title: Curvilinear grids for WENO methods in astrophysical simulations
Authors: Grimm-Strele, H.; Kupka, F.; Muthsam, H. J.
2014CoPhC.185..764G Altcode: 2013arXiv1308.3066G
We investigate the applicability of curvilinear grids in the context of
astrophysical simulations and WENO schemes. With the non-smooth mapping
functions from Calhoun et al. (2008), we can tackle many astrophysical
problems which were out of scope with the standard grids in numerical
astrophysics. We describe the difficulties occurring when implementing
curvilinear coordinates into our WENO code, and how we overcome
them. We illustrate the theoretical results with numerical data. The
WENO finite difference scheme works only for high Mach number flows
and smooth mapping functions, whereas the finite volume scheme gives
accurate results even for low Mach number flows and on non-smooth grids.
---------------------------------------------------------
Title: Pulsation - convection interaction
Authors: Kupka, F.; Mundprecht, E.; Muthsam, H. J.
2014IAUS..301..177K Altcode:
A lot of effort has been devoted to the hydrodynamical modelling
of Cepheids in one dimension. While the recovery of the most basic
properties such as the pulsational instability itself has been achieved
already a long time ago, properties such as the observed double-mode
pulsation of some objects and the red-edge of the classical instability
strip and their dependence on metallicity have remained a delicate
issue. The uncertainty introduced by adjustable parameters and further
physical approximations introduced in one-dimensional model equations
motivate an investigation based on numerical simulations which use
the full hydrodynamical equations. In this talk, results from such
two-dimensional numerical simulations of a short period Cepheid are
presented. The importance of a carefully designed numerical setup, in
particular of sufficient resolution and domain extent, is discussed. The
problematic issue of how to reliably choose fixed parameters for the
one-dimensional model is illustrated. Results from an analysis of
the interaction of pulsation with convection are shown concerning the
large-scale structure of the He ii ionization zone. We also address
the influence of convection on the atmospheric structure. Considering
the potential of hydrodynamical simulations and the wealth of ever
improving observational data an outlook on possible future work in
this field of research is given.
---------------------------------------------------------
Title: Multidimensional realistic modelling of Cepheid-like variables
- I. Extensions of the ANTARES code
Authors: Mundprecht, Eva; Muthsam, Herbert J.; Kupka, Friedrich
2013MNRAS.435.3191M Altcode: 2012arXiv1209.2952M; 2013MNRAS.tmp.2236M
We have extended the ANTARES code to simulate the coupling of pulsation
with convection in Cepheid-like variables in an increasingly realistic
way, in particular in multidimensions, 2D at this stage. Present-day
models of radially pulsating stars assume radial symmetry and have the
pulsation-convection interaction included via model equations containing
ad hoc closures and moreover parameters whose values are barely
known. We intend to construct ever more realistic multidimensional
models of Cepheids. In this paper, the first of a series, we describe
the basic numerical approach and how it is motivated by physical
properties of these objects which are sometimes more, sometimes less
obvious. For the construction of appropriate models a polar grid
comoving with the mean radial velocity has been introduced to optimize
radial resolution throughout the different pulsation phases. The grid is
radially stretched to account for the change of spatial scales due to
vertical stratification and a new grid refinement scheme is introduced
to resolve the upper, hydrogen ionization zone where the gradient of
temperature is steepest. We demonstrate that the simulations are not
conservative when the original weighted essentially non-oscillatory
method implemented in ANTARES is used and derive a new scheme which
allows a conservative time evolution. The numerical approximation of
diffusion follows the same principles. Moreover, the radiative transfer
solver has been modified to improve the efficiency of calculations on
parallel computers. We show that with these improvements, the ANTARES
code can be used for realistic simulations of the convection-pulsation
interaction in Cepheids. We discuss the properties of several numerical
models of this kind which include the upper 42 per cent of a Cepheid
along its radial coordinate and assume different opening angles. The
models are suitable for an in-depth study of convection and pulsation
in these objects.
---------------------------------------------------------
Title: Semi-convection
Authors: Zaussinger, F.; Kupka, F.; Muthsam, H. J.
2013LNP...865..219Z Altcode:
Double-diffusive processes play an important role in various
astrophysical and geophysical systems. Especially the case which
considers a thermally unstable configuration stabilized by mean
molecular weight and which is known as semi-convection has been
studied for several decades. Whether a semi-convective region in
a star should be treated layered or fully mixed has not yet been
conclusively answered. However, in recent years numerical simulations
have been used to investigate this fluid dynamical instability. With
high resolution methods we can now develop a better understanding
of this mixing process. For this review our intention is to present
semi-convection from different points of view. At first, a summary
of studies made during the last decades is used to demonstrate the
continuous importance of semi- convection for stellar evolution. The
physical process itself as well as linear stability criteria are
explained subsequently. Finally, models, experiments, and the study
through numerical simulations are discussed. Semi-convective mixing and
related questions in stellar evolution and, recently, also in exoplanet
science continue to be a vivid research field and have never ceased
to surprise.
---------------------------------------------------------
Title: Total-variation-diminishing implicit-explicit Runge-Kutta
methods for the simulation of double-diffusive convection in
astrophysics
Authors: Kupka, Friedrich; Happenhofer, Natalie; Higueras, Inmaculada;
Koch, Othmar
2012JCoPh.231.3561K Altcode: 2011arXiv1106.5672K
We put forward the use of total-variation-diminishing (or
more generally, strong stability preserving) implicit-explicit
Runge-Kutta methods for the time integration of the equations of
motion associated with the semiconvection problem in the simulation
of stellar convection. The fully compressible Navier-Stokes equation,
augmented by continuity and total energy equations, and an equation
of state describing the relation between the thermodynamic quantities,
is semi-discretized in space by essentially non-oscillatory schemes and
dissipative finite difference methods. It is subsequently integrated
in time by Runge-Kutta methods which are constructed such as to
preserve the total variation diminishing (or strong stability) property
satisfied by the spatial discretization coupled with the forward Euler
method. We analyse the stability, accuracy and dissipativity of the
time integrators and demonstrate that the most successful methods
yield a substantial gain in computational efficiency as compared to
classical explicit Runge-Kutta methods.
---------------------------------------------------------
Title: Divisions Iv-V / Working Group ap & Related Stars
Authors: Mathys, Gautier; Cunha, Margarida; Dworetsky, Michael;
Kochukhov, Oleg; Kupka, Friedrich; LeBlanc, Francis; Monier, Richard;
Paunzen, Ernst; Pintado, Olga; Piskunov, Nikolai; Ziznovsky, Jozef
2012IAUTA..28..203M Altcode:
The purpose of the Working Group on Ap and Related Stars (ApWG) is
to promote and facilitate research about stars in the spectral type
range from B to early F that exhibit surface chemical peculiarities
and related phenomena. This is a very active field of research, in
which a wide variety of new developments have taken place since 2009,
as illustrated by the following selected highlights.
---------------------------------------------------------
Title: Layered convection in double diffusive fluids
Authors: Zaussinger, F.; Kupka, F.; Muthsam, H. J.; Happenhofer, N.;
Grimm-Strele, H.
2012EGUGA..14.1830Z Altcode:
Double diffusive convection plays an important role in astrophysics
and oceanography where under certain conditions a thermally
unstable temperature gradient is counteracted by a stable solute
gradient. This configuration is well known from salt lakes, where
the salt concentration stabilizes convective motions and a layered
structure emerges. Similar conditions are found in stellar interiors,
where helium as the stabilizing component inhibits the development
of convection and the occurrence of double-diffusive staircases is
assumed. We investigate mixing timescales and stability conditions
using theoretical estimates and numerical simulations covering a
broad range of parameter sets by varying Prandtl-, Lewis- and Rayleigh
numbers. To shed light on the numerically inaccessible astrophysical
case we extrapolate to the relevant parameter range. We investigate
the initial layer formation process as well as the stability of evolved
layers by performing direct numerical simulations in 2D and 3D using the
Boussinesq approximation. A fitting formula for the Nusselt numbers and
the effective mixing rates is given. Finally, we present a semi-implicit
method to solve the compressible counterpart of the governing equations
which has the advantage to cover the entire relevant Mach number range.
---------------------------------------------------------
Title: Simulations of stellar convection, pulsation and semiconvection
Authors: Muthsam, Herbert J.; Kupka, Friedrich; Mundprecht, Eva;
Zaussinger, Florian; Grimm-Strele, Hannes; Happenhofer, Natalie
2011IAUS..271..179M Altcode: 2010arXiv1009.2409M
We report on modelling in stellar astrophysics with the ANTARES
code. First, we describe properties of turbulence in solar granulation
as seen in high-resolution calculations. Then, we turn to the first
2D model of pulsation-convection interaction in a cepheid. We discuss
properties of the outer and the HeII ionization zone. Thirdly, we
report on our work regarding models of semiconvection in the context
of stellar physics.
---------------------------------------------------------
Title: Vamdc as a Resource for Atomic and Molecular Data and the
New Release of Vald
Authors: Kupka, F.; Dubernet, M. -L.; VAMDC Collaboration
2011BaltA..20..503K Altcode: 2011OAst...20..503K; 2012arXiv1201.0154K
The Virtual Atomic and Molecular Data Centre (VAMDC) is an EU-FP7
project devoted to build a common electronic infrastructure for the
exchange and distribution of atomic and molecular data. Within VAMDC
scientists from many different disciplines in atomic and molecular
physics collaborate with users of their data and also with scientists
and engineers from the information and communication technology
community. In this presentation, an overview of the current status
of VAMDC and its capabilities is provided. In the second part of
the presentation I will focus on one of the databases which have
become a part of the VAMDC platform, the Vienna Atomic Line Data Base
(VALD). VALD has developed into a well-known resource of atomic data for
spectroscopy, particularly in astrophysics. A new release, VALD-3, will
provide numerous improvements over its predecessor. This particularly
relates to the data contents where new sets of atomic data for both
precision spectroscopy (i.e., with data for observed energy levels) as
well as opacity calculations (i.e., with data involving predicted energy
levels) have been included. Data for selected diatomic molecules have
been added, and a new system for data distribution and data referencing
provides more convenience in using the third release of VALD.
---------------------------------------------------------
Title: Asteroseismology of solar-type stars with Kepler I: Data
analysis
Authors: Karoff, C.; Chaplin, W. J.; Appourchaux, T.; Elsworth, Y.;
Garcia, R. A.; Houdek, G.; Metcalfe, T. S.; Molenda-Żakowicz, J.;
Monteiro, M. J. P. F. G.; Thompson, M. J.; Christensen-Dalsgaard, J.;
Gilliland, R. L.; Kjeldsen, H.; Basu, S.; Bedding, T. R.; Campante,
T. L.; Eggenberger, P.; Fletcher, S. T.; Gaulme, P.; Handberg, R.;
Hekker, S.; Martic, M.; Mathur, S.; Mosser, B.; Regulo, C.; Roxburgh,
I. W.; Salabert, D.; Stello, D.; Verner, G. A.; Belkacem, K.; Biazzo,
K.; Cunha, M. S.; Gruberbauer, M.; Guzik, J. A.; Kupka, F.; Leroy,
B.; Ludwig, H. -G.; Mathis, S.; Noels, A.; Noyes, R. W.; Roca Cortes,
T.; Roth, M.; Sato, K. H.; Schmitt, J.; Suran, M. D.; Trampedach,
R.; Uytterhoeven, K.; Ventura, R.
2010AN....331..972K Altcode: 2010arXiv1005.0507K
We report on the first asteroseismic analysis of solar-type stars
observed by Kepler. Observations of three G-type stars, made at
one-minute cadence during the first 33.5 days of science operations,
reveal high signal-to-noise solar-like oscillation spectra in all three
stars: About 20 modes of oscillation can clearly be distinguished
in each star. We discuss the appearance of the oscillation spectra,
including the presence of a possible signature of faculae, and the
presence of mixed modes in one of the three stars.
---------------------------------------------------------
Title: VAMDC: The Virtual Atomic and Molecular Data Centre: a Service
Orientated Data Infrastructure for e- Research
Authors: Mason, N. J.; Dubernet, M. L.; Benson, K. M.; Bureau, J.;
Heiter, U.; Kupka, F.; Le Sidaner, P.; Piskunov, N.; Rixon, G. T.;
Schlemmer, S.; Shih, A.; Tennyson, J.; Walton, N.; Witherick, D. W.
2010epsc.conf..861M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: ANTARES - A Numerical Tool for Astrophysical RESearch with
applications to solar granulation
Authors: Muthsam, H. J.; Kupka, F.; Löw-Baselli, B.; Obertscheider,
C.; Langer, M.; Lenz, P.
2010NewA...15..460M Altcode: 2009arXiv0905.0177M
We discuss the general design of the ANTARES code which is intended
for simulations in stellar hydrodynamics with radiative transfer and
realistic microphysics in 1D, 2D and 3D. We then compare the quality of
various numerical methods. We have applied ANTARES in order to obtain
high resolution simulations of solar granulation which we describe and
analyze. In order to obtain high resolution, we apply grid refinement
to a region predominantly occupied by an exploding granule. Strong,
rapidly rotating vortex tubes of small diameter (∼100km) generated
by the downdrafts and ascending into the photosphere near the granule
boundaries evolve, often entering the photosphere from below in an
arclike fashion. They essentially contribute to the turbulent velocity
field near the granule boundaries.
---------------------------------------------------------
Title: INTER-DIVISION IV-V WORKING GROUP on Ap and Related Stars
Authors: Cunha, Margarida S.; Weiss, Werner; Dworetsky, Mike;
Kochukhov, Oleg; Kupka, Friedrich; Leblanc, Francis; Monier, Richard;
Paunzen, Ernst; Piskunov, Nikolai; Shibahashi, Hiromoto; Smalley,
Barry; Ziznovsky, Jozef
2010IAUTB..27..205C Altcode:
The business meeting started at 11h00, in the presence of 18 members,
with a brief summary of the activities and achievements of the Working
group during the triennium 2006-2009.
