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Author name code: macgregor
ADS astronomy entries on 2022-09-14
author:"MacGregor, Keith B."

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Title: Frequency regularities of acoustic modes and multi-colour
    mode identification in rapidly rotating stars
Authors: Reese, D. R.; Lignières, F.; Ballot, J.; Dupret, M. -A.;
   Barban, C.; van't Veer-Menneret, C.; MacGregor, K. B.
2017A&A...601A.130R    Altcode: 2017arXiv170109164R
  Context. Mode identification has remained a major obstacle in the
  interpretation of pulsation spectra in rapidly rotating stars. This
  has motivated recent work on calculating realistic multi-colour mode
  visibilities in this type of star. <BR /> Aims: We would like to test
  mode identification methods and seismic diagnostics in rapidly rotating
  stars, using oscillation spectra that are based on these new theoretical
  predictions. <BR /> Methods: We investigate the auto-correlation
  function and Fourier transform of theoretically calculated
  frequency spectra, in which modes are selected according to their
  visibilities. Given that intrinsic mode amplitudes are determined by
  non-linear saturation and cannot currently be theoretically predicted,
  we experimented with various ad-hoc prescriptions for setting the mode
  amplitudes, including using random values. Furthermore, we analyse
  the ratios between mode amplitudes observed in different photometric
  bands to see up to what extent they can identify modes. <BR /> Results:
  When non-random intrinsic mode amplitudes are used, our results show
  that it is possible to extract a mean value for the large frequency
  separation or half its value and, sometimes, twice the rotation rate,
  from the auto-correlation of the frequency spectra. Furthermore,
  the Fourier transforms are mostly sensitive to the large frequency
  separation or half its value. The combination of the two methods may
  therefore measure and distinguish the two types of separations. When
  the intrinsic mode amplitudes include random factors, which seems more
  representative of real stars, the results are far less favourable. It is
  only when the large separation or half its value coincides with twice
  the rotation rate, that it might be possible to detect the signature
  of a frequency regularity. We also find that amplitude ratios are a
  good way of grouping together modes with similar characteristics. By
  analysing the frequencies of these groups, it is possible to constrain
  mode identification, as well as determine the large frequency separation
  and the rotation rate.

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Title: Properties of 42 Solar-type Kepler Targets from the
    Asteroseismic Modeling Portal
Authors: Metcalfe, T. S.; Creevey, O. L.; Doğan, G.; Mathur, S.;
   Xu, H.; Bedding, T. R.; Chaplin, W. J.; Christensen-Dalsgaard, J.;
   Karoff, C.; Trampedach, R.; Benomar, O.; Brown, B. P.; Buzasi, D. L.;
   Campante, T. L.; Çelik, Z.; Cunha, M. S.; Davies, G. R.; Deheuvels,
   S.; Derekas, A.; Di Mauro, M. P.; García, R. A.; Guzik, J. A.;
   Howe, R.; MacGregor, K. B.; Mazumdar, A.; Montalbán, J.; Monteiro,
   M. J. P. F. G.; Salabert, D.; Serenelli, A.; Stello, D.; Ste&şacute;
   licki, M.; Suran, M. D.; Yıldız, M.; Aksoy, C.; Elsworth, Y.;
   Gruberbauer, M.; Guenther, D. B.; Lebreton, Y.; Molaverdikhani, K.;
   Pricopi, D.; Simoniello, R.; White, T. R.
2014ApJS..214...27M    Altcode: 2014arXiv1402.3614M
  Recently the number of main-sequence and subgiant stars exhibiting
  solar-like oscillations that are resolved into individual mode
  frequencies has increased dramatically. While only a few such data
  sets were available for detailed modeling just a decade ago, the
  Kepler mission has produced suitable observations for hundreds of
  new targets. This rapid expansion in observational capacity has been
  accompanied by a shift in analysis and modeling strategies to yield
  uniform sets of derived stellar properties more quickly and easily. We
  use previously published asteroseismic and spectroscopic data sets
  to provide a uniform analysis of 42 solar-type Kepler targets from
  the Asteroseismic Modeling Portal. We find that fitting the individual
  frequencies typically doubles the precision of the asteroseismic radius,
  mass, and age compared to grid-based modeling of the global oscillation
  properties, and improves the precision of the radius and mass by about
  a factor of three over empirical scaling relations. We demonstrate
  the utility of the derived properties with several applications.

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Title: Oscillations and Surface Rotation of Red Giant Stars
Authors: Hedges, C.; Mathur, S.; Thompson, M. J.; MacGregor, K. B.
2013ASPC..479..197H    Altcode: 2013arXiv1308.0644H
  More than 15000 red giants observed by Kepler for a duration of almost
  one year became public at the beginning of this year. We analysed a
  subsample of 416 stars to determine the global properties of acoustic
  modes (mean large separation and frequency of maximum power). Using
  the effective temperature from the Kepler Input Catalog, we derived
  a first estimate of the masses and radii of these stars. Finally, we
  applied wavelets to look for a signature of surface rotation, which
  relies on the presence of spots or other surface features crossing
  the stellar visible disk.

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Title: Mode Visibilities and Frequency Patterns in Rapidly Rotating
    Stars
Authors: Reese, D. R.; Lignières, F.; Ballot, J.; Prat, V.; Barban,
   C.; van't Veer-Menneret, C.; MacGregor, K. B.
2013ASPC..479..545R    Altcode:
  One of the main obstacles in interpreting the pulsation spectra of
  rapidly rotating stars is mode identification. In order to address
  this issue, we generalise mode visibility calculations to rapidly
  rotating stars. These new calculations take into account gravity and
  limb darkening, as well as stellar deformation resulting from both
  the centrifugal force and the pulsation modes. Based on these new
  calculations, we produce synthetic pulsation spectra and study the
  frequency patterns present within. We then go on to discuss multi-colour
  photometry and its potential for mode identification.

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Title: Mode visibilities in rapidly rotating stars
Authors: Reese, D. R.; Prat, V.; Barban, C.; van 't Veer-Menneret,
   C.; MacGregor, K. B.
2013A&A...550A..77R    Altcode: 2012arXiv1212.2384R
  Context. Mode identification is a crucial step to comparing observed
  frequencies with theoretical ones. However, it has proven to be
  particularly difficult in rapidly rotating stars. An important reason
  for this is the lack of simple frequency patterns such as those present
  in solar-type pulsators. This problem is further aggravated in δ Scuti
  stars by their particularly rich frequency spectra. <BR /> Aims: As a
  first step to obtaining further observational constraints towards mode
  identification in rapid rotators, we aim to accurately calculate mode
  visibilities and amplitude ratios while fully taking into account the
  effects of rotation. <BR /> Methods: We derive the relevant equations
  for calculating mode visibilities in different photometric bands
  while fully taking into account the geometric distortion from both
  the centrifugal deformation and the pulsation modes, the variations
  in effective gravity, and an approximate treatment of the temperature
  variations, given the adiabatic nature of the pulsation modes. These
  equations are then applied to 2D oscillation modes, calculated using
  the TOP code (Two-dimension Oscillation Program), in fully distorted 2D
  models based on the self-consistent field (SCF) method. The specific
  intensities come from a grid of Kurucz atmospheres, thereby taking
  into account limb and gravity darkening. <BR /> Results: We obtain
  mode visibilities and amplitude ratios for 2 M<SUB>⊙</SUB> models
  with rotation rates ranging from 0 to 80% of the critical rotation
  rate. Based on these calculations, we confirm a number of results
  from earlier studies, such as the increased visibility of numerous
  chaotic modes at sufficient rotation rates, the simpler frequency
  spectra with dominant island modes for pole-on configurations,
  or the dependence of amplitude ratios on inclination and azimuthal
  order in rotating stars. In addition, we explain how the geometric
  shape of the star leads to a smaller contrast between pole-on and
  equator-on visibilities of equatorially-focused island modes. We
  also show that modes with similar (ℓ, |m|) values frequently
  have similar amplitude ratios, even in the most rapidly rotating
  models. <P />Appendices are available in electronic form at <A
  href="http://www.aanda.org">http://www.aanda.org</A>

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Title: Angular Momentum and Mass Loss From Magnetized Solar-Like Winds
Authors: Pinsonneault, Marc H.; Matt, S.; MacGregor, K. B.
2013AAS...22125206P    Altcode:
  We investigate angular momentum and mass loss from magnetized solar-like
  winds in cool stars. We present a physically motivated formulation and
  investigate two key phenomena: the F star transition from effective
  to ineffective spin down and the mass dependence of the spin down
  timescale in lower mass stars. We demonstrate that both phenomena are
  naturally explained within our framework. Tests of angular momentum
  loss models from upcoming Kepler data are discussed.

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Title: Signatures of rotation in oscillation spectra
Authors: Reese, D. R.; Prat, V.; Barban, C.; van't Veer-Menneret,
   C.; MacGregor, K. B.
2012sf2a.conf..211R    Altcode:
  Rotation makes oscillation spectra of rapidly rotating stars much more
  complicated. Hence, new strategies need to be developed in order to
  interpret such spectra. In what follows, we describe how multi-colour
  photometric mode visibilities can be generalised to rapidly rotating
  stars, while fully taking into account centrifugal deformation and
  gravity darkening. We then go on to describe some first results as
  well as a strategy for constraining mode identification.