---------------------------------------------------------
Title: The Asteroseismic Potential of Kepler: First Results for
Solar-Type Stars
Authors: Chaplin, W. J.; Appourchaux, T.; Elsworth, Y.; García,
R. A.; Houdek, G.; Karoff, C.; Metcalfe, T. S.; Molenda-Żakowicz,
J.; Monteiro, M. J. P. F. G.; Thompson, M. J.; Brown, T. M.;
Christensen-Dalsgaard, J.; Gilliland, R. L.; Kjeldsen, H.; Borucki,
W. J.; Koch, D.; Jenkins, J. M.; Ballot, J.; Basu, S.; Bazot, M.;
Bedding, T. R.; Benomar, O.; Bonanno, A.; Brandão, I. M.; Bruntt,
H.; Campante, T. L.; Creevey, O. L.; Di Mauro, M. P.; Doǧan,
G.; Dreizler, S.; Eggenberger, P.; Esch, L.; Fletcher, S. T.;
Frandsen, S.; Gai, N.; Gaulme, P.; Handberg, R.; Hekker, S.; Howe,
R.; Huber, D.; Korzennik, S. G.; Lebrun, J. C.; Leccia, S.; Martic,
M.; Mathur, S.; Mosser, B.; New, R.; Quirion, P. -O.; Régulo, C.;
Roxburgh, I. W.; Salabert, D.; Schou, J.; Sousa, S. G.; Stello, D.;
Verner, G. A.; Arentoft, T.; Barban, C.; Belkacem, K.; Benatti, S.;
Biazzo, K.; Boumier, P.; Bradley, P. A.; Broomhall, A. -M.; Buzasi,
D. L.; Claudi, R. U.; Cunha, M. S.; D'Antona, F.; Deheuvels, S.;
Derekas, A.; García Hernández, A.; Giampapa, M. S.; Goupil, M. J.;
Gruberbauer, M.; Guzik, J. A.; Hale, S. J.; Ireland, M. J.; Kiss,
L. L.; Kitiashvili, I. N.; Kolenberg, K.; Korhonen, H.; Kosovichev,
A. G.; Kupka, F.; Lebreton, Y.; Leroy, B.; Ludwig, H. -G.; Mathis, S.;
Michel, E.; Miglio, A.; Montalbán, J.; Moya, A.; Noels, A.; Noyes,
R. W.; Pallé, P. L.; Piau, L.; Preston, H. L.; Roca Cortés, T.;
Roth, M.; Sato, K. H.; Schmitt, J.; Serenelli, A. M.; Silva Aguirre,
V.; Stevens, I. R.; Suárez, J. C.; Suran, M. D.; Trampedach, R.;
Turck-Chièze, S.; Uytterhoeven, K.; Ventura, R.; Wilson, P. A.
2010ApJ...713L.169C Altcode: 2010arXiv1001.0506C
We present preliminary asteroseismic results from Kepler on three G-type
stars. The observations, made at one-minute cadence during the first
33.5 days of science operations, reveal high signal-to-noise solar-like
oscillation spectra in all three stars: about 20 modes of oscillation
may be clearly distinguished in each star. We discuss the appearance of
the oscillation spectra, use the frequencies and frequency separations
to provide first results on the radii, masses, and ages of the stars,
and comment in the light of these results on prospects for inference
on other solar-type stars that Kepler will observe.
---------------------------------------------------------
Title: Effects of resolution and helium abundance in A star surface
convection simulations
Authors: Kupka, F.; Ballot, J.; Muthsam, H. J.
2009CoAst.160...30K Altcode: 2009arXiv0905.0070K
We present results from 2D radiation-hydrodynamical simulations of fully
compressible convection for the surface layers of A-type stars with
the ANTARES code. Spectroscopic indicators for photospheric convective
velocity fields show a maximum of velocities near Teff ∼ 8000 K with
the largest values measured for the subgroup of Am stars. Thus far,
no prognostic model, neither theoret ical nor numerical, is able to
exactly reproduce the line profiles of sharp line A and Am stars in that
temperature range. As the helium abundance of A stars is not known from
observations, we have considered two extreme cases for our simulations:
a solar helium abundance as an upper limit and zero helium abundance
as a lower limit. The simulation for the helium free case is found to
differ from the case with solar helium abundance by larger velocities,
larger flow structures, and by a sign reversal of the flux of kinetic
energy inside the hydrogen ionisation zone. Both simulations show
extended shock fronts emerg- ing from the optical surface, as well
as mixing far below the region of partial ionisation of hydrogen, and
vertical oscillations emerging after initial perturba- tions have been
damped. We discuss problems related to the rapid radiative cooling at
the surface of A-type stars such as resolution and efficient relax-
ation. This includes identifying the radiative cooling rate Qrad ,
which poses the most severe time step limitation for the dynamical
equation for the evolution of energy density, as a stiff term. It is
thus a numerical rather than a physical constraint for the numerical
simulation of these objects. The present work is considered as a
step towards a systematic study of convection in A- to F-type stars,
encouraged by the new data becoming available for these objects from
both asteroseismological missions and from high resolution spectroscopy.
---------------------------------------------------------
Title: Atmospheric velocity fields in tepid main sequence stars
Authors: Landstreet, J. D.; Kupka, F.; Ford, H. A.; Officer, T.;
Sigut, T. A. A.; Silaj, J.; Strasser, S.; Townshend, A.
2009A&A...503..973L Altcode: 2009arXiv0906.3824L
Context: The line profiles of the stars with {v<SUB>e</SUB> sin i} below
a few km s<SUP>-1</SUP> can reveal direct signatures of local velocity
fields such as convection in stellar atmospheres. This effect is well
established in cool main sequence stars, and has been detected and
studied in three A stars. <BR />Aims: This paper reports observations
of main sequence B, A and F stars (1) to identify additional stars with
sufficiently low values of {v<SUB>e</SUB> sin i} to search for spectral
line profile signatures of local velocity fields and (2) to explore how
the signatures of the local velocity fields in the atmosphere depend
on stellar parameters such as effective temperature and peculiarity
type. <BR />Methods: We have carried out a spectroscopic survey of B and
A stars of low {v<SUB>e</SUB> sin i} at high resolution. Comparison of
model spectra with those observed allows us to detect signatures of the
local velocity fields such as asymmetric excess line wing absorption,
best-fit {v<SUB>e</SUB> sin i} parameter values that are found to be
larger for strong lines than for weak lines, and discrepancies between
observed and modelled line profile shapes. <BR />Results: Symptoms
of local atmospheric velocity fields are always detected through
a non-zero microturbulence parameter for main sequence stars having
{T<SUB>e</SUB>} below about 10 000 K, but not for hotter stars. Direct
line profile tracers of the atmospheric velocity field are found in six
very sharp-lined stars in addition to the three reported earlier. Direct
signatures of velocity fields are found to occur in A stars with and
without the Am chemical peculiarities, although the amplitude of the
effects seems larger in Am stars. Velocity fields are also directly
detected in spectral line profiles of A and early F supergiants, but
without significant line asymmetries. <BR />Conclusions: We confirm
that several atmospheric velocity field signatures, particularly excess
line wing absorption which is stronger in the blue line wing than
in the red, are detectable in the spectral lines of main sequence A
stars of sufficiently low {v<SUB>e</SUB> sin i}. We triple the sample
of A stars known to show these effects, which are found both in Am
and normal A stars. We argue that the observed line distortions are
probably due to convective motions reaching the atmosphere. These data
still have not been satisfactorily explained by models of atmospheric
convection, including numerical simulations. <P />Based in part on
observations obtained at the <P />Canada-France-Hawaii Telescope
(CFHT) which is operated by the <P />National Research Council of
Canada, the Institut National des <P />Sciences de l'Univers of the
Centre National de la Recherche <P />Scientifique of France, and the
University of <P />Hawaii. <P />Based in part on observations made at
<P />Observatoire de Haute Provence (CNRS), France.
---------------------------------------------------------
Title: Using p-mode excitation rates for probing convection in
solar-like stars
Authors: Kupka, F.; Belkacem, K.; Goupil, J. -M.; Samadi, R.
2009CoAst.159...24K Altcode:
We discuss how the possibility to measure mode excitation rates through
means of helio- and asteroseismology has improved our capabilities to
test convection models and numerical simulations of surface convection
and avoids ambiguities that have limited previous approaches.
---------------------------------------------------------
Title: 3D stellar atmospheres for stellar structure models and
asteroseismology.
Authors: Kupka, F.
2009MmSAI..80..701K Altcode: 2009arXiv0911.3353K
Convection is the most important physical process that determines the
structure of the envelopes of cool stars. It influences the surface
radiation flux and the shape of observed spectral line profiles and is
responsible for both generating and damping solar-like oscillations,
among others. 3D numerical simulations of stellar surface convection
have developed into a powerful tool to model and analyse the physical
mechanisms operating at the surface of cool stars. This review
discusses the main principles of 3D stellar atmospheres used for such
applications. The requirements from stellar structure and evolution
theory to use them as boundary conditions are analysed as well as the
capabilities of using helio- and asteroseismology to reduce modelling
uncertainties and probing the consistency and accuracy of 3D stellar
atmospheres as part of this process. Simulations for the solar surface
made by different teams are compared and some issues concerning the
uncertainties of this modelling approach are discussed.
---------------------------------------------------------
Title: An Introduction to Turbulence
Authors: Hillebrandt, W.; Kupka, F.
2009LNP...756....1H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Turbulent Convection and Numerical Simulations in Solar and
Stellar Astrophysics
Authors: Kupka, F.
2009LNP...756...49K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Interdisciplinary Aspects of Turbulence
Authors: Hillebrandt, Wolfgang; Kupka, Friedrich
2009LNP...756.....H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Inter-Division IV-V / Working Group Ap and Related Stars
Authors: Cunha, Margarida S.; Weiss, Werner W.; Dworetsky, Michael M.;
Kochukhov, Oleg; Kupka, Friedrich; Leblanc, Francis; Monier, Richard;
Paunzen, Ernst; Piskunov, Nikolai E.; Shibahashi, Hiromoto; Smalley,
Barry; Ziznovsky, Jozef
2009IAUTA..27..245C Altcode:
The diversity of physical phenomena embraced by the study of Chemically
Peculiar (CP) stars results in an associated research community
with interests that are equally diverse. This fact became once more
evident during the CP#Ap Workshop that took place in Vienna (Austria)
in September 2007, and which gathered over 80 members of this research
community. Besides the excellent scientific outcome of the meeting,
during the workshop the community had the opportunity to discuss its
organization and plans for the future. Following on those plans,
the Working Group has submitted a proposal for a Joint Discussion
during the IAU XXVII General Assembly, in Rio de Janeiro, which has
meanwhile been accepted. Moreover, through an ApN newsletter forum,
the Working Group has compiled requests from the community concerning
atomic and related data. These requests have been put together and
will be shared with Commission 14.
---------------------------------------------------------
Title: Vald
Authors: Obbrugger, M.; Heiter, U.; Kupka, F.; Lüftinger, T.;
Nesvacil, N.; Piskunov, N.; Ryabchikova, T. A.; Stempels, H. C.;
Stütz, Ch.; Weiss, W. W.
2008asvo.proc..215O Altcode:
VALD is a collection of atomic transition parameters and supporting
extraction software. VALD services are available via Email (VALD-EMS)
and the Web interface. Different kinds of requests are useful for
several needs like abundance analysis, radial velocity measurements,
or line identification. Since 1994, the early days of VALD, the database
has been constantly improved and the release of VALD-3 - incorporating,
e.g., molecular data and new line lists - is in preparation. With
meanwhile more than 5 000 000 requests VALD developed to a much
appreciated tool.
---------------------------------------------------------
Title: Analysing the Contributions in Moment Equations of Reynolds
Stress Models of Convection with Numerical Simulations
Authors: Kupka, F.; Muthsam, H. J.
2008IAUS..252..463K Altcode:
We discuss how 3D numerical simulations can be used to analyse the
different contributions within dynamical equations of non-local Reynolds
stress models of convection.
---------------------------------------------------------
Title: Shear Driven Turbulence and Coherent Structures in Solar
Surface Simulations
Authors: Kupka, F.
2008IAUS..252..451K Altcode:
Numerical simulations of convection near the solar surface are now
advanced enough to reproduce both a large set of observational data and
provide tests for convection models. We discuss the role of coherent
structures in models of solar p-mode excitation, for which the analysis
of numerical simulations has provided key inputs in the modelling. The
robustness of these simulations is shown by a comparison illustrating
the influence of boundary conditions on ensemble averaged quantities. In
a concluding example advanced high resolution simulations are shown
to resolve the onset of shear driven turbulence generated by up-
and downflow structures.
---------------------------------------------------------
Title: Modeling the excitation of acoustic modes in α Centauri A
Authors: Samadi, R.; Belkacem, K.; Goupil, M. J.; Dupret, M. -A.;
Kupka, F.
2008A&A...489..291S Altcode: 2008arXiv0806.0725S
From different seismic observations we infer the energy supplied
per unit of time by turbulent convection to the acoustic modes of α
Centauri A (HD 128620), a star that is similar but not identical to
the Sun. The inferred rates of energy supplied to the modes (i.e. mode
excitation rates) are found to be significantly higher than in the
Sun. They are compared with those computed with an excitation model that
includes two sources of driving, the Reynolds stress contribution and
the advection of entropy fluctuations. The model also uses a closure
model, the Closure Model with Plumes (CMP hereafter), that takes
the asymmetry between the up- and down-flows (i.e. the granules and
plumes, respectively) into account. Different prescriptions for the
eddy-time correlation function are also compared to observational
data. Calculations based on a Gaussian eddy-time correlation
underestimate excitation rates compared with the values derived from
observations for α Centauri A. On the other hand, calculations based
on a Lorentzian eddy-time correlation lie within the observational
error bars. This confirms results in the solar case. Compared to
the helioseismic data, those obtained for α Centauri A constitute
an additional support for our model of excitation. We show that mode
masses must be computed taking turbulent pressure into account. Finally,
we emphasize the need for more accurate seismic measurements in order
to distinguish between the CMP closure model and the quasi-normal
approximation in the case of α Centauri A, as well as to confirm or
not the need to include the excitation by the entropy fluctuations.
---------------------------------------------------------
Title: VALD — an atomic and molecular database for astrophysics
Authors: Heiter, U.; Barklem, P.; Fossati, L.; Kildiyarova, R.;
Kochukhov, O.; Kupka, F.; Obbrugger, M.; Piskunov, N.; Plez, B.;
Ryabchikova, T.; Stempels, H. C.; Stütz, Ch; Weiss, W. W.
2008JPhCS.130a2011H Altcode:
The VALD database of atomic and molecular data aims to ensure a robust
and consistent analysis of astrophysical spectra. We offer a convenient
e-mail and web-based user interface to a vast collection of spectral
line parameters for all chemical elements and in the future also for
molecules. An international team is working on the following tasks:
collecting line parameters from relevant theoretical and experimental
publications, computing line parameters, evaluating the data quality by
comparison of similar data from different sources and by comparison
with astrophysical observations, and incorporating the data into
VALD. A unique feature of VALD is its capability to provide the most
comprehensive spectral line lists for specific astrophysical plasma
conditions defined by the user.
---------------------------------------------------------
Title: High-resolution models of solar granulation: the
two-dimensional case
Authors: Muthsam, H. J.; Löw-Baselli, B.; Obertscheider, Chr.;
Langer, M.; Lenz, P.; Kupka, F.