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Title: Magnetic Braking Formulation for Sun-like Stars: Dependence
    on Dipole Field Strength and Rotation Rate
Authors: Matt, Sean P.; MacGregor, Keith B.; Pinsonneault, Marc H.;
   Greene, Thomas P.
2012ApJ...754L..26M    Altcode: 2012arXiv1206.2354M
  We use two-dimensional axisymmetric magnetohydrodynamic simulations
  to compute steady-state solutions for solar-like stellar winds from
  rotating stars with dipolar magnetic fields. Our parameter study
  includes 50 simulations covering a wide range of relative magnetic field
  strengths and rotation rates, extending from the slow- and approaching
  the fast-magnetic-rotator regimes. Using the simulations to compute
  the angular momentum loss, we derive a semi-analytic formulation for
  the external torque on the star that fits all of the simulations to
  a precision of a few percent. This formula provides a simple method
  for computing the magnetic braking of Sun-like stars due to magnetized
  stellar winds, which properly includes the dependence on the strength
  of the magnetic field, mass loss rate, stellar radius, surface gravity,
  and spin rate, and which is valid for both slow and fast rotators.

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Title: Self-Consistent Field Model Spectra and Images for the Rapid
    Rotator α Cephei
Authors: Aufdenberg, Jason P.; MacGregor, K.; Sola, M.
2012AAS...22013004A    Altcode:
  Non-LTE synthetic radiation fields have been coupled to Self-Consistent
  Field (SCF) rotating star models to predict images, interferometric
  observables (visibilities and closure phases), spectral energy
  distributions (SEDs), and high-resolution spectra for comparison with
  rapid rotater alpha Cephei (Alderamin). SCF models include differential
  rotation from the interior to the surface and differ from Roche models
  that assume a point-mass approximation of the gravitational potential
  and axially symmetric uniform rotation. SCF models are parametrized by a
  mass, the ratio of the axial rotation rate to the critical rate, and the
  degree and kind (solar or anti-solar) of differential rotation. Model
  spectra have been computed using a parallel interpolation algorithm
  (coded in Fortran90 with openMPI) which maps a radiation field
  database onto the rotationally distorted model star. The SCF model
  describes the surface shape and gravitational field from the pole to
  the equator. The luminosity and the von Zeipel exponent <P />specify
  the variation in effective temperature with stellar latitude. The
  radiation field is interpolated at each point on the star for
  each wavelength, emergent angle, local effective, and local surface
  gravity. <P />Model images are compared to the reconstructed images of
  Alderamin (Zhao et al. 2009) from the Michigan InfraRed Combiner (MIRC)
  at the CHARA Array. Model SEDs are compared to ultraviolet, visual and
  near-IR spectrophotometry. High-resolution model spectra are compared
  Alderamin's Mg II 4481 A line from the ELODIE spectral archive. We have
  found models near 2.2 solar masses with anti-solar differential rotation
  which match simultaneously the absolute magnitude, B-V color index, and
  projected axial ratio measured for Alderamin. The model images differ
  from the observations in brightness-temperature distribution over the
  projected stellar surface, the strength of the Mg II line profile, and
  the strength of the ultraviolet continuum. This work is partially funded
  by the McNair Scholars Program at Embry-Riddle Aeronautical University.

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Title: Reflection and Ducting of Gravity Waves Inside the Sun
Authors: MacGregor, K. B.; Rogers, T. M.
2011SoPh..270..417M    Altcode: 2011arXiv1104.4310M; 2011SoPh..tmp..103M
  Internal gravity waves excited by overshoot at the bottom of the
  convection zone can be influenced by rotation and by the strong
  toroidal magnetic field that is likely to be present in the solar
  tachocline. Using a simple Cartesian model, we show how waves with
  a vertical component of propagation can be reflected when traveling
  through a layer containing a horizontal magnetic field with a strength
  that varies with depth. This interaction can prevent a portion of
  the downward traveling wave energy flux from reaching the deep solar
  interior. If a highly reflecting magnetized layer is located some
  distance below the convection zone base, a duct or wave guide can
  be set up, wherein vertical propagation is restricted by successive
  reflections at the upper and lower boundaries. The presence of both
  upward and downward traveling disturbances inside the duct leads
  to the existence of a set of horizontally propagating modes that
  have significantly enhanced amplitudes. We point out that the helical
  structure of these waves makes them capable of generating an α-effect,
  and briefly consider the possibility that propagation in a shear of
  sufficient strength could lead to instability, the result of wave
  growth due to over-reflection.

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Title: Quantifying Magnetic Stellar Wind Torques
Authors: Matt, Sean; MacGregor, K. B.; Pinsonneault, M. H.; Greene,
   T. P.
2011AAS...21743420M    Altcode: 2011BAAS...4343420M
  In order to be able to understand the evolution of stellar spin rates
  and differential rotation, it is necessary to have a rigorous theory
  for predicting angular momentum loss via magnetic stellar winds that is
  applicable over a wide range of conditions. Based upon the results of
  multidimensional, numerical simulations and semi-analytic calculations,
  we present an improved formulation for predicting the stellar wind
  torque, which is valid for varying degrees of magnetization in the
  wind, as well as for stellar spin rates that range from the slow-
  to the fast-magnetic-rotator regimes.

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Title: Emission of Alfven Waves by Planets in Close Orbits
Authors: MacGregor, Keith B.; Pinsonneault, M. H.
2011AAS...21734323M    Altcode: 2011BAAS...4334323M
  We examine the electrodynamics of a conducting planet orbiting within
  a magnetized wind that emanates from its parent star. When the
  orbital motion differs from corotation with the star, an electric
  field exists in the rest frame of the planet, inducing a charge
  separation in its ionosphere. Because the planet is immersed in a
  plasma, this charge can flow away from it along the stellar magnetic
  field lines it successively contacts in its orbit. For sufficiently
  rapid orbital motion, a current system can be formed that is closed
  by Alfvenic disturbances that propagate along field lines away from
  the planet. Using a simple model for the wind from a Sun-like star,
  we survey the conditions under which Alfven wave emission can occur,
  and estimate the power radiated in the form of linear waves for a
  range of stellar, planetary, and wind properties. For a Jupiter-like
  planet in a close (a &lt; 0.10 AU) orbit about a solar-type star, the
  emitted wave power can be as large as 10<SUP>27</SUP> erg/s. While only
  a small influence on the planet's orbit, a wave power of this magnitude
  may have consequences for wind dynamics and localized heating of the
  stellar atmosphere. NCAR is sponsored by the NSF.

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Title: On the interaction of internal gravity waves with a magnetic
    field - II. Convective forcing
Authors: Rogers, T. M.; MacGregor, K. B.
2011MNRAS.410..946R    Altcode: 2010MNRAS.tmp.1507R; 2010arXiv1009.5933R
  We present results from numerical simulations of the interaction of
  internal gravity waves (IGW) with magnetic fields in the radiative
  interior of the Sun. In this second paper, the waves are forced
  self-consistently by an overlying convection zone and a toroidal
  magnetic field is imposed in the stably stratified layer just underneath
  the convection zone. Consistent with the results of previous analytic
  and simple numerical calculations, we find a strong wave-field
  interaction, in which waves are reflected in the field region. The
  wave-field interaction and wave reflection depend on the field strength
  as well as on the adopted values of the diffusivities. In some cases,
  wave reflection leads to an increased mean flow in the field region. In
  addition to reproducing some of the features of our simpler models,
  we find additional complex behaviours in these more complete and
  realistic calculations. First, we find that the spectrum of wave
  generation, both in magnetized and in unmagnetized models, is not
  generally well described by available analytic models, although some
  overlap does exist. Similarly, we find that the dissipation of waves is
  only partially described by the results of linear theory. We find that
  the distortion of the field by waves and convective overshoot leads to
  rapid decay and entrainment of the magnetic field which subsequently
  changes the wave-field interaction. In addition, the field alters the
  amount of wave energy propagating into the deep radiative interior,
  at times increasing the wave energy there and at others decreasing
  it. Because of the complexity of the problem and because the durations
  of these simulations are shorter than the anticipated time-scale for
  dynamical adjustment of the deep solar interior, we are unable to draw
  a definitive conclusion regarding the efficiency of angular momentum
  transport in the deep radiative interior by IGW in the presence of a
  magnetic field.

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Title: Atmospheres for Self-Consistent Field (SCF) Rotating Star
    Models
Authors: Aufdenberg, Jason P.; MacGregor, K. B.
2010AAS...21542708A    Altcode: 2010BAAS...42..343A
  A standard model detailed atmosphere for a rapidly rotating star
  makes the following assumptions: (1) a point-mass approximation of the
  gravitational potential (a Roche model) and (2) an axially symmetric
  uniform (non-differential) rotation. These assumptions are not made in
  the Self-Consistent Field (SCF) models of Jackson et al. (2004, 2005)
  and MacGregor et al. (2007). The SCF differentially rotating stellar
  structures use a mass distribution that is fully self consistent
  with the effective potential, which is the sum of the gravitational
  potential from Poisson's equation and the centrifugal potential. We
  investigate whether a SCF model can match observations of the pole-on
  rapid rotator and photometric standard Vega. Our first models match
  Vega's equatorial diameter and projected rotation velocity, but are
  not a very good match to the observed spectral energy distribution
  (SED). The photospheric shape under anti-solar conservative differential
  rotation yields a cool equatorial region which leaves the model SED too
  line blanketed in the ultraviolet. A further exploration of SCF model
  parameter space is outlined, including changes in the stellar mass,
  the rate of rotation and the kind and degree of differential rotation.