2007MNRAS.380.1335M Altcode: 2007arXiv0706.3349M; 2007MNRAS.tmp..777M
Using advanced numerical schemes and grid refinement, we present 2D
high-resolution models of solar granulation with particular emphasis on
downflowing plumes. In the high-resolution portion of our simulation,
a box measuring 1.97 × 2.58Mm<SUP>2</SUP> (vertical × horizontal),
the grid size is 1.82 × 2.84km<SUP>2</SUP>. Calculations at the
resolution usually applied in this type of simulations amount to only a
few horizontal gridpoints for a downflowing plume. Due to the increased
number of gridpoints in our high-resolution domain, the simulations
show the development of vigorous secondary instabilities of both the
plume's head and stem. The plume's head produces counterrotating vortex
patches, a topology due to the 2D nature of the simulations. Below a
depth of about 1Mm, the plume's head and stem instabilities produce,
in these 2D models, patches of low density, temperature, pressure
and high vorticity which may last for all of our simulation time,
~10min, and probably considerably longer. Centrifugal forces acting
in these patches counteract the strong inward pressure. Probably most
importantly, the plume's instabilities give rise to acoustic pulses
created predominantly down to ~1.5Mm. The pulses proceed laterally
as well as upwards and are ubiquitous. Ultimately, most of them
emerge into the photosphere. A considerable part of the photospheric
`turbulence' in these models is due to those pulses rather than to some
sort of eddies. The upflows in granules are smooth where they reach
the photosphere from below even in the present calculations; however,
the pulses may enter in the photosphere also in granular upflows.
---------------------------------------------------------
Title: Eclipsing binaries as a test for synthetic photometry
Authors: Heiter, U.; Smalley, B.; Stütz, Ch.; Kupka, F.; Kochukhov, O.
2007IAUS..240..328H Altcode:
Narrow band photometry is a viable tool to characterize large numbers
of stars. The connection between observed colors and astrophysical
parameters has to rely on synthetic photometry calculated from stellar
atmosphere models. Here, we present synthetic Hbeta indices calculated
from 1D model atmospheres, which implement various treatments of
convection. The calculated indices are transformed to the standard
system using observed medium-resolution spectra from recently published
stellar libraries. We test how well the synthetic photometry reproduces
observed indices by using a number of eclipsing binary systems. For
these stars, atmospheric parameters can be determined independently
from the models with highest possible accuracy. As a preliminary
conclusion, the computed indices deviate from the observed ones by an
amount expected from the observational errors and the accuracy of the
atmospheric parameters.
---------------------------------------------------------
Title: Convection in Astrophysics (IAU S239)
Authors: Kupka, Friedrich; Roxburgh, Ian; Chan, Kwing Lam
2007IAUS..239.....K Altcode:
Preface; Session A. Modelling convection and radiative transfer; Session
B. Observational probes of convection; Session C. Convection in planets
and brown dwarfs; Session D. Stellar evolution, nucleosynthesis and
convective mixing; Session E. Oscillations, mass loss and convection;
Session F. Convection and rotation; Session G. MHD convection and
dynamos.
---------------------------------------------------------
Title: Two-scale mass-flux closure models for turbulence: p-mode
amplitudes in solar-like stars
Authors: Belkacem, K.; Samadi, R.; Goupil, M. -J.; Kupka, F.; Dupret,
M. -A.
2007CoAst.150..153B Altcode:
A new closure model has been developed, which takes into account both
the skewness of the velocity distribution induced by the presence of
two flows in the convection zone, and the effects of turbulence onto
each flow (Belkacem et al. 2006a). Applied to the formalism of p-mode
excitation, it has been possible to validate this theoretical model
by a comparison with the observational excitation rates in the solar
case using GOLF data (see Belkacem et al. 2006b). The next step is
to consider α Cen A for which observations of the mode-damping rates
are available.
---------------------------------------------------------
Title: Reynolds stress models of convection in convective cores
Authors: Roxburgh, Ian W.; Kupka, Friedrich
2007IAUS..239...77R Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Probing Reynolds stress models of convection with numerical
simulations: I. Overall properties: fluxes, mean profiles
Authors: Kupka, F.; Muthsam, H. J.
2007IAUS..239...80K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Mixing length model of convection in stellar cores
Authors: Roxburgh, Ian W.; Kupka, Friedrich
2007IAUS..239...98R Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Some open questions concerning the modelling of non-locality
in Reynolds stress type models of stellar convection.
Authors: Kupka, F.
2007IAUS..239...92K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Coherent structures in granulation convection and their
importance for higher order closure models
Authors: Kupka, F.; Robinson, F. J.
2007IAUS..239...74K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A closure model for turbulent convection. Application to the
excitation of p modes
Authors: Belkacem, K.; Samadi, R.; Goupil, M. -J.; Kupka, F.;
Baudin, F.
2007IAUS..239..376B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Eclipsing binaries as a test for synthetic photometry
Authors: Heiter, U.; Smalley, B.; Stütz, Ch.; Kupka, F.; Kochukhov, O.
2007IAUS..239..169H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Modelling of solar granulation
Authors: Muthsam, H. J.; Löw-Baselli, B.; Obertscheider, Chr.;
Langer, M.; Lenz, P.; Kupka, F.
2007IAUS..239...89M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Probing Reynolds stress models of convection with numerical
simulations: II. Non-locality and third order moments.
Authors: Kupka, F.; Muthsam, H. J.
2007IAUS..239...83K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Solar-like oscillation amplitudes and line-widths as a probe
for turbulent convection in stars
Authors: Samadi, R.; Belkacem, K.; Goupil, M. -J.; Kupka, F.; Dupret,
M. -A.
2007IAUS..239..349S Altcode: 2006astro.ph.11760S
Excitation of solar-like oscillations is attributed to turbulent
convection and takes place at the upper-most part of the outer
convective zones. Amplitudes of these oscillations depend on the
efficiency of the excitation processes as well as on the properties
of turbulent convection. We present past and recent improvements on
the modeling of those processes. We show how the mode amplitudes and
mode line-widths can bring information about the turbulence in the
specific cases of the Sun and Alpha Cen A.
---------------------------------------------------------
Title: Probing Reynolds stress models of convection with numerical
simulations: III. Compressibility modelling and dissipation
Authors: Kupka, F.; Muthsam, H. J.
2007IAUS..239...86K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Effect of the microturbulence parameter on the Color-Magnitude
Diagram
Authors: Montalbán, J.; Nendwich, J.; Heiter, U.; Kupka, F.; Paunzen,
E.; Smalley, B.
2007IAUS..239..166M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Round table discussion of session A: modelling convection
and radiative transfer
Authors: Kupka, F.
2007IAUS..239...64K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On the effects of coherent structures on higher order moments
in models of solar and stellar surface convection
Authors: Kupka, F.; Robinson, F. J.
2007MNRAS.374..305K Altcode:
Non-local models of stellar convection usually rely on the assumption
that the transfer of convective heat flux, turbulent kinetic energy
and related quantities can be described as a diffusion process or that
the fourth-order moments of velocity and temperature fluctuations
follow a Gaussian distribution (quasi-normal approximation). The
latter is also assumed in models of solar p-mode excitation. <P />We
have used realistic numerical simulations of solar granulation and of
granulation in a K dwarf to test the quasi-normal approximation and
several alternatives. For the superadiabatic layer of the Sun and for
the quasi-adiabatic zone underneath, we find that the hypothesis of
quasi-normality is a rather poor approximation. In the superadiabatic
layer, it overestimates some of the fourth-order moments of vertical
velocity and temperature by up to a factor of 2 while it underestimates
them in the quasi-adiabatic layers by up to a factor of 3.5. The model
proposed by Gryanik & Hartmann and Gryanik et al. reduces the
discrepancies within the quasi-adiabatic zone to typically less than 30
per cent and is partially comparable and partially in better agreement
with the simulation data than two earlier models by Grossman &
Narayan. Simulation data for the K dwarf confirm these results. However,
none of the proposed models works well in the superadiabatic layer nor
in the photospheric layers above. For the Sun, we provide evidence that
the fourth-order moments of horizontal velocity fields can be estimated
to within about 30 per cent with the quasi-normal approximation despite
the complexity of the horizontal flow. Comparing our results to those
from solar simulations with idealized microphysics and with related
studies of geophysical convection zones confirms our conclusions about
the quasi-normal approximation and the new models. <P />The improvements
come from including the effects of coherent structures (such as granules
or plumes), while the limitations are tied to the transition regions
or boundaries such as the rapid radiative cooling that occurs at the
top of the convection zone. Incorporating the model of Gryanik &
Hartmann and Gryanik et al. into non-local convection models may well
produce a significant improvement in the modelling of convection or of
solar-like p-mode excitation in the quasi-adiabatic part of convection
zones. For application to entire convection zones, modifications are
necessary which can account for the change in background properties
of the convective medium near boundaries or transition regions.
---------------------------------------------------------
Title: A closure model with plumes. I. The solar convection
Authors: Belkacem, K.; Samadi, R.; Goupil, M. J.; Kupka, F.
2006A&A...460..173B Altcode: 2006astro.ph..7568B
Context: .Oscillations of stellar p modes, excited by turbulent
convection, are investigated. In the uppermost part of the solar
convection zone, radiative cooling is responsible for the formation
of turbulent plumes, hence the medium is modelled with downdrafts and
updrafts.<BR /> Aims: .We take into account the asymmetry of the up-
and downflows created by turbulent plumes through an adapted closure
model. In a companion paper, we apply it to the formalism of excitation
of solar p modes developed by Samadi & Goupil (2001).<BR /> Methods:
.Using results from 3D numerical simulations of the uppermost part of
the solar convection zone, we show that the two-scale mass-flux model
(TFM) is valid only for quasi-laminar or highly skewed flows (Gryanik
& Hartmann 2002) and does not reproduce turbulent properties
of the medium such as velocity-correlation products. We build a
generalized two-scale mass-flux Model (GTFM) model that takes both
the skew introduced by the presence of two flows and the effects of
turbulence in each flow into account. In order to apply the GTFM to
the solar case, we introduce the plume dynamics as modelled by Rieutord
& Zahn (1995) and construct a closure model with plumes (CMP).<BR
/> Results: .The CMP enables expressing the third- and fourth-order
correlation products in terms of second-order ones. When compared
with 3D simulation results, the CMP improves the agreement for the
fourth-order moments by a factor of two approximately compared with the
use of the quasi-normal approximation or a skewness computed with the
classical TFM.<BR /> Conclusions: .The asymmetry of turbulent convection
in the solar case has an important impact on the vertical-velocity
fourth-order moment, which has to be accounted for by models. The CMP
is a significant improvement and is expected to improve the modelling
of solar p-mode excitation.
---------------------------------------------------------
Title: A closure model with plumes. II. Application to the stochastic
excitation of solar p modes
Authors: Belkacem, K.; Samadi, R.; Goupil, M. J.; Kupka, F.; Baudin, F.
2006A&A...460..183B Altcode: 2006astro.ph..7570B
Context: . Amplitudes of stellar p modes result from a balance between
excitation and damping processes taking place in the uppermost part
of convective zones in solar-type stars and can therefore be used as
a seismic diagnostic for the physical properties of these external
layers. <BR /> Aims: . Our goal is to improve the theoretical modelling
of stochastic excitation of p modes by turbulent convection. <BR />
Methods: . With the help of the closure model with plume (CMP) developed
in a companion paper, we refine the theoretical description of the
excitation by the turbulent Reynolds stress term. The CMP is generalized
for two-point correlation products so as to apply it to the formalism
developed by Samadi & Goupil (2001, A&A, 370, 136). The
excitation source terms are then computed with this improvement, and a
comparison with solar data from the GOLF instrument is performed. <BR
/> Results: .The present model provides a significant improvement when
comparing absolute values of theoretical amplitudes with observational
data. It gives rise to a frequency dependence of the power supplied
to solar p modes, which agrees with GOLF observations. It is shown
that the asymmetry of the turbulent convection zone (up and downflows)
plays a major role in the excitation processes. Despite an increase in
the Reynolds stress term contribution due to our improved description,
an additional source of excitation, identified as the entropy source
term, is still necessary for reproducing the observational data. <BR />
Conclusions: .Theoretical excitation rates in the frequency range ν in
[2.5 mHz, 4 mHz] now are in agreement with the observational data from
the GOLF instrument. However, at lower frequencies, it exhibits small
discrepancies at the maximum level of a few per cent. Improvements are
likely to come from a better physical description of the excitation
by entropy fluctuations in the superadiabatic zone.
---------------------------------------------------------
Title: Seismic diagnostics inferred from the amplitudes of
stochastically-excited modes
Authors: Samadi, R.; Belkacem, K.; Goupil, M. -J.; Kupka, F.
2006IAUJD..17E..14S Altcode:
Solar-like oscillations have been detected in numerous main-sequence
stars as well as in red giant stars. The excitation of such oscillations
is ensured by turbulent convection at the upper most part of the
convective zones. From the measurement of the mode amplitude and
line-width it is possible to infer the power supplied to the modes by
turbulent convection. Apart from the Sun, α Cen A is the only star for
which both mode amplitudes and mode line-widths have been derived. I
will first present the diagnostics inferred from the measurement of the
amplitudes and line-widths in the Sun and in α Cen A. For other stars,
despite the lack of direct constraints on the excitation mechanisms,
I will show that the solar-like oscillations observed in different
stars can nevertheless offer interesting constraints on the properties
of turbulent convection.
---------------------------------------------------------
Title: Excitation of solar p modes. Effect of the asymmetry of the
convection zone
Authors: Belkacem, K.; Samadi, R.; Goupil, M. J.; Kupka, F.; Baudin, F.
2006sf2a.conf..523B Altcode:
Excitation of stellar p modes by turbulent convection is
investigated. The aim is to take into account the asymmetry of up-
and downflows created by turbulent plumes through an adapted closure
model. We built a generalized two scale mass flux model (GTFM) that
includes both the skew introduced by the presence of two flows and
the effect of turbulence within each flow. The plume dynamics modelled
according to Rieutord & Zahn (1995) is used to construct a closure
model with plumes (CMP). We apply it to the formalism of excitation
of stellar p modes developed by Samadi & Goupil (2001). The new
excitation model leads to a frequency dependence, of the power supplied
to solar p modes, which is in agreement with GOLF observations. Despite
an increase of the Reynolds stress contribution due to our improved
description, an additional source of excitation -identified as the
entropy source term- is still necessary to reproduce the maximum of
excitation rate. Our modelling including the entropy contribution
reproduces the maximum but <P />over-estimates, at low frequencies,
the power and calls for further theoretical improvements.
---------------------------------------------------------
Title: Prospects for population synthesis in the H band: NeMo grids
of stellar atmospheres compared to observations
Authors: Frémaux, J.; Kupka, F.; Boisson, C.; Joly, M.; Tsymbal, V.