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Title: Reflection and Ducting of Gravity Waves Inside the Sun
Authors: MacGregor, Keith B.; Rogers, T. M.
2010AAS...21542208M    Altcode: 2010BAAS...42..292M
  Internal gravity waves excited by overshoot at the bottom of the
  convection zone can be influenced by rotation and by the strong
  toroidal magnetic field that is likely to be present in the solar
  tachocline. Using a simple Cartesian model, we show how waves
  with a vertical component of propagation can be reflected when
  traveling through a layer containing a horizontal magnetic field
  with a strength that varies with depth. This interaction can prevent
  a portion of the downward traveling wave energy flux from reaching
  the deep solar interior. If a highly reflecting magnetized layer is
  located some distance below the convection zone base, a duct or wave
  guide can be set up, wherein vertical propagation is restricted by
  successive reflections at the upper and lower boundaries. The presence
  of both upward and downward traveling disturbances inside the duct
  leads to the existence of a set of horizontally propagating modes
  that have significantly enhanced amplitudes. We point out that the
  helical structure of these waves makes them capable of generating an
  alpha-effect, and briefly consider the possibility that propagation in
  a shear of sufficient strength could lead to instability, the result
  of wave growth due to over-reflection. <P />NCAR is sponsored by the
  National Science Foundation.

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Title: On the interaction of internal gravity waves with a magnetic
    field - I. Artificial wave forcing
Authors: Rogers, T. M.; MacGregor, K. B.
2010MNRAS.401..191R    Altcode: 2009MNRAS.tmp.1534R
  We present results from numerical simulations of the interaction of
  internal gravity waves (IGW) with a magnetic field. In accordance with
  the dispersion relation governing IGW in the presence of magnetism
  and rotation, when the IGW frequency is approximately that of the
  Alfvén frequency, strong reflection of the wave occurs. Such strong
  reflection markedly changes the angular momentum transport properties
  of the waves. In these simple models a strong, time-independent shear
  layer develops, in contrast to the oscillating shear layer that develops
  in the purely hydrodynamic case.

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Title: Pulsation Modes with High Azimuthal Orders in Stellar Models
    Based on the Self-Consistent Field Method
Authors: Reese, D. R.; MacGregor, K. B.; Jackson, S.; Skumanich, A.;
   Metcalfe, T. S.
2009ASPC..416..395R    Altcode:
  We investigate pulsation modes with high azimuthal orders in a
  uniformly and differentially rotating stellar model based on the
  SCF method. Our results show a more complicated m dependence of the
  pulsation frequencies than what was proposed in previous studies. These
  results allow a more precise determination of the effects of rapid
  uniform or differential rotation on stellar pulsations <P />thereby
  providing a better understanding of the underlying physics.

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Title: Mode identification in rapidly rotating stars
Authors: Reese, D. R.; Thompson, M. J.; MacGregor, K. B.; Jackson,
   S.; Skumanich, A.; Metcalfe, T. S.
2009A&A...506..183R    Altcode: 2009arXiv0905.4889R
  Context: Recent calculations of pulsation modes in rapidly rotating
  polytropic models and models based on the Self-Consistent Field method
  have shown that the frequency spectrum of low degree pulsation modes can
  be described by an empirical formula similar to Tassoul's asymptotic
  formula, provided that the underlying rotation profile is not too
  differential. <BR />Aims: Given the simplicity of this asymptotic
  formula, we investigate whether it can provide a means by which to
  identify pulsation modes in rapidly rotating stars. <BR />Methods:
  We develop a new mode identification scheme which consists in scanning
  a multidimensional parameter space for the formula coefficients which
  yield the best-fitting asymptotic spectra. This mode identification
  scheme is then tested on artificial spectra based on the asymptotic
  formula, on random frequencies and on spectra based on full numerical
  eigenmode calculations for which the mode identification is known
  beforehand. We also investigate the effects of adding random frequencies
  to mimic the effects of chaotic modes which are also expected to show
  up in such stars. <BR />Results: In the absence of chaotic modes,
  it is possible to accurately find a correct mode identification
  for most of the observed frequencies provided these frequencies are
  sufficiently close to their asymptotic values. The addition of random
  frequencies can very quickly become problematic and hinder correct mode
  identification. Modifying the mode identification scheme to reject the
  worst fitting modes can bring some improvement but the results still
  remain poorer than in the case without chaotic modes.

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Title: Pulsation modes in rapidly rotating stellar models based on
    the self-consistent field method
Authors: Reese, D. R.; MacGregor, K. B.; Jackson, S.; Skumanich, A.;
   Metcalfe, T. S.
2009A&A...506..189R    Altcode: 2009arXiv0903.4854R
  Context: New observational means such as the space missions CoRoT and
  Kepler and ground-based networks are and will be collecting stellar
  pulsation data with unprecedented accuracy. A significant fraction of
  the stars in which pulsations are observed are rotating rapidly. <BR
  />Aims: Our aim is to characterise pulsation modes in rapidly rotating
  stellar models so as to be able to interpret asteroseismic data from
  such stars. <BR />Methods: A new pulsation code is applied to stellar
  models based on the self-consistent field (SCF) method. <BR />Results:
  Pulsation modes in SCF models follow a similar behaviour to those
  in uniformly rotating polytropic models, provided that the rotation
  profile is not too differential. Pulsation modes fall into different
  categories, the three main ones being island, chaotic, and whispering
  gallery modes, which are rotating counterparts to modes with low,
  medium, and high l-|m| values, respectively. The frequencies of the
  island modes follow an asymptotic pattern quite similar to what was
  found for polytropic models. Extending this asymptotic formula to higher
  azimuthal orders reveals more subtle behaviour as a function of m and
  provides a first estimate of the average advection of pulsation modes
  by rotation. Further calculations based on a variational principle
  confirm this estimate and provide rotation kernels that could be
  used in inversion methods. When the rotation profile becomes highly
  differential, it becomes more and more difficult to find island and
  whispering gallery modes at low azimuthal orders. At high azimuthal
  orders, whispering gallery modes, and in some cases island modes,
  reappear. <BR />Conclusions: The asymptotic formula found for
  frequencies of island modes can potentially serve as the basis of a
  mode identification scheme in rapidly rotating stars when the rotation
  profile is not too differential.

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Title: RECENT DIRECTIONS IN ASTROPHYSICAL QUANTITATIVE SPECTROSCOPY
AND RADIATION HYDRODYNAMICS: Proceedings of the International
    Conference in Honor of Dimitri Mihalas for His Lifetime Scientific
    Contributions on the Occasion of His 70th Birthday
Authors: Hubeny, Ivan; Stone, James M.; MacGregor, Keith; Werner, Klaus
2009AIPC.1171.....H    Altcode:
  No abstract at ADS

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Title: Comparison of pulsation modes in rapidly rotating polytropic
    and SCF models
Authors: Reese, D.; MacGregor, K. B.; Jackson, S.; Skumanich, A.;
   Metcalfe, T. S.
2009CoAst.158..264R    Altcode:
  In this talk, I will show numerical calculations of pulsation
  modes in rapidly differentially rotating stellar models based on the
  self-consistent field (SCF) method {(e.g. MacGregor et al. 2007)}. The
  pulsation calculations are based on the numerical method presented in
  {Lignières et al. (2006)} and {Reese et al. (2006)}. I will compare
  these results with previous calculations based on simpler polytropic
  models, and discuss how the structure of the frequency spectrum is
  affected by differential rotation and other stellar parameters. In
  particular, differential rotation can lead to a breakdown of
  regularities in the frequency spectrum in favour of a more chaotic
  behaviour.

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Title: Magneto-Thermohaline Mixing in Red Giants
Authors: Denissenkov, Pavel A.; Pinsonneault, Marc; MacGregor, Keith B.
2009ApJ...696.1823D    Altcode: 2008arXiv0806.4346D
  We revise a magnetic buoyancy model that has recently been proposed
  as a mechanism for extra mixing in the radiative zones of low-mass
  red giants. The most important revision is our accounting of the heat
  exchange between rising magnetic flux rings and their surrounding
  medium. This increases the buoyant rising time by five orders of
  magnitude; therefore, the number of magnetic flux rings participating
  in the mixing has to be increased correspondingly. On the other hand,
  our revised model takes advantage of the fact that the mean molecular
  weight of the rings formed in the vicinity of the hydrogen burning
  shell has been reduced by <SUP>3</SUP>He burning. This increases
  their thermohaline buoyancy (hence, decreases the total ring number)
  considerably, making it equivalent to the pure magnetic buoyancy
  produced by a frozen-in toroidal field with B <SUB>phiv</SUB> ≈
  10 MG. We emphasize that some toroidal field is still needed for the
  rings to remain cohesive while rising. Besides, this field prevents
  the horizontal turbulent diffusion from eroding the μ contrast between
  the rings and their surrounding medium. We propose that the necessary
  toroidal magnetic field is generated by differential rotation of the
  radiative zone that stretches a preexisting poloidal field around the
  rotation axis, and that magnetic flux rings are formed as a result of
  its buoyancy-related instability.