2006A&A...449..109F Altcode: 2005astro.ph.11125F
Context: .For applications in population synthesis, libraries of
theoretical stellar spectra are often considered an alternative
to template libraries of observed spectra, because they allow a
complete sampling of stellar parameters. Most of the attention in
published theoretical spectral libraries has been devoted to the
visual wavelength range.Aims.The goal of the present work is to
explore the near-infrared range where few observed fully calibrated
spectra and no theoretical libraries are available.Methods.We make
a detailed comparison of theoretical spectra in the range 1.57-1.67
μm for spectral types from A to early M and for giant and dwarf
stars, with observed stellar spectra at resolutions around 3000,
which would be sufficient to disentangle the different groups of
late-type stars. We selected the NeMo grids of stellar atmospheres
to perform this comparison.Results.We first demonstrate that observed
spectral flux distributions can be matched very well with theoretical
ones for almost the entire parameter range covered by the NeMo grids at
moderate resolution in the visual range. In the infrared range, although
the overall shape of the observed flux distributions still matches
reasonably well, the individual spectral features are reproduced by the
theoretical spectra only for stars earlier than mid F type. For later
spectral types the differences increase, and theoretical spectra of K
type stars have systematically weaker line features than those found
in observations. These discrepancies are traced back to stem primarily
from incomplete data on neutral atomic lines, although some of them are
also related to molecules.Conclusions.Libraries of theoretical spectra
for A to early M type stars can be successfully used in the visual
regions for population synthesis, but their application in the infrared
is restricted to early and intermediate type stars. Improving atomic
data in the near infrared is a key element in making the construction
of reliable libraries of stellar spectra feasible in the infrared.
---------------------------------------------------------
Title: Limb-darkening coefficients for the purpose of pulsation mode
identification for A-F stars. .
Authors: Barban, C.; Goupil, M. J.; van't Veer-Menneret, C.; Garrido,
R.; Heiter, U.; Kupka, F.
2006MmSAI..77..101B Altcode:
Limb-darkening coefficients are computed from a set of model atmospheres
with: a solar chemical composition, 6000 K< T<SUB>eff</SUB> <
8500 K (Delta T_eff=250 K), 2.5 < logg < 4.5 (Delta log g=0.1)
and a microturbulent velocity of 2 km/s. Convection is included assuming
either the turbulent convection approach of \citet{cm} or the classical
mixing length prescription with alpha =0.5 and 1.25. Four limb-darkening
laws have been used: quadratic, cubic, square root and the one of
\citet{cl}. We compare the ATLAS 9 intensities and the ones computed
from these laws. We find that Claret's law is the best law for almost
all the models, independently of the convection prescription used.
---------------------------------------------------------
Title: Influence of local treatments of convection upon solar p mode
excitation rates
Authors: Samadi, R.; Kupka, F.; Goupil, M. J.; Lebreton, Y.; van't
Veer-Menneret, C.
2006A&A...445..233S Altcode: 2005astro.ph..7243S
We compute the rates P at which acoustic energy is injected into
the solar radial p modes for several solar models. The solar models
are computed with two different local treatments of convection: the
classical mixing-length theory (MLT) and the formulation by Canuto et
al. (1996, ApJ, 473, 550, CGM). Among the models investigated here,
our best models reproduce both (i) the solar radius and the solar
luminosity at solar age and (ii) the observed Balmer line profiles. For
the MLT treatment, the rates P do significantly depend on the properties
of the atmosphere, whereas for the CGM treatment, the dependence of
P on the properties of the atmosphere is found to be smaller than
the error bars attached to the seismic measurements. The excitation
rates P for modes associated with the MLT models are significantly
underestimated compared with the solar seismic constraints. The CGM
models yield values for P closer to the seismic data than do the MLT
models. We conclude that the solar p-mode excitation rates provide
valuable constraints and, according to the present investigation,
clearly favor the CGM treatment with respect to the MLT, although
neither of them yields values of P as close to the observations as
recently found for 3D numerical simulations.
---------------------------------------------------------
Title: Workshop on "Interdisciplinary Aspects of Turbulence"
Authors: Kupka, F.; Hillebrandt, W.
2005iat..work.....K Altcode:
The Workshop on Interdisciplinary Aspects of Turbulence was organized
within the framework of the Max-Planck-Institute for Astrophysics in
Garching, Germany, and held at Ringberg Castle. It brought together
researchers from the fields of Astrophysics, Atmospheric and Ocean
Physics, Engineering and Laboratory Turbulence Research, as well
as Non-linear Dynamics and Statistical Mechanics who share a common
interest in turbulence and its properties of mixing, self-organisation
of large scale structures, and related issues and the interdisciplinary
aspects underlying these topics. The proceedings volume contains a
collection of extended abstracts as well as short review papers based
on 30 from a total of 40 contributions presented at the workshop as well
as links and references to further material discussed at the workshop.
---------------------------------------------------------
Title: Turbulent convection in astrophysics and geophysics -
a comparison
Authors: Kupka, F.
2005iat..work..141K Altcode:
Energy transport by turbulent convection is studied in both astrophysics
and geophysics. We briefly outline the physics of convection as well
as the differences between astrophysical and geophysical turbulent
convective flows. The case of convection in stars including our
sun is described in more detail. Reynolds stress models have been
suggested for quantitative predictions for both stellar and geophysical
convection. One of the main problems of such models is how to account
for the influence of the observed large scale coherent structures. We
discuss a possible solution which has first been suggested for the
convective planetary boundary layer and show how it performs when
applied to convection in our sun.
---------------------------------------------------------
Title: Spectrum synthesis of sharp-line B, A and F stars
Authors: Silaj, J.; Townshend, A.; Kupka, F.; Landstreet, J.; Sigut, A.
2005EAS....17..345S Altcode:
The spectra of a number of reportedly sharp-line B, A and F stars
have been observed in a 60-Å window with a resolving power of
120 000. As a first step in analysis of these data, the spectra have
been synthesized to see how well simple models fit. The information
obtained from both successful modelling and from discrepant fits
is discussed.
---------------------------------------------------------
Title: Direct Simulations of Radiative and Convective Zones
Authors: Kupka, F.
2005EAS....17..177K Altcode:
Numerical simulations have become one of the main tools in the research
on stellar convection zones and they can be expected to become very
important for the study of the convection-diffusion interaction as
well. In this review, I will first provide some historical background
and then select a few sample problems where numerical simulations of
stellar convection have provided useful results or are expected to do
so in the near future: solar granulation, interacting convection zones,
and semi-convection.
---------------------------------------------------------
Title: Some Physics We Can Learn from Spectroscopy of A-Type Stars
Authors: Kupka, F.
2005PhST..119...20K Altcode:
Stars of spectral type A are distinguished from others by the
interesting fact that several physical processes are of equal importance
in their outer envelope. The competing processes include diffusion
driven by either gravitational settling or radiation pressure, thermal
convection driven by partial ionisation of hydrogen and/or helium, their
interaction with a global stellar magnetic field in some cases, and
large rotation rates or pulsational instabilities in others. Whether one
of them dominates over the others is also determined by their initial
and present environment. Most of the information on the physics of
these processes is obtained from spectroscopy of the surface layers of
these stars. In spectroscopic studies of A-stars we can either attempt
to obtain a better understanding of the physical processes, if our
atomic data are good enough, or alternatively, if the stellar physics
provides strong enough constraints, we can learn about atomicphysics:
wavelengths, oscillator strengths, pressure broadening, and other
quantities which characterise electronic transitions in neutral and
ionised atoms within the stellar plasma. <P />Professor Charles R
Cowley has made many important contributions to the spectroscopy
of A-type stars throughout his research work and he is also very
active in this field today. I have enjoyed being able to work with
him on this subject on several occasions. In this paper I present an
overview as to why the spectroscopy of A-type stars is of interest
to both astrophysicists and laboratory spectroscopists, and I will
use various opportunities to illustrate the selected topics through
contributions made by Professor Cowley to this field.
---------------------------------------------------------
Title: Observational signatures of atmospheric velocity fields in
Main Sequence stars
Authors: Kupka, F.; Landstreet, J. D.; Sigut, A.; Bildfell, C.; Ford,
A.; Officer, T.; Silaj, J.; Townshend, A.
2004IAUS..224..573K Altcode:
In stars with sufficiently small projected rotational velocities
(less than a few km s<SUP>-1</SUP>), it is often possible to detect
signatures of the atmospheric velocity field in line profiles. These
signatures may be as subtle as small asymmetries in the profile ("line
bisector curvature") or as obvious as profile shapes that strongly
depart from those predicted by simple microturbulence models. We have
recently carried out a high resolution survey of sharp-line stars
to search for these symptoms of local velocity fields. We report the
first results of a comparison of models with the observed profiles.
---------------------------------------------------------
Title: Convection in stars
Authors: Kupka, F.
2004IAUS..224..119K Altcode:
Convection is one of the most intricate processes studied in stellar
astrophysics and has challenged both theorists and observers since
the beginnings of astrophysics. But during the last two decades
observational data of unprecedented resolution and accuracy have
been collected in solar and stellar research which permit a new look
at the field. An enormous increase of computer speed now permits
solving more complete model equations with more accurate numerical
approximations. Modelling and theoretical understanding of convection,
however, are lagging behind observational progress and are still
wanting.
---------------------------------------------------------
Title: Stellar model atmospheres with emphasis on velocity dynamics
Authors: Stütz, Ch.; Kupka, F.
2004IAUS..224..570S Altcode:
Recently a formalism for solving the open set of non-local hydrodynamic
moment equations has been applied to the envelopes of A-stars by
Kupka & Montgomery (2002). Tests on numerical simulations and
selected properties derived from stellar spectra such as average
surface velocities and velocity skewness have shown the applicability
of this RSM approach. In addition, a variety of improvements over
existing modeling standards have been developed within and around
the AMS group at the Institute for Astronomy in Vienna during the
last couple of years. For example, individual elemental abundance
patterns (Piskunov & Kupka 2001) or stratification as an observed
parameter (Shuliak et al. 2004) can be taken into account in our model
atmospheres. Once the underlying programs, which currently operate as
stand alone applications, are combined with a model atmosphere code,
we will obtain a powerful and efficient tool that will allow us the
investigation of a number of open problems in the physics of A-stars,
such as more realistic models of the temperature gradients of cool
CP2 stars.
---------------------------------------------------------
Title: Panel discussion section I
Authors: Kupka, F.
2004IAUS..224..465K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Effect of Local Treatments of Convection upon the Solar P-Mode
Excitation Rates
Authors: Samadi, R.; Goupil, M. J.; Lebreton, Y.; van't Veer, C.;
Kupka, F.
2004ESASP.559..611S Altcode: 2004soho...14..611S; 2004astro.ph..9324S
We compute, for several solar models, the rates P at which the
solar radial p modes are expected to be excited. The solar models
are computed with two different local treatments of convection : the
classical mixing-length theory (MLT hereafter) and Canuto, Goldmann
and Mazzitelli(1996, CGM hereafter)'s formulation. For one set of
solar models (EMLT and ECGM models), the atmosphere is gray and
assumes Eddington's approximation. For a second set of models (KMLT
and KCGM models), the atmosphere is built using a T(tau) law which has
been obtained from a Kurucz's model atmosphere computed with the same
local treatment of convection. The mixing-length parameter in the model
atmosphere is chosen so as to provide a good agreement between synthetic
and observed Balmer line profiles, while the mixing-length parameter
in the interior model is calibrated so that the model reproduces
the solar radius at solar age. For the MLT treatment, the rates P do
depend significantly on the properties of the atmosphere. On the other
hand, for the CGM treatment, differences in P between the ECGM and the
KCGM models are very small compared to the error bars attached to the
seismic measurements. The excitation rates P for modes from the EMLT
model are significantly under-estimated compared with the solar seismic
constraints. The KMLT model results in intermediate values for P and
shows also an important discontinuity in the temperature gradient and
the convective velocity. On the other hand, the KCGM model and the ECGM
model yield values for P closer to the seismic data than the EMLT and
KMLT models. We conclude that the solar p-mode excitation rates provide
valuable constraints and according to the present investigation cleary
favor the CGM treatment with respect to the MLT.
---------------------------------------------------------
Title: The 5200-Åflux depression of chemically peculiar stars -
II. The cool chemically peculiar and λ Bootis stars
Authors: Kupka, F.; Paunzen, E.; Iliev, I. Kh.; Maitzen, H. M.
2004MNRAS.352..863K Altcode: 2004MNRAS.tmp..204K
After establishing the synthetic Δa photometric system in the first
paper of this series, we now present model atmospheres computed with
individual abundances for a representative sample of chemically
peculiar (CP) stars and either confirm or redetermine their input
parameters through comparisons with photometric, spectrophotometric
and high-resolution spectroscopic data. The final models obtained from
this procedure were used to compute synthetic Δa indices which were
compared with observations. The observed behaviour of Δa is reproduced
for several types of CP stars: models for Am stars show negligible
(or marginally positive) values of a few mmag, while for λ Bootis
stars - and for metal deficient A stars in general - we obtain negative
values (as low as -12 mmag in one case). For the coolest CP2 stars with
effective temperatures below about 8500 K, we obtain mild (~+10 mmag)
to moderately large (~+30 mmag) flux depressions in agreement with
observations. However, Δa values for slightly hotter members of the CP2
group (for which still T<SUB>eff</SUB> < 10000 K) are underestimated
from these new models. The effect of the microturbulence parameter on
the Δa index is revisited and its different role in various types of
CP stars for reproducing the flux depression at 5200 Åis explained. We
also provide reasons why models based on enhanced microturbulence and
scaled solar abundance could not explain the observed flux depression
for all types of CP stars. We discuss potential improvements of
the current models including the possibility of still missing line
opacities (unidentified and autoionization lines), modifications due
to an explicit account of a global stellar magnetic field, and the
effect of vertical abundance stratification.
---------------------------------------------------------
Title: Interpolation of Stellar Model Grids and Application to the
NEMO Grid
Authors: Nendwich, J.; Heiter, U.; Kupka, F.; Nesvacil, N.; Weiss,
W. W.
2004CoAst.144...43N Altcode: 2004astro.ph..6381N
NEMO Grid of Stellar Atmospheres is a DVD with 91,520 stellar model
atmospheres representing a 5D grid of modified ATLAS atmospheres;
the purpose of the modifications was to include different treatments
of convection and higher vertical resolution. In addition, for every
model fluxes are provided and color indices for 14 different photometric
systems. Because the model grid contained gaps due to non-converging
models, we developed and applied a set of 4D interpolation routines to
complete the grid. All the data, which will continuously be updated,
can be found on the NEMO homepage http://ams.astro.univie.ac.at/nemo/
and are available via DVD.
---------------------------------------------------------
Title: White dwarf envelopes: further results of a non-local model
of convection
Authors: Montgomery, M. H.; Kupka, F.
2004MNRAS.350..267M Altcode: 2004astro.ph..1489M
We present results of a fully non-local model of convection for white
dwarf envelopes. We show that this model is able to reproduce the
results of numerical simulations for convective efficiencies ranging
from very inefficient to moderately efficient; this agreement is made
more impressive given that no closure parameters have been adjusted
in going from the previously reported case of A-stars to the present
case of white dwarfs. For comparison, in order to match the peak
convective flux found in numerical simulations for both the white dwarf
envelopes discussed in this paper and the A-star envelopes discussed
in our previous work requires changing the mixing length parameter of
commonly used local models by a factor of 4. We also examine in detail
the overshooting at the base of the convection zone, both in terms of
the convective flux and in terms of the velocity field; we find that
the flux overshoots by ~1.25 H<SUB>P</SUB> and the velocity by ~2.5
H<SUB>P</SUB>. Because of the large amount of overshooting found at the
base of the convection zone, the new model predicts the mixed region
of white dwarf envelopes to contain at least 10 times more mass than
local mixing length theory (MLT) models having similar photospheric
temperature structures. This result is consistent with the upper limit
given by numerical simulations which predict an even larger amount of
mass to be mixed by convective overshooting. Finally, we attempt to
parametrize some of our results in terms of local MLT-based models,
insofar as is possible given the limitations of MLT.
---------------------------------------------------------
Title: Convection in the atmospheres and envelopes of Pre-Main
Sequence stars
Authors: Montalbán, J.; D'Antona, F.; Kupka, F.; Heiter, U.
2004A&A...416.1081M Altcode: 2003astro.ph.10690M
The T<SUB>eff</SUB> location of Pre-Main Sequence (PMS) evolutionary
tracks depends on the treatment of over-adiabaticity (D'Antona
& Mazzitelli \cite{Antona1994}, \cite{Antona1998}). Since the
convection penetrates into the stellar atmosphere, also the treatment
of convection in the modeling of stellar atmospheres will affect the
location of the Hayashi tracks. In this paper we present new non-grey
PMS tracks for T<SUB>eff</SUB>,>4000 K. We compute several grids of
evolutionary tracks varying: i) the treatment of convection: either
the Mixing Length Theory (MLT) or Canuto et al. (\cite{Canuto1996e},
CGM) formulation of a Full Spectrum of Turbulence; ii) the atmospheric
boundary conditions: we use the new Vienna grids of ATLAS9 atmospheres
(Heiter et al. \cite{Heiter2002a}), which were computed using either MLT
(with α=Λ/H<SUB>p</SUB>=0.5) or CGM treatments. For comparison, we
also compute grids of models with the NextGen (Allard & Hauschildt
\cite{Allard1997}, AH97) atmosphere models, and a 1 {M<SUB>⊙</SUB>}
grey MLT evolutionary track using the α calibration based on
2D-hydrodynamical models (Ludwig et al. \cite{Ludwig1999}). These
different grids of models allow us to analyze the effects of convection
modeling on the non-grey PMS evolutionary tracks. We disentangle
the effect of the wavelength dependent opacity on a self-consistent
treatment of convection in the atmosphere from the role of the
convection model itself in the atmosphere and in the interior. While
for some parts of the HR diagram (e.g., A stars) a low efficiency of
atmospheric convection is clearly indicated by the data, for others the
evidence is conflicting, showing the weaknesses of all the presently
adopted local convection models. Nevertheless, the assumption of a
low photospheric efficiency permits us to reproduce a larger amount of
data and we have hence restricted our study to this case and draw the
following conclusions for it: i) in spite of the solar calibration,
if MLT convection is adopted a large uncertainty results in the shape
and location of PMS tracks, and the MLT calibration loses sense. ii)
As long as the model of convection is not the same in the interior
and in the atmosphere, the optical depth at which we take the boundary
conditions is an additional parameter of the models. iii) Furthermore,
very different sub-atmospheric structures are obtained (for MS and PMS
stellar models) depending not only on the treatment of convection,
but also on the optical depth at which the boundary conditions are
taken. iv) The comparison between NextGen based models and ATLAS9 based
models shows that in the T<SUB>eff</SUB> domain they have in common
(4000-10 000 K) the improved opacities in NextGen atmosphere models have
no relevant role on the PMS location, this being determined mainly by
the treatment of the over-adiabatic convection. v) In the framework of
standard stellar modeling (i.e., non-magnetic, non-rotating, spherical
models), the comparison between theoretical models and observational
data in very young binary systems indicates that, for both treatments
of convection (MLT and CGM) and for any of the atmosphere grids
(including those based on the 2D-hydrodynamical atmosphere models),
the same assumption for convection cannot be used in PMS and MS: either
the models fit the MS - and the Sun in particular - or they fit the
PMS. Convection in the PMS phase appears to be less efficient than
what is necessary to fit the Sun.
---------------------------------------------------------
Title: New grids of ATLAS9 atmospheres. II. Limb-darkening
coefficients for the Strömgren photometric system for A-F stars
Authors: Barban, C.; Goupil, M. J.; Van't Veer-Menneret, C.; Garrido,
R.; Kupka, F.; Heiter, U.
2003A&A...405.1095B Altcode:
Using up-to-date model atmospheres (Heiter et al. \cite{Heiter02})
with the turbulent convection approach developed by Canuto et
al. (\cite{Canuto96}, CGM), quadratic, cubic and square root limb
darkening coefficients (LDC) are calculated with a least square
fit method for the Strömgren photometric system. This is done for a
sample of solar metallicity models with effective temperatures between
6000 and 8500 K and with log g between 2.5 and 4.5. A comparison is
made between these LDC and the ones computed from model atmospheres
using the classical mixing length prescription with a mixing length
parameter alpha =1.25 and alpha =0.5. For CGM model atmospheres,
the law which reproduces better the model intensity is found to
be the square root one for the u band and the cubic law for the v
band. The results are more complex for the b and y bands depending
on the temperature and gravity of the model. Similar conclusions are
reached for MLT alpha =0.5 models. As expected much larger differences
are found between CGM and MLT with alpha =1.25. In a second part,
the weighted limb-darkening integrals, b<SUB>l</SUB>, and their
derivatives with respect to temperature and gravity, are then computed
using the best limb-darkening law. These integrals are known to be
very important in the context of photometric mode identification of
non-radial pulsating stars. The effect of convection treatment on these
quantities is discussed and as expected differences in the b<SUB>l</SUB>
coefficients and derivatives computed with CGM and MLT alpha =0.5 are
much smaller than differences obtained between computations with CGM
and MLT alpha =1.25. <P />Table 1 is only available in electronic form
at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)
or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/405/1095
---------------------------------------------------------
Title: Observations of Rapid Radial Velocity Variations of Spectral
Lines in Rapidly Oscillating Ap (roAp) Stars
Authors: Landstreet, J. D.; Kochukhov, O.; Kupka, F.; Ryabchikova,
T.; Weiss, W. W.
2003aahd.conf..347L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: VizieR Online Data Catalog: Limb-darkening coefficients from
ATLAS9 models (Barban+, 2003)
Authors: Barban, C.; Goupil, M. J.; van't Veer-Menneret, C.; Garrido,
R.; Kupka, F.; Heiter, U.
2003yCat..34051095B Altcode:
Using up-to-date model atmospheres (Heiter et
al. 2002A&A...392..619H) with the turbulent convection approach
developed by Canuto, Goldman & Mazzitelli (1996ApJ...473..550C,
CGM), quadratic, cubic and square root limb darkening coefficients
(LDC) are calculated with a least square fit method for the Stroemgren
photometric system. This is done for a sample of solar metallicity
models with effective temperatures between 6000 and 8500K and with logg
between 2.5 and 4.5. A comparison is made between these LDC and the
ones computed from model atmospheres using the classical mixing length
prescription with a mixing length parameter α=1.25 and α=0.5. For CGM
model atmospheres, the law which reproduces better the model intensity
is found to be the square root one for the u band and the cubic law for
the v band. The results are more complex for the b and y bands depending
on the temperature and gravity of the model. Similar conclusions are
reached for Mixing Length Theory (MLT) α=0.5 models. As expected much
larger differences are found between CGM and MLT with α=1.25. In a
second part, the weighted limb-darkening integrals, b<SUB>ell</SUB>,
and their derivatives with respect to temperature and gravity, are then
computed using the best limb-darkening law. These integrals are known
to be very important in the context of photometric mode identification
of non-radial pulsating stars. The effect of convection treatment
on these quantities is discussed and as expected differences in the
b<SUB>ell</SUB> coefficients and derivatives computed with CGM and MLT
α=0.5 are much smaller than differences obtained between computations
with CGM and MLT α=1.25. <P />The limb darkening coefficients are
given here for the u, v, b and y bands and for CGM models, MLT α=0.5
models and MLT α=1.25 models. <P />(3 data files).
---------------------------------------------------------
Title: The 5200-Å flux depression of chemically peculiar stars -
I. Synthetic Δa photometry: the normality line
Authors: Kupka, F.; Paunzen, E.; Maitzen, H. M.
2003MNRAS.341..849K Altcode: 2003astro.ph..3225K
The Δa photometric system provides an efficient observational method
to identify and distinguish magnetic and several other types of
chemically peculiar (CP) stars of spectral types B to F from other
classes of stars in the same range of effective temperatures. We
have developed a synthetic photometric system that can be used to
explore the capability of model atmospheres with individual element
abundances to predict photometric Δa magnitudes, which measure the
extent of the flux depression around 5200 Å found in different types
of CP stars. In this first paper, we confirm the observed dependency
of the a index as a function of various colour indices sensitive to
the effective temperature of stars as well as its average scatter
expected from surface gravity variations within the main-sequence
band. The behaviour of the so-called `normality line' of Δa systems
used in photometric observations of CP stars is well reproduced. The
metallicity dependence of the normality line of the Δa system was
computed for several grids of model atmospheres where the abundances
of elements heavier than He had been scaled +/-0.5 dex with respect to
the solar value. We estimate a lowering of Δa magnitudes for CP stars
within the Magellanic Clouds by ~-3 mmag relative to those in the solar
neighbourhood assuming an average metallicity of [Fe/H]=-0.5 dex. Using
these results on the metallicity bias of the Δa system we find the
observational systems in use suitable to identify CP stars in other
galaxies or distant regions of our own Galaxy and capable to provide
data samples on a statistically meaningful basis. In turn, the synthetic
system is suitable to test the performance of model atmospheres for CP
stars. This work will be presented in follow-up papers of this series.
---------------------------------------------------------
Title: Colors of ATLAS9 Atmospheres and Their Interpolation
Authors: Nendwich, J.; Nesvacil, N.; Weiss, W. W.; Heiter, U.;
Kupka, F.
2003IAUS..210P..A9N Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Problems and Difficulties in Building up Stellar Models with
Non-grey Boundary Conditions
Authors: Montalbán, J.; D'Antona, F.; Kupka, F.
2003IAUS..210P..C6M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Non-local Convection Models for Stellar Atmospheres and
Envelopes
Authors: Kupka, F.
2003IAUS..210..143K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Pre-Main Sequence and Main Sequence Models Using the Vienna
Grids of ATLAS9 Atmospheres
Authors: Montalbán, J.; Kupka, F.; D'Antona, F.; Heiter, U.
2003IAUS..210P..C5M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Convection in A Stars
Authors: Kupka, F.
2003ASPC..305..190K Altcode: 2003mfob.conf..190K
No abstract at ADS
---------------------------------------------------------
Title: Application of New ATLAS9 Model Atmosphere Grids
Authors: Heiter, U.; Kupka, F.; Samadi, R.; Barban, C.; van't-Veer
Menneret, C.; Lebreton, Y.; Goupil, M. -J.; Garrido, R.; Weiss, W. W.
2003IAUS..210P.E10H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Using Balmer Line Profiles to Investigate Convection in A
and F Stars
Authors: Smalley, B.; Kupka, F.
2003IAUS..210P.C10S Altcode: 2002astro.ph..7388S
Balmer lines are an important diagnostic of stellar atmospheric
structure, since they are formed at a wide range of depths within the
atmosphere. The different Balmer lines are formed at slightly different
depths making them useful atmospheric diagnostics. The low sensitivity
to surface gravity for stars cooler than ~8000 K makes them excellent
diagnostics in the treatment of atmospheric convection. For hotter
stars Balmer profiles are sensitive to both effective temperature and
surface gravity. Provided we know the surface gravity of these stars
from some other method (e.g. from eclipsing binary systems), we can
use them to determine effective temperature. In previous work, we have
found no significant systematic problems with using uvby photometry
to determine atmospheric parameters of fundamental (and standard)
stars. In fact, uvby was found to be very good for obtaining both Teff
and log g. Using Halpha and Hbeta profiles, we have found that both
the Canuto & Mazzitteli and standard Kurucz mixing-length theory
without approximate overshooting are both in agreement to within the
uncertainties of the fundamental stars. Overshooting models were always
clearly discrepant. Some evidence was found for significant disagreement
between all treatments of convection and fundamental values around
8000~9000K, but these results were for fundamental stars without
fundamental surface gravities. We have used stars with fundamental
values of both Teff and log g to explore this region in more detail.
---------------------------------------------------------
Title: Tools and Methods for Abundance Analysis
Authors: Knoglinger, P.; Nesvacil, N.; Kupka, F.; Mittermayer, P.;
Piskunov, N.; Weiss, W. W.; Bruntt, H.
2003IAUS..210P.E66K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Discovery of rapid radial velocity variations in the roAp
star 10 Aql and possible pulsations of β CrB
Authors: Kochukhov, O.; Landstreet, J. D.; Ryabchikova, T.; Weiss,
W. W.; Kupka, F.
2002MNRAS.337L...1K Altcode:
We report discovery of radial velocity variations in rare earth spectral
lines of the roAp star 10 Aql with amplitudes of between 30 and 130 m
s<SUP>-1</SUP> and periods of about 11 min. Radial velocity variations
with amplitude 70 m s<SUP>-1</SUP> may also have been detected in
one spectral line of Fe I in β CrB. If confirmed, our results may
indicate that all Ap stars in a certain temperature range pulsate,
which means that roAp stars do not exist as a separate class but are
only distinguished by higher pulsational amplitudes.
---------------------------------------------------------
Title: On the anomaly of Balmer line profiles of A-type
stars. Fundamental binary systems
Authors: Smalley, B.; Gardiner, R. B.; Kupka, F.; Bessell, M. S.