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Title: Structural Models for Stars with Solar-like Differential
    Rotation
Authors: MacGregor, Keith B.; Metcalfe, T. S.; Cameron, M.
2009AAS...21340611M    Altcode: 2009BAAS...41..200M
  Rapid rotation can profoundly affect the structure of stars,
  changing basic stellar properties like the radiative luminosity and
  effective temperature and causing the photospheric shape to deviate
  from sphericity. We have computed a grid of models for chemically
  homogeneous, uniformly and differentially rotating stars with masses in
  the range from 1 to 2 solar masses, using a recently developed method
  for obtaining fully consistent solutions to the equations governing
  the stellar structure and gravitational potential. A reformulation
  of the so-called self-consistent field method, this new approach
  yields two-dimensional, axisymmetric, equilibrium configurations
  for conservative rotation laws (i.e., internal angular velocity
  distributions with the property that the associated centrifugal
  acceleration can be derived from a potential). We present results
  for 1 and 2 solar-mass models with solar-like differential rotation
  profiles in which the stellar equator rotates more rapidly than the
  poles. The models have flattened, oblate shapes with equatorial
  (polar) radii that are larger (smaller) than the radius of the
  corresponding non-rotating models. Rotationally induced modifications
  of the thermodynamic conditions in the deep interior lead to reduced
  luminosities, and the model surface temperature distributions are
  latitude-dependent with polar (equatorial) regions that are hotter
  (cooler) than the effective temperature of a non-rotating model for
  the same mass. We compare the properties of these models with those
  of previously computed 1 and 2 solar-mass models having anti-solar
  differential rotation (MacGregor et al. 2007), and enumerate several
  possible consequences of the structural changes arising from rapid
  rotation. NCAR is sponsored by the National Science Foundation.

---------------------------------------------------------
Title: Pulsations of rapidly rotating stellar models based on the
Self-Consistent-Field method: numerical aspects and accuracy
Authors: Reese, D.; MacGregor, K. B.; Jackson, S.; Skumanich, A.;
   Metcalfe, T. S.
2008sf2a.conf..531R    Altcode:
  We use the numerical method developed in tet{Lignieres2006} and
  tet{Reese2006} to calculate pulsation modes in stellar models generated
  by the Self-Consistent-Field method described in tet{Jackson2005}
  and tet{MacGregor2007}. A discussion on the numerical method and its
  accuracy is given, followed by a very brief description of some of
  the results.

---------------------------------------------------------
Title: What Prevents Internal Gravity Waves from Disturbing the
    Solar Uniform Rotation?
Authors: Denissenkov, Pavel A.; Pinsonneault, Marc; MacGregor, Keith B.
2008ApJ...684..757D    Altcode: 2008arXiv0801.3622D
  Internal gravity waves (IGWs) are naturally produced by convection
  in stellar envelopes, and they could be an important mechanism
  for transporting angular momentum in the radiative interiors of
  stars. Prior work has established that they could operate over a short
  enough timescale to explain the internal solar rotation as a function
  of depth. We demonstrate that the natural action of IGWs is to produce
  large-scale oscillations in the solar rotation as a function of depth,
  which is in marked contrast to the nearly uniform rotation in the outer
  radiative envelope of the Sun. An additional angular momentum transport
  mechanism is therefore required, and neither molecular nor shear-induced
  turbulent viscosity is sufficient to smooth out the profile. Magnetic
  processes, such as the Tayler-Spruit dynamo, could flatten the rotation
  profile. We therefore conclude that IGWs must operate in conjunction
  with magnetic angular momentum transport processes if they operate
  at all. Furthermore, both classes of mechanisms must be inhibited to
  some degree by mean molecular weight gradients in order to explain the
  recent evidence for a rapidly rotating embedded core in the Sun, should
  it be confirmed by a further analysis of solar g-mode oscillations.

---------------------------------------------------------
Title: Non-linear dynamics of gravity wave driven flows in the solar
    radiative interior
Authors: Rogers, T. M.; MacGregor, K. B.; Glatzmaier, G. A.
2008MNRAS.387..616R    Altcode: 2008arXiv0804.1987R; 2008MNRAS.tmp..615R
  We present results of non-linear numerical simulations of gravity wave
  driven shear flow oscillations in the equatorial plane of the solar
  radiative interior. These results show that many of the assumptions
  of quasi-linear theory are not valid. When only two waves are forced
  (prograde and retrograde), oscillatory mean flow is maintained;
  but critical layers often form and are dynamically important. When a
  spectrum of waves is forced, the non-linear wave-wave interactions are
  dynamically important, often acting to decrease the maintenance of a
  mean flow. The (in)coherence of such wave-wave interactions must be
  taken into account when describing wave-driven mean flows.

---------------------------------------------------------
Title: Absorption Line Profiles for Differentially Rotating Stellar
    Models
Authors: MacGregor, Keith B.; Casini, R.; Flanagan, W.; Jackson, S.;
   Skumanich, A.
2007AAS...21110310M    Altcode: 2007BAAS...39..921M
  We have computed absorption line profiles for rapidly rotating 2
  solar-mass stars, using structural models of chemically homogeneous,
  uniformly and differentially rotating stars of this mass to specify
  photospheric physical conditions. The models were constructed
  with a recent reformulation of the self-consistent field method, an
  iterative procedure yielding two-dimensional, axisymmetric, equilibrium
  configurations that are consistent solutions to the stellar structure
  equations and Poisson's equation for the gravitational potential,
  for a specified conservative angular velocity distribution. The
  simulated line profiles display a variety of shapes, with morphological
  characteristics that depend on the properties of the surface rotational
  velocity distribution, the angle of inclination of the line of sight
  relative to the rotation axis, and on rotational modifications to the
  structure of the star, including the difference between the surface
  temperatures of the poles and the equator and deviations of the stellar
  shape from sphericity. Some features of the Doppler-broadened profiles
  reflect details of the distribution of the projected rotation speed
  over the visible surface of the star, and may thus provide the means
  for distinguishing between uniform and differential rotation of the
  stellar photosphere. For example, differentially rotating models can
  exhibit absorption profiles with flat or even convex-upward bottoms, as
  opposed to the rounded, concave-upward profiles that are indicative of
  uniform rotation. We present preliminary results from an ongoing effort
  to use simulated line profiles in conjunction with the techniques of
  Principal Component Analysis to infer stellar rotational properties
  from observations.

---------------------------------------------------------
Title: On the Structure and Properties of Differentially Rotating,
    Main-Sequence Stars in the 1-2 M<SUB>solar</SUB> Range
Authors: MacGregor, K. B.; Jackson, Stephen; Skumanich, Andrew;
   Metcalfe, Travis S.
2007ApJ...663..560M    Altcode: 2007arXiv0704.1275M
  We present models for chemically homogeneous, differentially rotating,
  main-sequence stars with masses in the range 1-2 M<SUB>solar</SUB>. The
  models were constructed using a code based on a reformulation of the
  self-consistent field method of computing the equilibrium stellar
  structure for a specified conservative internal rotation law. Relative
  to nonrotating stars of the same mass, these models all have reduced
  luminosities and effective temperatures, and flattened photospheric
  shapes (i.e., decreased polar radii) with equatorial radii that
  can be larger or smaller, depending on the degree of differential
  rotation. For a fixed ratio of the axial rotation rate to the surface
  equatorial rotation rate, increasingly rapid rotation generally deepens
  convective envelopes, shrinks convective cores, and can lead to the
  presence of a convective core (envelope) in a 1 (2) M<SUB>solar</SUB>
  model, a feature that is absent in a nonrotating star of the same
  mass. The positions of differentially rotating models for a given mass
  in the H-R diagram can be shifted in such a way as to approximate the
  nonrotating ZAMS for lower mass stars. Implications of these results
  include (1) possible ambiguities arising from similarities between
  the properties of rotating and nonrotating models of different masses,
  (2) a reduced radiative luminosity for a young, rapidly rotating Sun,
  (3) modified rates of lithium destruction by nuclear processes in the
  layers beneath an outer convective envelope, and (4) the excitation of
  solar-like oscillations and the operation of a solar-like hydromagnetic
  dynamo in some 1.5-2 M<SUB>solar</SUB> stars.

---------------------------------------------------------
Title: Penetration of Dynamo-generated Magnetic Fields into the
    Sun's Radiative Interior
Authors: Dikpati, Mausumi; Gilman, Peter A.; MacGregor, Keith B.
2006ApJ...638..564D    Altcode:
  Any large-scale magnetic fields present in solar/stellar radiative
  interiors have so far been thought to be primordial or residuals
  from extinct dynamos. We show that a regular cyclic dynamo can
  also be the origin of strong magnetic fields in the solar radiative
  tachocline and interior below. By exploiting a kinematic, mean-field
  flux-transport dynamo, we show that for a wide range of core-diffusivity
  values, from 10<SUP>9</SUP> cm<SUP>2</SUP> s<SUP>-1</SUP> down to a
  molecular diffusivity of 10<SUP>3</SUP> cm<SUP>2</SUP> s<SUP>-1</SUP>,
  oscillatory dynamo fields penetrate below the tachocline. Amplitudes
  of these fields are in the range of ~1 kG to 3×10<SUP>3</SUP> kG,
  depending on core diffusivity value, when the dynamo produces ~100
  kG peak toroidal fields in the overshoot tachocline. For a low enough
  core diffusivity (&lt;~10<SUP>7</SUP> cm<SUP>2</SUP> s<SUP>-1</SUP>),
  there is also a steady (nonreversing) dynamo in the radiative tachocline
  and below, which generates strong toroidal field of amplitude ~1 kG to
  3×10<SUP>3</SUP> kG or more there. The key elements in this dynamo
  are the low diffusivity, the differential rotation near the bottom
  of the tachocline, and an assumed tachocline α-effect. The Lorentz
  force feedback may limit oscillatory dynamo fields to ~30 kG, for
  which the mean nonreversing toroidal fields is still ~300 kG, for the
  lowest core diffusivity value. The presence of strong oscillatory and
  steady toroidal fields in the radiative tachocline implies that there
  cannot be a slow tachocline; the dynamics should always be fast there,
  dominated by MHD. These results are obtained using solar parameters,
  but they should also apply generally to stars with convecting shells
  and perhaps also with convective cores.