2002A&A...395..601S Altcode: 2002astro.ph..9239S
In previous work, Gardiner et al. (\cite{GKS99}) found evidence
for a discrepancy between the T<SUB>eff</SUB> obtained from Balmer
lines with that from photometry and fundamental values for A-type
stars. An investigation into this anomaly is presented using Balmer
line profiles of stars in binary system with fundamental values of both
T<SUB>eff</SUB> and log g. A revision of the fundamental parameters
for binary systems given by Smalley & Dworetsky (\cite{SD95}) is
also presented. The T<SUB>eff</SUB> obtained by fitting Hα and Hβ
line profiles is compared to the fundamental values and those obtained
from uvby photometry. We find that the discrepancy found by Gardiner et
al. (\cite{GKS99}) for stars in the range 7000 K <~ T<SUB>eff</SUB>
<~ 9000 K is no longer evident. Partly based on DENIS data obtained
at the European Southern Observatory.
---------------------------------------------------------
Title: New grids of ATLAS9 atmospheres I: Influence of convection
treatments on model structure and on observable quantities
Authors: Heiter, U.; Kupka, F.; van't Veer-Menneret, C.; Barban, C.;
Weiss, W. W.; Goupil, M. -J.; Schmidt, W.; Katz, D.; Garrido, R.
2002A&A...392..619H Altcode: 2002astro.ph..6156H
We present several new sets of grids of model stellar atmospheres
computed with modified versions of the ATLAS9 code. Each individual set
consists of several grids of models with different metallicities ranging
from [M/H] = -2.0 to +1.0 dex. The grids range from 4000 to 10 000 K
in T<SUB>eff</SUB> and from 2.0 to 5.0 dex in log {g}. The individual
sets differ from each other and from previous ones essentially in the
physics used for the treatment of the convective energy transport, in
the higher vertical resolution of the atmospheres and in a finer grid
in the (T<SUB>eff</SUB>, log {g}) plane. These improvements enable
the computation of derivatives of color indices accurate enough for
pulsation mode identification. In addition, we show that the chosen
vertical resolution is necessary and sufficient for the purpose of
stellar interior modelling. To explain the physical differences between
the model grids we provide a description of the currently available
modifications of ATLAS9 according to their treatment of convection. Our
critical analysis of the dependence of the atmospheric structure and
observable quantities on convection treatment, vertical resolution and
metallicity reveals that spectroscopic and photometric observations are
best represented when using an inefficient convection treatment. This
conclusion holds whatever convection formulation investigated here is
used, i.e. MLT(alpha =0.5), CM and CGM are equivalent. We also find that
changing the convection treatment can lead to a change in the effective
temperature estimated from Strömgren color indices from 200 to 400 K.
---------------------------------------------------------
Title: Mode identification using the exoplanetary camera
Authors: Garrido, R.; Moya, A.; Goupil, M. J.; Barban, C.; van't
Veer-Menneret, C.; Kupka, F.; Heiter, U.
2002CoAst.141...48G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A-star envelopes: a test of local and non-local models of
convection
Authors: Kupka, F.; Montgomery, M. H.
2002MNRAS.330L...6K Altcode: 2001astro.ph.12401K
We present results of a fully non-local, compressible model of
convection for A-star envelopes. This model quite naturally reproduces
a variety of results from observations and numerical simulations which
local models based on a mixing length do not. Our principal results,
which are for models with T <SUB>eff</SUB> between 7200 and 8500K, are
the following. First, the photospheric velocities and filling factors
are in qualitative agreement with those derived from observations of
line profiles of A-type stars. Secondly, the Heii and Hi convection
zones are separated in terms of convective flux and thermal interaction,
but joined in terms of the convective velocity field, in agreement with
numerical simulations. In addition, we attempt to quantify the amount
of overshooting in our models at the base of the Heii convection zone.
---------------------------------------------------------
Title: Colors in Eddington: implications for mode identification
Authors: Garrido, R.; Claret, A.; Moya, A.; Kupka, F.; Heiter, U.;
Barban, C.; Goupil, M. -J.; van't Veer-Menneret, C.
2002ESASP.485..103G Altcode: 2002sshp.conf..103G
Theoretical radial and non-radial modes of low n orders in main sequence
stars, like δ Scuti and β Cephei, do not show regular pattern in their
power spectra. Although Barban et al. (2001) have shown that assuming an
amplitude distribution certain patterns could be recovered, a method to
identify the mode is needed in order to perform real asteroseismology of
these objects. Examples are given in the literature where for several
stars showing many periods: FG Vir (24): Breger et al. (1999), XX Pyx
(22): Handler et al. (2000), 4 CVn (30): Breger et al. (1999) and BI CMi
(20): Breger et al. (2002), a physical modeling was not successful,
the reason being the mode identification. Multicolor photometry,
as well as line profile spectroscopy, have been the usual methods to
identify modes. In this work we will focus on the capabilities of color
information to discriminate the different angular degrees l of the
spherical harmonic associated to a radial or non-radial mode excited
in a given δ Scuti pulsating star. Some numerical simulations are
also presented for the exoplanetary camera in the space mission COROT
where it is shown that, under some physical assumptions, color can be
very important in order to identify the angular degree l. Furthermore
colors can give relevant information of the non-adiabatic properties
of some pulsating stars which, in the case of the δ Scuti stars,
are related with the treatment of the convection. Arguments are also
given to support the inclusion of color information in Eddington.
---------------------------------------------------------
Title: The Böhm-Vitense Gap: The Role of Turbulent Convection
Authors: D'Antona, F.; Montalbán, J.; Kupka, F.; Heiter, U.
2002ApJ...564L..93D Altcode:
“Böhm-Vitense gaps,” discontinuities in the color distribution
of A-F type stars along the main sequence, have traditionally
been attributed to the abrupt onset of strong convection (8000
K>=T<SUB>eff</SUB>>=6400 K) in stellar atmospheres and
envelopes. Using the full spectrum of turbulence model to describe
convective transport (Canuto, Goldman, & Mazzitelli) both in
the interior and in the atmosphere yields a very sharp transition
between structures that are convective only in the surface layers
and structures that show a well-developed convection also in the
interior. This produces a sudden change in the T<SUB>eff</SUB> of stars,
around T<SUB>eff</SUB>~=6800 K. Using numerical simulations, we show
that this feature produces a stellar depletion that is consistent with
the gap at 0.33<~B-V<~0.38. The standard mixing-length theory
does not show this behavior. Consequently, this particular gap is a
“T<SUB>eff</SUB>” gap and not a “color” gap. In fact, it is also
present in the V-I color distribution of the Hyades stars.
---------------------------------------------------------
Title: TEMPLOGG for determining stellar parameters of MONS targets
Authors: Kupka, F.; Bruntt, H.
2001JAD.....7Q...8K Altcode:
We describe the TEMPLOGG tool which has been developed at the Institute
of Astronomy in Vienna for the determination of fundamental stellar
parameters from photometric observations. TEMPLOGG provides a very
convenient access to a variety of calibrations for the Strömgren-β
and the Geneva photometric systems. Using observed photometric indices
the program automatically selects the most suitable transformation
between observed colours and stellar fundamental parameters. We have
used it here for a parameter determination with a preliminary list of
primary MONS targets. The results of this study are presented as an
example for the application and capabilities of TEMPLOGG.
---------------------------------------------------------
Title: Using TEMPLOGG for determining stellar parameters of MONS
targets
Authors: Kupka, F.; Bruntt, H.
2001fcm..book...39K Altcode:
We describe the TEMPLOGG tool which has been developed at the Institute
of Astronomy in Vienna for the determination of fundamental stellar
parameters from photometric observations. TEMPLOGG provides a very
convenient access to a variety of calibrations for the Stromgren-beta
and the Geneva photometric systems. Using observed photometric indices
the program automatically selects the most suitable transformation
between observed colours and stellar fundamental parameters. We have
used it here for a parameter determination with a preliminary list of
primary MONS targets. The results of this study are presented as an
example for the application and capabilities of TEMPLOGG.
---------------------------------------------------------
Title: Convection in the atmospheres and envelopes of turnoff and
giant branch stars of globular clusters
Authors: Montalbán, J.; Kupka, F.; D'Antona, F.; Schmidt, W.
2001A&A...370..982M Altcode:
We explore the dependence of {T_eff}'s and colors of stellar
models on the treatment of over-adiabatic convection, both in the
atmosphere and in the interior. We compute main sequence, turnoff,
and subgiant models for low metallicity stars (Z=2 10<SUP>-4</SUP>)
using as boundary conditions two new sets of model atmospheres by the
Wien group (Kupka \cite{kup96}; Smalley & Kupka \cite{sma97};
Heiter et al. \cite{hei01}). In these models convection is treated
either in the Mixing Length Theory (MLT) or in the Full Spectrum of
Turbulence (FST) formulation. We check the dependence of the {T_eff}
location of the HR diagram both on the optical depth tau at which the
atmospheric boundary conditions are fixed, and on the convective model
adopted in the atmosphere and interior. Obviously, full selfconsistency
of the result is obtained only if the treatment of convection is the
same in the outer layers and in the interior. We show that it is not
appropriate to use MLT convection in the atmosphere and FST in the
interior; if we wish to test the effect of changing the l/H_p in the
MLT, the atmospheric integration must be limited to tau =1. We construct
isochrones for ages of 10 Gyr and greater, and transform the theoretical
{T_eff} and gravity values to the Johnson B and V magnitudes. The two
sets of model atmospheres give small differences (up to ~ 0.03 mag) for
the B-V color, a result of the different temperature stratification in
the model atmosphere with FST convection treatment compared to that one
based on MLT. The FST boundary conditions provide relative locations
of turnoff and giant branch which differ from the MLT solutions, and
are in better agreement with the HR diagram morphology of the Globular
Cluster M 92.
---------------------------------------------------------
Title: Model Atmospheres with Individualized Abundances
Authors: Piskunov, N.; Kupka, F.
2001ApJ...547.1040P Altcode:
We describe a new method for computing opacity distribution functions
(ODFs) for model atmosphere calculations. The method is tailored to
model the atmospheres of individual stars on a modern workstation. Our
goal is the computation of model atmospheres for stars with abundances
significantly different from the solar or scaled solar composition
typically used for grid calculations. As a consistency test, we show
that the new procedure is able to reproduce the ODFs and existing model
atmospheres for solar abundances, and we describe models for stars
with peculiar abundances. We demonstrate that while mild chemical
peculiarities can be well represented by scaled solar models, the
extreme cases result in a very different atmospheric structure with no
analogs in scaled solar grids. Such a structure influences the emerging
spectrum as is clearly seen both in the observed flux distribution and
in the line ratios that are much better represented by the new models.
---------------------------------------------------------
Title: A New Project for Theoretical Colors of Globular Cluster Stars
Authors: Montalban, J.; D'Antona, F.; Kupka, F.; Schmidt, W.
2001coev.conf..243M Altcode:
The following sections are included: <P />* Summary <P />* References
---------------------------------------------------------
Title: Abundances in Przybylski's star
Authors: Cowley, C. R.; Ryabchikova, T.; Kupka, F.; Bord, D. J.;
Mathys, G.; Bidelman, W. P.
2000MNRAS.317..299C Altcode:
We have derived abundances for 54 elements in the extreme roAp star
HD101065. ESO spectra with a resolution of about 80000, and S/N of 200
or more were employed. The adopted model has T<SUB>eff</SUB>=6600K, and
log(g)=4.2. Because of the increased line opacity and consequent low
gas pressure, convection plays no significant role in the temperature
structure. Lighter elemental abundances through the iron group scatter
about standard abundance distribution (SAD) (solar) values. Iron
and nickel are about one order of magnitude deficient while cobalt
is enhanced by 1.5dex. Heavier elements, including the lanthanides,
generally follow the solar pattern but enhanced by 3 to 4dex. Odd-Z
elements are generally less abundant than their even-Z neighbours. With
a few exceptions (e.g. Yb), the abundance pattern among the heavy
elements is remarkably coherent, and resembles a displaced solar
distribution.
---------------------------------------------------------
Title: Abundance analysis of roAp stars. V. HD 166473
Authors: Gelbmann, M.; Ryabchikova, T.; Weiss, W. W.; Piskunov, N.;
Kupka, F.; Mathys, G.
2000A&A...356..200G Altcode:
This fifth paper in a sequence on abundance analyses of roAp stars
features several improvements and complements over the previous
investigations: i. The new VALD-2 atomic data base was used which
significantly improves the analysis of C, N, O, and rare earth
elements (REE) and in particular includes also some doubly ionized
REE species. ii. An individual opacity distribution function table
was generated for a successful synthesis of photometric indices of
this very peculiar star. iii. The influence of a (rather strong)
magnetic field on abundance determinations is studied and presented
for 30 elements. iv. Our investigation of 33 elements (45 ions) is
the hitherto most complete chemical analysis of a chemically peculiar
star, using modern tools. Similar to the four roAp stars analysed
by us so far (alpha Cir: Kupka et al. 1996, HD 203932: Gelbmann
et al. 1997, gamma Equ: Ryabchikova et al. 1997a, and HD 24712:
Ryabchikova et al. 1997b) we find nearly solar abundances of Fe and
Ni, and a definite overabundance of Cr and especially Co. Rare earth
elements have large overabundances, whereas C and O are underabundant
relative to the Sun. This pattern seems to be a common property of the
chemically peculiar (CP2, Ap) stars. A new and most striking result
is the discovery of the anomalous line strengths of the second ions
of REE resulting in an abundance increase of up to +1.5 dex, compared
to values obtained from lines of the first ions. This anomaly is not
found in non-roAp and “normal” stars. Based on observations obtained
at the European Southern Observatory (La Silla, Chile)
---------------------------------------------------------
Title: Spectroscopic Survey of Rapidly Oscillating Ap Stars
Authors: Weiss, W. W.; Ryabchikova, T. A.; Kupka, F.; Lueftinger,
T. R.; Savanov, I. S.; Malanushenko, V. P.
2000ASPC..203..487W Altcode: 2000ilss.conf..487W; 2000IAUCo.176..487W
A spectroscopic survey of roAp stars has been initiated in Vienna
in order to determine their fundamental astrophysical parameters and
abundances. We report here on our attempt to confirm and elaborate an
atmospheric peculiarity recently discovered (Ryabchikova et al. 1999)
which should allow to identify roAp stars with a single spectrum.
---------------------------------------------------------
Title: VALD-2 -- The New Vienna Atomic Line Database
Authors: Kupka, F. G.; Ryabchikova, T. A.; Piskunov, N. E.; Stempels,
H. C.; Weiss, W. W.
2000BaltA...9..590K Altcode: 2000OAst....9..590K
We provide a brief outline of the concepts and facilities of the Vienna
Atomic Line DataBase in its new version 2. A summary of contents and
recommendations how to use the VALD-2 are given. We conclude by a few
applications planned for future releases of VALD.
---------------------------------------------------------
Title: Line Blanketing in Przybylski's Star
Authors: Cowley, C. R.; Kupka, F.; Mathys, G.