---------------------------------------------------------
Title: Generating Magnetic Fields in Early-type Stars
Authors: MacGregor, K. B.
2005ASPC..337...28M    Altcode:
  Although magnetic fields have been detected in many upper main
  sequence stars, the physical origins of this magnetism are not well
  understood. After surveying the magnetic properties of these stars and
  reviewing some of the arguments for and against a fossil origin for the
  observed fields, we examine the possibility that they are produced by a
  hydromagnetic dynamo, operating within the central convective core. We
  describe model results suggesting that dynamo action can take place in
  such an environment, but that the high electrical conductivity and large
  spatial extent of the overlying, stably stratified, radiative envelope
  are significant obstacles to the emergence of the generated fields
  at the stellar surface. Advection by internal circulatory flows is,
  by itself, an ineffective transport mechanism, but the buoyant rise of
  tube-like concentrations of magnetic flux may, under certain conditions,
  contribute to bringing the fields from the core to the photosphere. We
  briefly discuss some alternative models, in which these problems are
  circumvented by field generation processes that take place closer to
  the stellar surface.

---------------------------------------------------------
Title: Constraints on the Applicability of an Interface Dynamo to
    the Sun
Authors: Dikpati, Mausumi; Gilman, Peter A.; MacGregor, Keith B.
2005ApJ...631..647D    Altcode:
  Taking into account the helioseismically inferred interior structure,
  we show that a pure interface-type dynamo does not work for the
  Sun if the skin effect for poloidal fields does not allow them to
  penetrate the tachocline. Using a simple mean-field kinematic α-Ω
  dynamo model, we demonstrate that, in the absence of tachocline radial
  shear participating in the dynamo process, a latitudinal differential
  rotation can provide the necessary Ω-effect to drive an oscillation
  in an interface dynamo, but it alone cannot produce the latitudinal
  migration. We show that to make an interface dynamo work with the
  constraints of interior structure and skin depth, a meridional
  circulation is essential. We conclude that a flux-transport dynamo
  driven by both the Babcock-Leighton and interface/bottom α-effects
  is a robust large-scale dynamo for the Sun.

---------------------------------------------------------
Title: Comments on "Full-sphere simulations of circulation-dominated
solar dynamo: Exploring the parity issue"
Authors: Dikpati, M.; Rempel, M.; Gilman, P. A.; MacGregor, K. B.
2005A&A...437..699D    Altcode:
  Using two distinct simulation codes that respectively apply
  semi-implicit and fully explicit schemes, we perform calculations
  of a 2D kinematic Babcock-Leighton type flux-transport dynamo with
  Chatterjee et al.'s parameter settings. We show that their solutions are
  diffusion-dominated, rather than circulation-dominated as their title
  implies. We also have been unable to reproduce several properties of
  their dynamo solutions, namely we obtain a much faster cycle with ~
  4 times shorter period than theirs, with highly overlapping cycles;
  a polar field value of ∼ 2 kG if one has to produce a ~ 100 kG
  toroidal field at convection zone base; and quadrupolar parity as
  opposed to Chatterjee et al.'s dipolar parity solutions.

---------------------------------------------------------
Title: Dynamos with feedback of of j x B force on meridional flow
    and differential rotation
Authors: Rempel, M.; Dikpati, M.; MacGregor, K.
2005ESASP.560..913R    Altcode: 2005csss...13..913R
  No abstract at ADS

---------------------------------------------------------
Title: On the Use of the Self-consistent-Field Method in the
    Construction of Models for Rapidly Rotating Main-Sequence Stars
Authors: Jackson, Stephen; MacGregor, Keith B.; Skumanich, Andrew
2005ApJS..156..245J    Altcode:
  A new formulation of the self-consistent-field (SCF) method for
  computing models of rapidly, differentially rotating stars is
  described. The angular velocity is assumed to depend only on the
  distance from the axis of rotation. In the modified SCF iterative
  scheme, normalized distributions of two thermodynamic variables-pressure
  and temperature-are used as trial functions, while the central values
  of the pressure and temperature are adjusted by a Newton-Raphson
  iteration. A two-dimensional trial density distribution, which is
  needed to compute the gravitational potential, is readily obtained
  from the pressure and temperature through the equation of state in
  conjunction with a third trial function specifying the two-dimensional
  shape of the constant-density surfaces. Rotating models of chemically
  homogeneous main-sequence stars have been computed as necessary
  in order to illustrate the algorithm and to make comparisons with
  existing models. Unlike previous implementations of the SCF method,
  the method described here is not limited to the upper main sequence:
  it converges for all main-sequence masses, including those well below
  9 M<SUB>solar</SUB>. Moreover, the method converges for values of
  the parameter t=T/|W| (the ratio of rotational kinetic energy to
  gravitational potential energy) that are at least as high as those
  obtained by Clement's relaxation technique. The method is also capable
  of producing models with deep concavities about the poles as well
  as models with extreme oblateness (far greater than that possible
  in uniformly rotating stars). For cases with moderate degrees of
  differential rotation (say for Ω<SUB>0</SUB>/Ω<SUB>e</SUB>&lt;10,
  where Ω<SUB>0</SUB> and Ω<SUB>e</SUB> denote the angular velocity at
  the center and at the equator, respectively), the method has been found
  to be remarkably robust. For higher degrees of differential rotation,
  models are restricted to a portion of parameter space away from two
  regions of nonconvergence, inside which some of the models evidently
  develop toroidal level surfaces.

---------------------------------------------------------
Title: Models for the Rapidly Rotating Be Star Achernar
Authors: Jackson, Stephen; MacGregor, Keith B.; Skumanich, Andrew
2004ApJ...606.1196J    Altcode:
  We present models for the Be star Achernar (α Eri), which recent
  interferometric observations have shown has a photospheric shape that
  is significantly distorted by the effects of rotation. The models are
  two-dimensional, axisymmetric configurations, constructed using a new
  version of the self-consistent field (SCF) method for computing the
  structure of a rapidly, differentially rotating star. Our revised SCF
  technique does not suffer from the computational difficulties that
  affected previous implementations of the method, yielding converged
  stellar models regardless of mass. For models with masses like those of
  main sequence stars of mid- to early-B spectral type, it is possible to
  reproduce Achernar's inferred equatorial and polar dimensions through a
  combination of rotational flattening/distension and suitable inclination
  of the rotation axis. However, while matching Achernar's apparent shape,
  these models are discrepant in other respects, being (on average)
  cooler and more rapidly rotating than observations indicate.

---------------------------------------------------------
Title: Dynamos with feedback of jxB Force on Meridional Flow and
    Differential Rotation
Authors: Rempel, M.; Dikpati, M.; MacGregor, K.
2004AAS...204.8802R    Altcode: 2004BAAS...36..819R
  Recently, flux-transport dynamos have been successful in
  reproducing various observed features of the large scale solar
  magnetic fields. However, these studies addressed the transport of
  magnetic fields by the meridional circulation in a purely kinematic
  regime. The toroidal field strength at the base of the solar convection
  zone inferred from studies of rising magnetic flux tubes is around
  100 KG and thus orders of magnitude larger than the equipartition
  field strength estimated from a meridional flow velocity of a few
  m/s. Therefore it is crucial for flux-transport dynamos to address
  the feedback of the jxB on the meridional flow. We present a "dynamic"
  dynamo model, in which we couple a mean-field Reynolds-stress approach
  for the differential rotation and meridional circulation with the
  axisymmetric dynamo equations. This provides a self-consistent model
  that allows to study the back-reaction of the mean-field Lorentz force
  of the dynamo generated field on differential rotation and meridional
  circulation. This model gives an estimate of the magnetic field strength
  up to which a transport of magnetic field by the weak meridional
  flow and amplification by the shear in the differential rotation
  is possible. Additional to this the model also provides solar cycle
  variations in differential rotation and meridional circulation, which
  can be compared to helioseismic data. We also show that the feedback
  of the Lorentz-force on the meridional flow can be included into a
  kinematic dynamo model in terms of a "quenching" of the stream function,
  which deflects the flow from regions of strong toroidal magnetic
  field. From both studies we conclude that flux-transport dynamos work
  even with strong feedback of the jxB force, primarily because of two
  reasons: 1) The transport of the weak poloidal magnetic field, which
  is the sources of strong toroidal field, is not affected strongly. 2)
  The meridional flow results from a small difference between large
  forces, so that the transport capability is much larger than a simple
  estimate based on equipartition argument. <P />This work is partially
  supported by NASA grants W-10107 and W-10175. The National Center for
  Atmospheric Research is sponsored by the National Science Foundation.

---------------------------------------------------------
Title: Models for the Rapidly Rotating Be Star Achernar
Authors: Skumanich, A.; Macgregor, K. B.; Jackson, S.
2004AAS...204.0704S    Altcode: 2004BAAS...36..785S
  We present models for the Be star Achernar (α Eridani), which recent
  interferometric observations have shown has a photospheric shape that
  is significantly distorted by the effects of rotation. The models are
  two-dimensional, axisymmetric configurations, constructed using a new
  version of the self-consistent-field (SCF) method for computing the
  the structure of a rapidly, differentially rotating star, Jackson
  et al. (2004). Our revised SCF technique does not suffer from the
  computational difficulties that affected previous implementations of
  the method, yielding converged stellar models regardless of mass. For
  models with masses like those of main sequence stars of mid to early
  B spectral type, it is possible to reproduce Achernar's inferred
  equatorial and polar dimensions through a combination of rotational
  flattening/distension and suitable inclination of the rotation
  axis. However, while matching Achernar's apparent shape, these
  models are discrepant in other respects, being (on average) cooler
  and more rapidly rotating than observations indicate. <P />Jackson,
  S., Macgregor, K. B. &amp; Skumanich, A., in preparation <P />The NCAR
  (National Center for Atmospheric Research) is sponsored by the National
  Science Foundation.

---------------------------------------------------------
Title: Angular Momentum Transport between a T Tauri Star and an
    Accretion Disk
Authors: Barnes, B.; MacGregor, K. B.
2003csss...12..747B    Altcode:
  We present results from time dependent numerical simulations of
  the hydromagnetic interaction between a rotating T Tauri star and a
  magnetically coupled Keplerian accretion disk. For a diffusive disk,
  we find that most of the toroidal component of the magnetic field
  is generated within a thin, shearing boundary layer that forms along
  the interface between the disk and the magnetosphere. We describe the
  properties of the system when it has attained a rotational equilibrium
  state in which the stellar spin-up produced by accretion and structural
  changes is compensated for by the spin-down torque arising from the
  magnetic connection between the star and the disk.

---------------------------------------------------------
Title: Gravity Wave-driven Flows in the Solar
    Tachocline. II. Stationary Flows
Authors: Kim, Eun-jin; MacGregor, K. B.
2003ApJ...588..645K    Altcode:
  The effects of gravity waves on the mean radial differential rotation
  profile in the solar tachocline are studied, including the effect of
  a uniform, toroidal magnetic field. Vertical transport of horizontal
  momentum arises from the radiative damping of inwardly traveling waves
  that are generated by low-frequency, convective fluid motions. By
  considering two-wave and one-wave interactions, the radiatively
  damped gravity waves are shown to accentuate the shear in the mean
  radial differential rotation. In the presence of a strong horizontal
  magnetic field, internal gravity waves become nearly Alfvénic and
  cannot propagate downward through the tachocline. For a magnetic field
  that is weak enough to permit wave propagation, the mean shear profile
  is shown to be smoother than that obtained in the case of purely
  hydrodynamic waves. The implications of our results for gravity-wave
  forcing of the internal solar rotation are discussed.

---------------------------------------------------------
Title: Magnetic Fields in Massive Stars. II. The Buoyant Rise of
    Magnetic Flux Tubes through the Radiative Interior
Authors: MacGregor, K. B.; Cassinelli, J. P.
2003ApJ...586..480M    Altcode: 2002astro.ph.12224M
  We present results from an investigation of the dynamical behavior
  of buoyant magnetic flux rings in the radiative interior of a
  uniformly rotating, early-type star. Our physical model describes
  a thin, axisymmetric, toroidal flux tube that is released from the
  outer boundary of the convective core and is acted on by buoyant,
  centrifugal, Coriolis, magnetic tension, and aerodynamic drag
  forces. We find that rings emitted in the equatorial plane can
  attain a stationary equilibrium state that is stable with respect to
  small displacements in radius, but is unstable when perturbed in the
  meridional direction. Rings emitted at other latitudes travel toward
  the surface along trajectories that largely parallel the rotation axis
  of the star. Over much of the ascent, the instantaneous rise speed is
  determined by the rate of heating by the absorption of radiation that
  diffuses into the tube from the external medium. Since the timescale
  for this heating varies like the square of the tube cross-sectional
  radius, for the same field strength, thin rings rise more rapidly than
  do thick rings. For a reasonable range of assumed ring sizes and field
  strengths, our results suggest that buoyancy is a viable mechanism
  for bringing magnetic flux from the core to the surface, being capable
  of accomplishing this transport in a time that is generally much less
  than the stellar main-sequence lifetime.

---------------------------------------------------------
Title: Gravity Waves in the Radiative Zone and Tachocline
Authors: MacGregor, K. B.
2003PADEU..13....9M    Altcode:
  We review the properties of internal gravity waves under physical
  conditions like those of the solar radiative interior, and consider
  a few of the ways in which such disturbances might influence the
  dynamical structure of the tachocline region.

---------------------------------------------------------
Title: Solar dynamo models
Authors: MacGregor, Keith
2002ocnd.confE..19M    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Magnetic Fields in Massive Stars. I. Dynamo Models
Authors: Charbonneau, Paul; MacGregor, Keith B.
2001ApJ...559.1094C    Altcode:
  Motivated by mounting evidence for the presence of magnetic fields
  in the atmospheres of “normal” early-type main-sequence stars,
  we investigate the various possible modes of dynamo action in
  their convective core. Working within the framework of mean field
  electrodynamics, we compute α<SUP>2</SUP> and α<SUP>2</SUP>Ω
  dynamo models and demonstrate that the transition from the former
  class to the latter occurs smoothly as internal differential rotation
  is increased. Our models also include a magnetic diffusivity contrast
  between the core and radiative envelope. The primary challenge facing
  such models is to somehow bring the magnetic field generated in the
  deep interior to the stellar surface. We investigate the degree to which
  thermally driven meridional circulation can act as a suitable transport
  agent. In all models with strong core-to-envelope magnetic diffusivity
  contrast-presumably closest to reality- whenever circulation is strong
  enough to carry a significant magnetic flux, it is also strong enough
  to prevent dynamo action. Estimates of typical meridional circulation
  speeds indicate that this regime is likely not attained in the interior
  of early-type main-sequence stars. Dynamo action then remains highly
  probable, but an alternate mechanism must be sought to carry the
  magnetic field to the surface.

---------------------------------------------------------
Title: Gravity Wave-driven Flows in the Solar Tachocline
Authors: Kim, Eun-jin; MacGregor, K. B.
2001ApJ...556L.117K    Altcode:
  We present results from time-dependent hydrodynamic calculations of
  the interaction between internal gravity waves and the mean radial
  differential rotation in the solar tachocline. Such waves are thought
  to be generated by turbulent fluid motions at the base of the convection
  zone. Our simplified model treats the effects of wave forcing, produced
  by radiative damping of downward propagating disturbances, on the
  rotational shear flow in the region immediately below the convection
  zone. We have used the model to investigate the dependence of the
  computed flow properties on the values assumed for the wave frequency,
  the horizontal component of the wavevector, the initial wave velocity
  amplitude, and the viscosity of the background medium. Our results
  indicate that if the first three of these quantities are held fixed,
  stationary shear flow solutions are obtained for viscosities larger than
  a parameter-dependent critical value. If the viscosity is continuously
  decreased from this value, the flow undergoes a succession of dramatic
  transformations, first becoming periodic, then quasi-periodic, and
  ultimately chaotic when the viscosity is made sufficiently small. We
  discuss the implications of these results for the recently reported
  time variability of the angular velocity of rotation within the solar
  tachocline.

---------------------------------------------------------
Title: Spin-Down of Young Stars: The Role of Magnetic Fields
Authors: MacGregor, K. B.
2001ASPC..248..505M    Altcode: 2001mfah.conf..505M
  No abstract at ADS

---------------------------------------------------------
Title: Angular momentum transport by internal gravity waves
Authors: Kim, Eun-Jin; MacGregor, Keith B.
2000AIPC..537..256K    Altcode: 2000wdss.conf..256K
  We discuss the effects of internal gravity waves on the mean radial
  differential rotation profile in the solar tachocline. Vertical
  transport of horizontal momentum arises from the radiative damping of
  inwardly traveling waves that are generated by low-frequency, convective
  fluid motions. For a viscosity typical of the radiative layers below
  the convection zone, the equilibrium profile of radial differential
  rotation is demonstrated to be unstable, possibly leading to turbulent
  mixing in the tachocline. The effect of a uniform, toroidal magnetic
  field is discussed. .

---------------------------------------------------------
Title: The Rise of a Magnetic Flux Tube through the Radiative Envelope
    of a 9 M_solar Star
Authors: Cassinelli, J. P.; MacGregor, K. B.
2000ASPC..214..337C    Altcode: 2000IAUCo.175..337C; 2000bpet.conf..337C
  No abstract at ADS

---------------------------------------------------------
Title: Magnetic fields and light element depletion in the Sun
Authors: Charbonneau, P.; Barnes, G.; MacGregor, K. B.
2000IAUJD...5E..14C    Altcode:
  I will first briefly review some important similarities and differences
  in models for the spin-down of solar-type stars, with or without
  internal magnetic fields in their radiative interior. This will
  be followed by a presentation of some simple calculations for
  the main-sequence depletion of Lithium and Beryllium in the Sun,
  in a regime where magnetic fields provide the chief mechanism for
  the internal redistribution of angular momentum. In this model the
  transport of light elements still occurs in response to shear-induced
  small-scale turbulence, following various commonly used prescriptions
  for the transport coefficients. For some (physically reasonable)
  values of model parameters, both internal differential rotation and
  light element abundances end up solar-like at 4.5Gyr. Within this
  framework light element depletion is a sensitive function of the
  strength of the assumed internal magnetic field.

---------------------------------------------------------
Title: Angular momentum evolution: a comparison of internal transport
    mechanisms
Authors: MacGregor, K. B.
2000ASPC..198..329M    Altcode: 2000scac.conf..329M
  No abstract at ADS

---------------------------------------------------------
Title: On the magnetohydrodynamics of a conducting fluid between
    two flat plates
Authors: Barnes, G.; MacGregor, K. B.
1999PhPl....6.3030B    Altcode:
  The time-dependent flow of a viscous, electrically conducting fluid
  contained within the space between two parallel, semi-infinite,
  perfectly conducting plates is considered. A uniform magnetic field
  directed perpendicular to the plate surfaces is assumed to pervade
  the fluid. Oscillatory motion of one of the plates in its own plane
  is induced through the application of a prescribed acceleration, the
  magnitude and direction of which vary sinusoidally in time. For a system
  forced in this manner, the resulting flow and transverse field component
  are solved for, as well as for the motion of the plate as a function
  of time. The magnetic and viscous stresses exerted on the boundary
  plate by the contiguous field and fluid are explicitly incorporated
  into the treatment of its motion. The physical properties and behavior
  of this system are investigated by examining analytic and numerical
  solutions obtained for a range of forcing periods, Reynolds numbers,
  and plate mass column densities. The relevance of these results to the
  interpretation of a model for Alfvénic torsional oscillations in the
  solar interior are discussed.