1999AAS...195.5002C Altcode: 1999BAAS...31.1447C
Przybylski's star (HD 101065) may be the most heavily blanketed
star known. It therefore provides a test of our techniques for line
blanketing. The current abstract draws on a paper in preparation by
CRC, T. Ryabchikova, F. Kupka, G. Mathys, and D. J. Bord, based on
ESO spectra obtained by GM. Unfortunately, the atomic species that
provide the majority of the line blanketing in Przybylski's star
does not have enough atomic data for realistic calculations of the
blanketing. We therefore discuss three models in which iron-group
elements were articifically elevated in abundance in the calculation
of opacity used to construct the models. We thank Drs. R. L. Kurucz,
and Bengt Edvardsson for calculating respectively Models 1 (dashed
[Fe/H]=+3) and 2 (dot-dash, [Fe/H]=+2) at our request. Model 3
(line, [Fe/H]) was calculated by FK, using the Canuto-Mazzitelli
formalism. Figure 1 (www.astro.lsa.umich.edu/usrs/cowley/models.gif),
shows these 3 models in good agreement with one another, and clearly
different from a standard solar-abundance Atlas9 model (dashed)
with the same effective temperature. All three models are scaled to
Te=6600K. The blanketed models have little or no convection, and show
the lowered boundary temperature of classical picket-fence models. The
true boundary temperature may be still lower than in these numerical
models. Abundances from Pr I and Nd I are systematically higher than
those from the corresponding second spectra, as are those from Pr III
and Nd III. It was noted long ago by Przybylski and others that the
Balmer profiles had cores indicative of temperatures of some 6000K;
the wings could be fit with much higher temperatures--perhaps
as high as 7500K. Molecular species have been sought but not
identified. Calculations show CN and CH lines would be very weak, even
if the temperature between log(tau5000)=-3.5 and -5.4 were allowed to
drop to 3000K.
---------------------------------------------------------
Title: The Hydrodynamic Moment Equations: An Alternative Treatment
For Stellar Convection
Authors: Kupka, F.
1999POBeo..65...13K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Turbulent Convection: Comparing the Moment Equations to
Numerical Simulations
Authors: Kupka, F.
1999ApJ...526L..45K Altcode: 1999astro.ph..9331K
The nonlocal hydrodynamic moment equations for compressible convection
are compared to numerical simulations. Convective and radiative flux
typically deviate less than 20% from the three-dimensional simulations,
while mean thermodynamic quantities are accurate to at least 2%
for the cases we have investigated. The moment equations are solved
in minutes rather than days as required on standard workstations. We
conclude that this convection model has the potential to considerably
improve the modeling of convection zones in stellar envelopes and cores,
in particular those of A and F stars.
---------------------------------------------------------
Title: VALD - The Vienna Atomic Line Database: A Survey
Authors: Kupka, F.; Ryabchikova, T. A.
1999POBeo..65..223K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Interacting convection zones
Authors: Muthsam, Herbert J.; Göb, Wolfgang; Kupka, Friedrich;
Liebich, Wolfgang
1999NewA....4..405M Altcode:
3D Numerical simulations of convection zones separated by a stable
layer (according to the Schwarzschild criterion) are presented. The
compressible case is considered. We make use of idealized microphysics
closely related to polytropes. Decreasing the importance of the
separating stable layer by diminishing its vertical extent in a series
of models we investigate how the two convection zones merge into
one. In our parameter range it is the upper zone which increases in
size and ultimately squeezes the lower convection zone more or less
out of existence. Properties of various fluxes and other physical
quantities are discussed.
---------------------------------------------------------
Title: VALD-2: Progress of the Vienna Atomic Line Data Base
Authors: Kupka, F.; Piskunov, N.; Ryabchikova, T. A.; Stempels, H. C.;
Weiss, W. W.
1999A&AS..138..119K Altcode:
We describe the updated version of the Vienna Atomic Line Data
Base (VALD, \cite[Piskunov et al. 1995)]{pis95} which represents a
considerable improvement over the first installation from 1994. The
original line lists have been complemented with critically evaluated
data obtained from experimental measurements and theoretical
calculations which are necessary for computing state-of-the-art line
opacities in stellar atmospheres, as well as for synthesizing spectra
for high precision analyses. In this paper, we present new and improved
data sets for neutral species and ions of Si, P, Sc, Ti, V, Cr, Mn, Fe,
Co, Ni, Cu, Zn, Y, Zr, Ru, Xe, La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er,
Tm, Yb, Lu, Re, Pt, Au, Hg, and Pb. For some species data are available
in VALD for the first time. We explain our choice of quality rankings by
reviewing the literature for the new data and by comparison with source
lists included into VALD. For some cases, we produced new line data
by weighted averaging of data from different sources with individual
error estimates in order to increase the reliability of VALD line
lists. Software modifications allow remote users of VALD to specify
individual extraction parameters as an alternative to the default
settings of the VALD team and to have direct control over the quality
ranking of line data. A World-Wide-Web interface is described which
provides easy access to all new features. To simplify proper crediting
of all authors of atomic data, VALD now includes a compilation of all
publications used in each type of reply. Finally, we briefly discuss
the future roadmap of VALD developments, including the incorporation
of molecular transitions and integration with external data
bases. http://www.astro.univie.ac.at/~vald http://www.astro.uu.se/~vald
---------------------------------------------------------
Title: Testing convection theories using Balmer line profiles of A,
F, and G stars
Authors: Gardiner, R. B.; Kupka, F.; Smalley, B.
1999A&A...347..876G Altcode:
We consider the effects of convection on the Balmer line profiles
({H_α} and {H_β}) of A, F, and G stars. The standard mixing-length
theory (MLT) atlas9 models of Kurucz (1993), with and without
overshooting, are compared to atlas9 models based on the turbulent
convection theory proposed by Canuto & Mazzitelli (1991, 1992)
and implemented by Kupka (1996), and the improved version of this model
proposed by Canuto et al. (1996) also implemented by Kupka. The Balmer
line profiles are a useful tool in investigating convection because
they are very sensitive to the parameters of convection used in the
stellar atmosphere codes. The {H_α} and {H_β} lines are formed at
different depths in the atmosphere. The {H_α} line is formed just
above the convection zone. The {H_β} line, however, is partially
formed inside the convection zone. We have calculated the {T_eff} of
observed stars by fitting Balmer line profiles to synthetic spectra
and compared this to: (i) the {T_eff} of the fundamental stars; (ii)
the {T_eff} of stars determined by the Infra-Red Flux Method and (iii)
the {T_eff} determined by Geneva photometry for the stars in the Hyades
cluster. We find that the results from the {H_α} and {H_β} lines are
different, as expected, due to the differing levels of formation. The
tests are inconclusive between three of the four models; MLT with no
overshooting, CM and CGM models, which all give results in reasonable
agreement with fundamental values. The results indicate that for the
MLT theory with no overshooting it is necessary to set the mixing
length parameter alpha equal to 0.5 for stars with {T_eff <= 6000}
K or {T_eff >= 7000} K. However for stars with {6000} K{<= T_eff
<= 7000} K the required value for the parameter is {alpha >=
1.25}. Models with overshooting are found to be clearly discrepant,
consistent with the results with uvby photometry by Smalley & Kupka
(1997). Based on observations made at the Observatorio del Roque de
los Muchachos using the Richardson-Brealey Spectrograph on the 1.0m
Jacobs Kapteyn Telescope.
---------------------------------------------------------
Title: Eu III identification and Eu abundance in CP stars
Authors: Ryabchikova, T.; Piskunov, N.; Savanov, I.; Kupka, F.;
Malanushenko, V.
1999A&A...343..229R Altcode:
We report the first identification of the Eu iii lambda 6666.347 line
in optical spectra of CP stars. This line is clearly present in the
spectra of HR 4816, 73 Dra, HR 7575, beta CrB, and alpha (2) CVn,
while it is marginally present or absent in spectra of the roAp stars
(rapidly oscillating Ap stars, cf. Kurtz 1990) alpha Cir, gamma Equ,
HD 203932, GZ Lib (33 Lib), and HD 24712. Careful synthetic spectrum
calculations for the Eu ii lambda 6645.11 line taking into account
hyperfine, isotopic, and magnetic splittings allow us to obtain more
accurate Eu abundances in the atmospheres of 9 CP stars. In most
cases the derived abundances are significantly lower than the previous
results reported for some of the stars based on coarse analysis of the
famous blue Eu ii lines. Assuming an ionization balance in the stellar
atmospheres we give an estimate of the astrophysical oscillator strength
log (gf)=1.18 +/- 0.14 for the Eu iii lambda 6666.347 line. This value
is obtained without taking into account a possible hyperfine-splitting
which is unknown for this Eu iii line. We also provide astrophysical
gf-values for Eu iii lambda lambda 7221.838, 7225.151, and 8079.071.
---------------------------------------------------------
Title: Observational Evidence for Convection in Main Sequence Star
Atmospheres
Authors: Weiss, W. W.; Kupka, F.
1999ASPC..173...21W Altcode: 1999sstt.conf...21W
No abstract at ADS
---------------------------------------------------------
Title: Testing Convection Theories Using Balmer Line Profiles of A,
F and G Stars
Authors: Gardiner, R.; Smalley, B.; Kupka, F.
1999ASPC..173..213G Altcode: 1999sstt.conf..213G
No abstract at ADS
---------------------------------------------------------
Title: Computing Solar and Stellar Overshooting with Turbulent
Convection Models. First Tests of a Fully Non-Local Model
Authors: Kupka, F.
1999ASPC..173..157K Altcode: 1999sstt.conf..157K
No abstract at ADS
---------------------------------------------------------
Title: The acoustic cut-off frequency of roAp stars
Authors: Audard, N.; Kupka, F.; Morel, P.; Provost, J.; Weiss, W. W.
1998A&A...335..954A Altcode: 1997astro.ph.12126A
Some of the rapidly oscillating (roAp) stars, have frequencies which are
larger than the acoustic cut-off frequency determined from published
stellar models which usually assume a grey atmosphere. As the cut-off
frequency depends on the T(tau ) relation, we have computed models
and adiabatic frequencies for pulsating Ap stars with more realistic
atmospheres which include a frequency dependent treatment of radiative
transfer, take blanketing effects into account, and which have a better
treatment of the radiative pressure. In addition, we are using opacity
distribution functions specific to the atmospheric composition. With
these improvements over the classical stellar models the theoretical
acoustic cut-off frequency for roAp stars are increased by about
200 mu Hz, which brings them close to the observations. We restrict
the comparison of our computations with observations to those two
`pathological' roAp stars for which more reliable astrophysical
parameters are available, HD 24712 and alpha Cir, and comment briefly
on a third one, HD 134214. For alpha Cir we find models which have
indeed a cut-off frequency beyond the largest observed frequency and
which are well within the T<SUB>eff</SUB> - L/L_⊙ error box. For HD
24712 only models which are hotter by about 100 K and less luminous
by nearly 10% than what is actually the most probable value derived by
spectroscopy would have an acoustic cut-off frequency large enough. HD
134214 fits our models best, however, the error box for T<SUB>eff</SUB>
- L/L_⊙ is the largest of all three stars. One may thus speculate
that the old controversy about a mismatch between observed largest
frequencies and theoretical cut-off frequencies of roAp star models
is resolved. Based on hipparcos data.
---------------------------------------------------------
Title: Abundance analysis of the lambda Bootis stars HD 192640,
HD 183324, and HD 84123
Authors: Heiter, U.; Kupka, F.; Paunzen, E.; Weiss, W. W.; Gelbmann, M.
1998A&A...335.1009H Altcode:
The classification of lambda Bootis stars based on photometric indices
or low resolution spectra is not sufficient for a final decision
concerning a membership to this group. A detailed spectroscopic
investigation is necessary to rule out stars with similar classification
spectra but different abundance patterns. Therefore a program on
abundance analyses of lambda Bootis candidates was established, which
makes use of a software package that enables the analysis of high
signal-to-noise spectra with large resolution. In this paper we present
the results of the application of these tools on two lambda Bootis stars
(HD 192640 and HD 183324) for which the derived abundances agree very
well with the literature. For a third star (HD 84123), which shows a
very low projected rotational velocity, the newly determined abundance
pattern confirms its membership to the lambda Bootis group. We also
investigated the effect of using several different codes for the
calculation of the model atmospheres on the synthetic spectra of the
program stars, which span a wide range in effective temperature, gravity
and metallicity. The substitution of opacity distribution functions,
which where pretabulated for metal abundances scaled according to the
solar composition, by ones calculated for the individual abundance
patterns does not change the synthetic spectra significantly. On the
other hand, the derived abundances are sensitive to the treatment of
overshooting within the calculation of the convective flux for cool
stars. Based on observations obtained at the Observatoire de Haute
Provence, the Osservatorio Astronomico di Padua-Asiago and with the
Hipparcos satellite
---------------------------------------------------------
Title: CP star atmospheres based on individual ODFs
Authors: Kupka, F.; Piskunov, N. E.
1998CoSka..27..228K Altcode: 1998astro.ph..5057K
We describe a new method for the computation of opacity distribution
functions (ODFs) useful to calculate one-dimensional model atmospheres
in local thermal equilibrium (LTE). The new method is fast enough to
be applied on current workstations and allows the computation of model
atmospheres which deviate significantly from (scaled) solar chemical
composition. It has reproduced existing ODFs and model atmospheres
for solar abundances. Depending on the type of chemical peculiarity
the "individual" model atmosphere may have a structure and surface
fluxes similar to atmospheres based on (scaled) solar abundances or
deviate in a way that cannot be reproduced by any of the conventional
models. Examples are given to illustrate this behavior. The availability
of models with "individualized" abundances is crucial for abundance
analyses and Doppler imaging of extreme CP stars.
---------------------------------------------------------
Title: The effects of convection on the colours of A and F stars
Authors: Smalley, B.; Kupka, F.
1998CoSka..27..233S Altcode: 1998astro.ph..5058S
We present a discussion on the effects of convection on the uvby colours
of A and F stars. The mixing-length theory used in ATLAS9 is compared to
the turbulent convection theory of Canuto & Mazzitelli. Comparison
with fundamental stars reveals that colours calculated using the Canuto
& Mazzitelli convection theory are generally in better agreement
than those obtained using mixing-length theory.
---------------------------------------------------------
Title: EU III identification and EU abundance in cool CP stars
Authors: Ryabchikova, T.; Piskunov, N.; Savanov, I.; Kupka, F.
1998CoSka..27..359R Altcode: 1998astro.ph..5206R
We report the first identification of the Eu III lambda 6666.317
line in optical spectra of CP stars. This line is clearly present in
the spectra of HR 4816, 73 Dra, HR 7575, and beta CrB, while it is
marginally present or absent in spectra of the roAp stars alpha Cir,
gamma Equ, BI Mic, 33 Lib, and HD 24712.