---------------------------------------------------------
Title: Angular Momentum Transport in Magnetized Stellar Radiative
    Zones. IV. Ferraro's Theorem and the Solar Tachocline
Authors: MacGregor, K. B.; Charbonneau, P.
1999ApJ...519..911M    Altcode:
  We consider the circumstances under which the latitudinal differential
  rotation of the solar convective envelope can (or cannot) be imprinted
  on the underlying radiative core through the agency of a hypothetical
  weak, large-scale poloidal magnetic field threading the solar
  radiative interior. We do so by constructing steady, two-dimensional
  axisymmetric solutions to the coupled momentum and induction equations
  under the assumption of a purely zonal flow and time-independent
  poloidal magnetic field. Our results show that the structure of the
  interior solutions is entirely determined by the boundary conditions
  imposed at the core-envelope interface. Specifically, in the high
  Reynolds number regime a poloidal field having a nonzero component
  normal to the core-envelope interface can lead to the transmission
  of significant differential rotation into the radiative interior. In
  contrast, for a poloidal field that is contained entirely within
  the radiative core, any differential rotation is confined to a
  thin magnetoviscous boundary layer located immediately beneath the
  interface, as well as along the rotation/magnetic axis. We argue
  that a magnetically decoupled configuration is more likely to be
  realized in the solar interior. Consequently, the helioseismically
  inferred lack of differential rotation in the radiative core does not
  necessarily preclude the existence of a weak, large-scale poloidal
  field therein. We suggest that such a field may well be dynamically
  significant in determining the structure of the solar tachocline.

---------------------------------------------------------
Title: Angular Momentum Transport in Magnetized Stellar Radiative
    Zones. III. The Solar Light-Element Abundances
Authors: Barnes, G.; Charbonneau, P.; MacGregor, K. B.
1999ApJ...511..466B    Altcode:
  We calculate the depletion of the trace elements lithium and beryllium
  within a solar-mass star during the course of its evolution from
  the zero-age main sequence to the age of the present-day Sun. In the
  radiative layers beneath the convection zone, we assume that these
  elements are transported by the turbulent fluid motions that result from
  instability of the shear flow associated with internal differential
  rotation. This turbulent mixing is modeled as a diffusion process,
  using a diffusion coefficient that is taken to be proportional to the
  gradient of the angular velocity distribution inside the star. We study
  the evolution of the light-element abundances produced by rotational
  mixing for models in which internal angular momentum redistribution
  takes place either by hydrodynamic or by hydromagnetic means. Since
  models based on these alternative mechanisms for angular-momentum
  transport predict similar surface rotation rates late in the evolution,
  we explore the extent to which light-element abundances make it possible
  to distinguish between them. In the case of an internally magnetized
  star, our computations indicate that both the details of the surface
  abundance evolution and the magnitude of the depletion at solar age can
  depend sensitively on the assumed strength and configuration of the
  poloidal magnetic field inside the star. For a configuration with no
  direct magnetic coupling between the radiative and convective portions
  of the stellar interior, the depletion of lithium calibrated to the
  solar lithium depletion at the solar age is similar at all ages to
  the lithium depletion of a model in which angular-momentum transport
  occurs solely by hydrodynamical processes. However, the two models can
  be distinguished on the basis of their respective beryllium depletions,
  with the depletion of the magnetic model being significantly smaller
  than that of the nonmagnetic model.

---------------------------------------------------------
Title: Gravity Waves in a Magnetized Shear Layer
Authors: Barnes, G.; MacGregor, K. B.; Charbonneau, P.
1998ApJ...498L.169B    Altcode:
  We use the equations governing the propagation of a gravity wave
  in the presence of a background flow and magnetic field to derive,
  in the Boussinesq approximation, dispersion relations for plane wave
  solutions in certain special cases. We show how, under conditions
  typical of the interior of the Sun, the addition of a magnetic field
  may prevent certain wavevectors from propagating and alter the existence
  and position of any critical layer that might absorb the gravity wave.

---------------------------------------------------------
Title: Angular Momentum Transport in Magnetized Stellar Radiative
Zones: The Solar Light Element Abundances
Authors: Barnes, G.; Charbonneau, P.; MacGregor, K. B.
1998ASPC..154..886B    Altcode: 1998csss...10..886B
  We calculate the depletion of the trace elements lithium and beryllium
  within a solar mass star, during the course of its evolution from
  the zero-age main sequence to the age of the present-day Sun. In the
  radiative layers beneath the convection zone, we assume that these
  elements are transported by the turbulent fluid motions that result from
  the instability of the shear flow associated with internal differential
  rotation. This turbulent mixing is modeled as a diffusive process,
  using a diffusion coefficient that is taken to be proportional to the
  gradient of the angular velocity distribution inside the star. We study
  the evolution of the light element abundances produced by rotational
  mixing for models in which internal angular momentum redistribution
  takes place either by hydrodynamic or by hydromagnetic means. Since
  models based on these alternative mechanisms for angular momentum
  transport predict similar surface rotation rates late in the evolution,
  we explore the extent to which light element abundances make it possible
  to distinguish between them. In the case of an internally magnetized
  star, our computations indicate that both the details of the surface
  abundance evolution and the magnitude of the depletion at solar age can
  depend sensitively on the assumed strength and configuration of the
  poloidal magnetic field inside the star. For a configuration with no
  direct magnetic coupling between the radiative and convective portions
  of the stellar interior, the depletion of lithium as a function of
  age is similar to that of a model in which angular momentum transport
  occurs solely by hydrodynamical processes. However, the two models can
  be distinguished on the basis of their respective beryllium depletions,
  with the depletion of the magnetic model being significantly smaller
  than that of the non-magnetic model.

---------------------------------------------------------
Title: Acceleration Mechanisms for Cool Star Winds
Authors: MacGregor, K. B.
1998cvsw.conf....3M    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Mixing in Low-Mass Stars: The Lithium-Rotation Connection
Authors: Balachandran, Suchitra C.; Garcia Lopez, R. J.; Kraft, R. P.;
   MacGregor, K. B.; Barnes, G.; Martin, E. L.; Pinsonneault, Marc H.
1998ASPC..154..111B    Altcode: 1998csss...10..111B
  We have known for over three decades that the Sun has depleted
  its surface lithium. During this period it has become increasingly
  evident that mixing, unaccounted for by the standard models, occurs
  in the stellar interior. There is some conjecture that this mixing
  may be driven by rotation and thus be dependent upon the rotational
  history of the star. In this discussion session, we will examine the
  observational connection between mixing and rotation and critically
  evaluate current models.

---------------------------------------------------------
Title: Solar Interface Dynamos. I. Linear, Kinematic Models in
    Cartesian Geometry
Authors: MacGregor, K. B.; Charbonneau, P.
1997ApJ...486..484M    Altcode:
  We describe a simple, kinematic model for a dynamo operating in
  the vicinity of the interface between the convective and radiative
  portions of the solar interior. The model dynamo resides within
  a Cartesian domain, partioned into an upper, convective half and
  lower, radiative half, with the magnetic diffusivity η of the
  former region (η<SUB>2</SUB>) assumed to exceed that of the latter
  (η<SUB>1</SUB>). The fluid motions that constitute the α-effect
  are confined to a thin, horizontal layer located entirely within the
  convective half of the domain; the vertical shear is nonzero only
  within a second, nonoverlapping layer contained inside the radiative
  half of the domain. We derive and solve a dispersion relation that
  describes horizontally propagating dynamo waves. For sufficiently large
  values of a parameter analogous to the dynamo number of conventional
  models, growing modes can be found for any ratio of the upper and
  lower magnetic diffusivities. However, unlike kinematic models in
  which the shear and α-effect are uniformly distributed throughout
  the same volume, the present model has wavelike solutions that grow
  in time only for a finite range of horizontal wavenumbers. <P />An
  additional consequence of the assumed dynamo spatial structure is that
  the strength of the azimuthal magnetic field at the location of the
  α-effect layer is reduced relative to the azimuthal field strength
  at the shear layer. When the jump in η occurs close to the α-effect
  layer, it is found that over one period of the dynamo's operation,
  the ratio of the maximum strengths of the azimuthal fields at these
  two positions can vary as the ratio (η<SUB>1</SUB>/η<SUB>2</SUB>)
  of the magnetic diffusivities.