---------------------------------------------------------
Title: Atmospheric structure and acoustic cut-off frequency of
roAp stars
Authors: Audard, N.; Kupka, F.; Morel, P.; Provost, J.; Weiss, W. W.
1998CoSka..27..304A Altcode: 1998astro.ph..5091A
Some of the rapidly oscillating (CP2) stars, have frequencies which are
larger than the theoretical acoustic cut-off frequency. As the cut-off
frequency depends on the T(tau) relation in the atmosphere, we have
computed models and adiabatic frequencies for pulsating Ap stars with
T(tau) laws based on Kurucz model atmospheres and on Hopf's purely
radiative relation. The frequency-dependent treatment of radiative
transfer as well as an improved calculation of the radiative pressure
in Kurucz model atmospheres increase the theoretical acoustic cut-off
frequency by about 200 microHz, which is closer to the observations. For
alpha Cir we find models with Kurucz atmospheres which have indeed a
cut-off frequency beyond the largest observed frequency and which are
well within the T(eff) - L error box. For HD 24712 only models which
are hotter by about 100 K and less luminous by nearly 10% than what
is actually the most probable value would have an acoustic cut-off
frequency large enough. One may thus speculate that the old controversy
about a mismatch between observed largest frequencies and theoretical
cut-off frequencies of roAp star models is resolved. However, the
observational errors for the astrophysical fundamental parameters have
to be reduced further and the model atmospheres refined.
---------------------------------------------------------
Title: The acoustic cut-off frequency of A to F stars
Authors: Audard, N.; Kupka, F.; Morel, P.; Provost, J.; Weiss, W. W.
1998IAUS..185..299A Altcode:
For some Ap stars, frequencies are observed which are larger than the
theoretical acoustic cut-off frequency, which strongly depends on the T
tau relation in the atmosphere. We have computed models and eigenmodes
for pulsating Ap stars. The internal stellar structure is obtained
with the CESAM code, and improved model atmospheres are constructed
with T tau laws derived from Kurucz's model atmospheres. These
models are compared to models whose atmospheres are derived from
the purely radiative Hopf's T tau law. Our main result is that the
frequency-dependent treatment of radiative transfer in Kurucz's model
atmospheres induces an increase of the theoretical acoustic cut-off
frequency, which brings us closer to the observations. As the cut-off
frequency is determined by the very outer layers, we point out the
necessity to compute very accurate model atmospheres.
---------------------------------------------------------
Title: The role of convection on the UVBY colours of A, F, and G stars
Authors: Smalley, B.; Kupka, F.
1997A&A...328..349S Altcode:
We discuss the effects of convection on the theoretical uvby colours of
A, F, and G stars. The standard mixing-length theory atlas9 models of
Kurucz (1993), with and without approximate overshooting, are compared
to models using the turbulent convection theory proposed by Canuto &
Mazzitelli (1991, 1992) and implemented by Kupka (1996a). Comparison
with fundamental T_eff and log g stars reveals that the Canuto &
Mazzitelli models give results that are generally superior to standard
mixing-length theory (MLT) without convective overshooting. MLT
models with overshooting are found to be clearly discrepant. This is
supported by comparisons of non-fundamental stars, with T_eff obtained
from the Infrared Flux Method and log g from stellar evolutionary
models for open cluster stars. The Canuto & Mazzitelli theory
gives values of (b-y)_0 and c_0 that are in best overall agreement
with observations. Investigations of the m_0 index reveal that all
of the treatments of convection presented here give values that are
significantly discrepant for models with T_eff < 6000 K. It is
unclear as to whether this is due to problems with the treatment of
convection, missing opacity, or some other reason. None of the models
give totally satisfactory m_0 indices for hotter stars, but the Canuto
& Mazzitelli models are in closest overall agreement above 7000
K. Grids of uvby colours, based on the CM treatment of convection,
are presented. These grids represent an improvement over the colours
obtained from models using the mixing-length theory. The agreement with
fundamental stars enables the colours to be used directly without the
need for semi-empirical adjustments that were necessary with the earlier
colour grids. Table~5 is only available at the CDS via anonymous ftp
130.79.128.5 or via http://cdsweb.u-strasbg.fr/Abstract.html
---------------------------------------------------------
Title: VizieR Online Data Catalog: Role of Convection in A, F,
and G stars (Smalley+ 1997)
Authors: Smalley, B.; Kupka, F.
1997yCat..33280349S Altcode:
We discuss the effects of convection on the theoretical uvby
colours of A, F, and G stars. The standard mixing-length theory
atlas9 models of Kurucz (1993, ATLAS9, SAO, Cambridge, USA), with
and without approximate overshooting, are compared to models using
the turbulent convection theory proposed by Canuto & Mazzitelli
(1991ApJ...370..295C, 1992ApJ...389..724C) and implemented by Kupka
(1996, ASPConf. Proc. 108, 73). Comparison with fundamental Teff
and log g stars reveals that the Canuto & Mazzitelli models give
results that are generally superior to standard mixing-length theory
(MLT) without convective overshooting. MLT models with overshooting
are found to be clearly discrepant. This is supported by comparisons
of non-fundamental stars, with Teff obtained from the Infrared Flux
Method and log g from stellar evolutionary models for open cluster
stars. The Canuto & Mazzitelli theory gives values of (b-y)0 and c0
that are in best overall agreement with observations. Investigations
of the m0 index reveal that all of the treatments of convection
presented here give values that are significantly discrepant for
models with Teff<6000K. It is unclear as to whether this is
due to problems with the treatment of convection, missing opacity,
or some other reason. None of the models give totally satisfactory
m0 indices for hotter stars, but the Canuto & Mazzitelli models
are in closest overall agreement above 7000K. Grids of uvby colours,
based on the CM treatment of convection, are presented. These grids
represent an improvement over the colours obtained from models using
the mixing-length theory. The agreement with fundamental stars enables
the colours to be used directly without the need for semi-empirical
adjustments that were necessary with the earlier colour grids. For a
description of the uvby photometric system, see e.g. <GCPD/04>
(6 data files).
---------------------------------------------------------
Title: (Erratum) Abundance analysis of roAp stars.
Authors: Gelbmann, M.; Kupka, F.; Weiss, W. W.; Mathys, G.
1997A&A...322.1026G Altcode:
Erratum to Astron. Astrophys. 319, 630-636 (1997).
---------------------------------------------------------
Title: Abundance analysis of roAp stars. II. HD 203932
Authors: Gelbmann, M.; Kupka, F.; Weiss, W. W.; Mathys, G.
1997A&A...319..630G Altcode:
A new tool to simplify abundance analyses which is based on stand-alone
programs has been applied to the rapidly oscillating Ap star HD
203932 (BI Mic, CD -30 18600, SAO 212996; Ap(SrEu), V=8.82mag). The
spectroscopically determined T_eff_=7450+/-100K and logg=4.3+/-0.15 put
this star close to the ZAMS. Other fundamental atmospheric parameters
are v_micro_<0.6km/s and the total abundance of all iron peak
elements [M/H]=0.0+/-0.1. The fundamental parameters put HD 203932 in a
region of the HR-diagram where convection starts becoming efficient and
the standard mixing length theory models lead to severe problems in the
determination of the atmospheric parameters. The difference between the
upper limit for logg obtained from several variants of the mixing length
theory and the Canuto-Mazzitelli model indicates that the choice of a
particular convection model can influence the determination of basic
stellar parameters. For the first time abundances were determined for
HD 203932 showing a pattern for the 35 investigated elements which is
similar to α Cir (Kupka et al. 1996A&A...308..886K, Paper I). Fe
and Ni have about solar abundance, Cr and especially Co are clearly
overabundant as well as rare earth elements. The most underabundant
element is Sc, followed by C, N, and O, which is a common property of
CP2 stars. The lack of a correlation in our data between individual line
abundances and their effective Lande factors implies a mean magnetic
field modulus not exceeding few kG. Compared to the last homogeneous
spectroscopic investigation of a large sample of chemically peculiar
stars (21 cool Ap stars, Adelman 1973ApJ...183...95A), our analysis
is based on data with higher spectral resolution and signal-to-noise
ratio. Even more important, we are using a much larger atomic line
data base with more precise atomic parameters than available more than
twenty years ago.
---------------------------------------------------------
Title: The Vienna Atomic Line Database : Present State and Future
Development
Authors: Ryabchikova, T. A.; Piskunov, N. E.; Kupka, F.; Weiss, W. W.
1997BaltA...6..244R Altcode: 1997OAst....6..244R
We describe the main structure of the Vienna Atomic Line Database,
the tools provided for the users and the statistics of its use in
the last two years. Our plans for future developments of the database
are discussed.
---------------------------------------------------------
Title: Abundance analysis of roAp stars. I. α Circini.
Authors: Kupka, F.; Ryabchikova, T. A.; Weiss, W. W.; Kuschnig, R.;
Rogl, J.; Mathys, G.
1996A&A...308..886K Altcode:
Based on high resolution, low noise spectroscopy in the spectral region
from 4200A to 6700A we derived T_eff_=7900+/-200K, logg=4.2+/-0.15,
vsin(i)=12.5(-0.5,+1.5)km/s, and elemental abundances for the rapidly
oscillating (ro)Ap star αCir(HD128898, HR5463). We used the spectrum
synthesis and the equivalent width technique and confirmed the presence
of a magnetic field in αCir. The underabundance of C, N, and O, and
the overabundances of rare-earth and some other heavy elements are
comparable to other cool Ap-stars. The most important peculiarity is
that of Co, which is almost as overabundant as Cr, the most overabundant
iron peak element in cool Ap-stars.
---------------------------------------------------------
Title: New models for the convective flux in stellar atmospheres
Authors: Kupka, F.
1996IAUS..176..557K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Beyond Mixing Length Theory
Authors: Kupka, F.
1996ASPC..108...73K Altcode: 1996mass.conf...73K
No abstract at ADS
---------------------------------------------------------
Title: Introduction
Authors: Griffin, R. E. M.; Kupka, F.
1996ASPC..108..299G Altcode: 1996mass.conf..299G
No abstract at ADS
---------------------------------------------------------
Title: M.A.S.S., Model Atmospheres and Spectrum Synthesis
Authors: Adelman, Saul J.; Kupka, Friedrich; Weiss, Warner W.
1996ASPC..108.....A Altcode: 1996mass.conf.....A
No abstract at ADS
---------------------------------------------------------
Title: M.A.S.S. Model atmospheres and stellar spectra. 5th Vienna
workshop
Authors: Adelman, Saul J.; Kupka, Friedrich; Weiss, Werner W.
1996mssm.conf.....A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: VALD: The Vienna Atomic Line Data Base.
Authors: Piskunov, N. E.; Kupka, F.; Ryabchikova, T. A.; Weiss, W. W.;
Jeffery, C. S.
1995A&AS..112..525P Altcode:
The "Vienna Atomic Line Data Base" (VALD) consists of a set of
critically evaluated lists of astrophysically important atomic
transition parameters and supporting extraction software. VALD contains
about 600000 entries and is one of the largest collections of accurate
and homogeneous data for atomic transitions presently available. It
also includes specific tools for extracting data for spectrum synthesis
and model atmosphere calculations. The different accuracies of data
available in the literature made it necessary to introduce a ranking
system and to provide a flexible method for extracting the best possible
set of atomic line parameters for a given transition from all the
available sources. The data base is presently restricted to spectral
lines which are relevant for stars in which the LTE approximation is
sufficient and molecular lines do not have to be taken into account. The
provision was made that these requirements should not restrict the
general design of VALD and the possibility of future expansion. In
this paper we describe the structure of VALD, the available data sets
and specific retrieval tools. The electronic-mail interface (VALD-EMS)
created to allow remote access to VALD is also described. Both users
and producers of atomic data are invited to explore the database,
and to collaborate in improving and extending its contents.
---------------------------------------------------------
Title: Fine Analysis of Pulsating CP Stars
Authors: Kupka, F.; Gelbmann, M.; Heiter, U.; Kuschnig, R.; Weiss,
W. W.; Ryabchikova, T. A.
1995ASPC...83..317K Altcode: 1995IAUCo.155..317K; 1995aasp.conf..317K
No abstract at ADS
---------------------------------------------------------
Title: A numerical study of compressible convection.
Authors: Muthsam, H. J.; Goeb, W.; Kupka, F.; Liebich, W.; Zoechling,
J.
1995A&A...293..127M Altcode:
We perform numerical investigations of (mainly 3D) compressible
convection by using simple microphysics (closely related to piecewise
polytropes). The convective layer is embedded between stable layers
at the top and the bottom. Overshooting above is found to be quite
effective within the present parameter range, overshooting below
less so. However, consistent with prior 2D calculation the induced
motions outside of the directly convectively influenced region are more
developed below than above the convection zone. Typical features of the
lower parts of the convection zone leading to a downwardly pointing
convective flux are discussed as is the existence of a specific cell
system near the lower boundary of the convection zone proper. Also,
the influence of parameters is discussed. 2D convection tends to lead to
more intensive convection than 3D (the physical parameters being equal).
---------------------------------------------------------
Title: The Vienna Atomic Line Data-Base
Authors: Piskunov, N. E.; Kupka, F.; Ryabchikova, T. A.; Weiss, W. W.;
Jeffery, C. S.
1995ASPC...81..610P Altcode: 1995lahr.conf..610P
No abstract at ADS
---------------------------------------------------------
Title: The Evolutionary Status of Lambda Boo Stars
Authors: Paunzen, E.; Gelbmann, M.; Heiter, U.; Kupka, F.; Kuschnig,
R.; Weiss, W. W.
1995ASPC...83..315P Altcode: 1995IAUCo.155..315P; 1995aasp.conf..315P
No abstract at ADS
---------------------------------------------------------
Title: Abundance analysis of cool oscillating CP stars
Authors: Kupka, F.; Ryabchikova, T.; Bolgova, G.; Kuschnig, R.; Weiss,
W. W.; Mathys, G.; Le Contel, J. M.
1994cpms.conf..130K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Interaction of convection zones: the nonmagnetic case
Authors: Muthsam, H. J.; Göb, W.; Kupka, F.; Liebich, W.
1994smf..conf..152M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The peculiar binary system HR 8891 (ET And)
Authors: Kuschnig, R.; Weiss, W. W.; Piskounov, N.; Ryabchikova,
T.; Kreidl, T. J.; Alvarez, M.; Bedolla, S. G.; Bus, S. J.; Guo, Z.;
Hao, J.; Huang, L.; Kupka, F.; Le Contel, D.; Le Contel, J. M.; Osip,
D. J.; Panov, K.; Polosukchina, N.; Sareyan, J. P.; Schneider, H.;
Valtier, J. C.; Zboril, M.; Ziznovsky, J.; Zverko, J.
1994IAUS..162...43K Altcode:
No abstract at ADS