---------------------------------------------------------
Title: Solar Interface Dynamos. II. Linear, Kinematic Models in
    Spherical Geometry
Authors: Charbonneau, P.; MacGregor, K. B.
1997ApJ...486..502C    Altcode:
  Numerical models of interface dynamos are constructed, and their
  properties discussed in some detail. These models are extensions in
  spherical geometry of the Cartesian interface models considered by
  Parker and in the first paper of this series. The models are cast in
  the framework of classical mean-field electrodynamics and make use of
  a realistic solar-like internal differential rotation profile. The
  magnetic diffusivity is assumed to vary discontinously by orders of
  magnitude across the core-envelope interface. This allows the buildup of
  very strong toroidal magnetic fields below the interface, as apparently
  required by recent models of erupting bipolar magnetic regions. <P
  />Distinct dynamo modes powered either by the latitudinal or radial
  shear can coexist and, under certain conditions, interfere destructively
  with one another. Hybrid modes, relying on the latitudinal shear both
  in the envelope and below it, are most easily excited in some portions
  of parameter space, and represent a class of dynamo solutions distinct
  from the true interface modes previously investigated in Cartesian
  geometry. Which mode is preferentially excited depends primarily
  on the assumed ratio of magnetic diffusivities on either side of
  the core-envelope interface. For an α-effect having a simple cos θ
  latitudinal dependency, the interface mode associated with the radial
  shear below the polar regions of the interface is easier to excite than
  its equatorial counterpart. In analogy with more conventional dynamo
  models, interface modes propagate equatorward if the product of the
  radial shear (∂Ω/∂r) and α-effect coefficient (C<SUB>α</SUB>)
  is negative, and poleward if that product is positive. <P />Interface
  dynamo modes powered by the positive radial shear localized below the
  core-envelope interface in the equatorial regions can be produced
  by artificially restricting the α-effect to low latitudes. For
  negative dynamo number, those modes are globally dipolar, propagate
  toward the equator, and are characterized by a phase relationship
  between poloidal and toroidal magnetic field components that is in
  agreement with observations. <P />While the models discussed in this
  paper are linear and kinematic, and consequently rather limited in
  their predictive power, results obtained so far certainly suggest
  that interface dynamos represent a very attractive alternative to
  conventional solar mean-field dynamo models.

---------------------------------------------------------
Title: Angular Momentum Evolution in Late-Type Stars
Authors: Charbonneau, P.; Schrijver, C. J.; MacGregor, K. B.
1997cwh..conf..677C    Altcode: 2006mslp.conf..677C
  No abstract at ADS

---------------------------------------------------------
Title: ALFVÉN Wave-Driven Winds
Authors: MacGregor, K. B.; Charbonneau, P.
1997cwh..conf..327M    Altcode: 2006mslp.conf..327M
  No abstract at ADS

---------------------------------------------------------
Title: On the Generation of Equipartition-Strength Magnetic Fields
    by Turbulent Hydromagnetic Dynamos
Authors: Charbonneau, P.; MacGregor, K. B.
1996ApJ...473L..59C    Altcode:
  The generation of a mean magnetic field by the action of small-scale
  turbulent fluid motions, the alpha -effect, is a fundamental ingredient
  of mean-field dynamo theory. However, recent mathematical models and
  numerical experiments are providing increasingly strong support to the
  notion that at high magnetic Reynolds numbers, the alpha -effect is
  strongly impeded long before the mean magnetic field has reached energy
  equipartition with the driving fluid motions. Taken at face value,
  this raises serious doubt as to whether the solar magnetic field is
  produced by a turbulent hydromagnetic dynamo after all, since it is an
  observed fact that the Sun does possess a structured, large-scale mean
  magnetic field of strength comparable to equipartition. In this Letter
  we demonstrate that the class of mean-field turbulent hydromagnetic
  models known as interface dynamos can produce equipartition-strength
  mean magnetic fields even in the presence of strong alpha -quenching.

---------------------------------------------------------
Title: Nonlinear interface dynamos with α-quenching.
Authors: Charbonneau, P.; MacGregor, K. B.
1996BAAS...28..935C    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Nonlinear interface dynamos with alpha -quenching
Authors: Charbonneau, P.; MacGregor, K. B.
1996AAS...188.6902C    Altcode: 1996BAAS...28Q.935C
  There exist various mechanisms capable of limiting the magnitude of the
  (presumably) dynamo-generated, large-scale solar magnetic field. One
  such mechanism is the so-called “alpha -quenching”. The underlying
  idea is that the Lorentz force associated with the dynamo-generated
  magnetic fields impedes the small scale, turbulent fluid motions
  giving rise to the so-called “alpha -effect” (the production of
  poloidal from toroidal fields in the framework of mean-field dynamo
  theory). In mean-field models, a popular ---yet essentially ad hoc---
  prescription for alpha -quenching consists in replacing the coefficient
  (alpha ) of the alpha -effect source term in the dynamo equations by an
  expression of the form alpha -&gt; alpha (B) =alpha_0 /(1+(|B|/B_eq)(2))
  , where alpha_0 is a measure of the strength of the alpha -effect in the
  linear regime, and B_eq is the equipartition field strength, based on
  the kinetic energy of the turbulent, convective fluid motions (B_eq ~
  10(4) G at the base of the solar convection zone). In principle, such
  “Weak Quenching” allows the production of magnetic fields of roughly
  equipartition strength, as demonstrated by the numerous conventional
  mean-field dynamo models making use of eq. (1), or some close variant,
  published to date. Vainshtein &amp; Cattaneo (1992, ApJ 393, 165)
  and Gruzinov &amp; Diamond (1995, Phys. Plasmas 2, 1941) have argued,
  however, that alpha -quenching should be described by alpha -&gt;
  alpha (B) =alpha_0 /(R_m(|B|/B_eq)(2)) where R_m is a magnetic
  Reynolds number based on the microscopic properties of the flow
  (R_m&gt;&gt; 1 for solar interior conditions). This now describes
  a much stronger form of alpha -quenching, and, with R_m&gt;&gt;
  1, could be fatal to large-scale dynamo action, in the sense that
  the dynamo could only produce magnetic fields of strength &lt;&lt;
  B_eq. This is in marked contradiction with the demands set by recent
  models of bipolar magnetic region emergence, which require field
  strengths of order 10x B_eq ~ 10(5) G for the observed latitudes and
  tilt of emergence to be adequately reproduced. In this contribution, we
  investigate the circumstances under which interface dynamos can avoid
  alpha -quenching, either in the “Weak” or “Strong” forms defined
  above. In interface dynamos the alpha -effect is assumed to operate
  within the solar convective envelope, while the strongest magnetic
  fields are generated by shearing below the core-envelope interface
  (Parker 1993, ApJ 408, 707; Charbonneau &amp; MacGregor, submitted to
  ApJ). This spatial segregation of the alpha -effect source region is
  the key to avoiding alpha -quenching. This is illustrated using a few
  nonlinear, kinematic interface dynamo solutions applicable to the Sun.

---------------------------------------------------------
Title: Stellar winds.
Authors: MacGregor, K. B.
1996ASIC..481..301M    Altcode:
  The author considers some of the ways in which MHD processes can
  contribute to the acceleration of wind-type outflows from stars. He
  first summarizes the measured properties of the "average" solar wind,
  and reviews the evidence for the existence of analogous flows from
  solar-type stars in general. The influence of magnetic fields on wind
  dynamics is then studied by considering how a simple, stationary,
  thermally driven wind model is modified by the inclusion of several
  different MHD effects. Specifically, the author examines how the radial
  acceleration of such a wind is influenced by the incorporation of (1)
  magnetically controlled, non-spherical expansion, (2) the Lorentz
  force associated with a large-scale, stellar magnetic field, and
  (3) the force arising from outwardly propagating, short-wavelength
  Alfvén waves into the basic model. He subsequently considers how these
  processes might affect the dynamical structure of a radiatively driven
  wind from a luminous, hot, OB star.

---------------------------------------------------------
Title: Waves in Radiating Fluids
Authors: Bogdan, T. J.; Knoelker, M.; MacGregor, K. B.; Kim, E. -J.
1996ApJ...456..879B    Altcode:
  We derive from first principles the equations which govern the behavior
  of small-amplitude fluctuations in a homogeneous and isotropic
  radiating fluid. Products of the fluctuating quantities are shown
  to obey a wave-energy conservation law from which it follows that
  all perturbations must ultimately decay in time. Under fairly general
  circumstances the governing equations may be solved through the use of
  integral transforms which affords an accounting of the various wave
  modes supported by the radiating fluid. In addition to the familiar
  radiatively modified acoustic mode, the radiation-diffusion mode, the
  radiative-relaxation mode, and the isotropization and exchange modes
  which constitute the discrete spectrum of the differential equation,
  we find a continuous spectrum of wave modes associated with the
  "collisionless" nature of the photons on timescales short compared
  to the photon lifetime. This continuous spectrum is eliminated
  if an Eddington approximation is used to close the hierarchy of
  equations that relate the fluctuating angular moments of the radiation
  field. Quantitative results are obtained for the simple case in which
  the opacity may be regarded as being independent of the frequency of
  the photon and the source function may be approximated by the (local)
  Planck function.

---------------------------------------------------------
Title: Stellar Winds with Non-WKB Alfven Waves. II. Wind Models for
    Cool, Evolved Stars
Authors: Charbonneau, P.; MacGregor, K. B.
1995ApJ...454..901C    Altcode:
  We construct Alfvén wave-driven wind models for physical conditions
  appropriate to the expanding envelopes of cool, evolved stars. To
  derive wind properties, we assume steady, isothermal, spherically
  symmetric flow, but do not use the WKB (i.e., short-wavelength)
  approximation to calculate the wave amplitudes. Instead, we make
  use of the formalism developed in the first paper of this series
  (MacGregor &amp; Charbonneau 1994), which describes wave reflection
  and associated modifications to the wave force, and consistently
  incorporates these effects into the treatment of wind dynamics. <P
  />For flows containing undamped Alfvén waves of arbitrarily long
  wavelength we find that the occurrence of wave reflection has profound
  consequences for wind acceleration and mass loss. Specifically, in all
  of our computed models, the outward-directed wave force near the base
  of the flow is significantly reduced relative to that in comparable
  WKB models. As a result, the initial expansion speeds and mass flux
  densities of model winds that include non-WKB effects are smaller than
  those of corresponding WKB winds. Moreover, at large distances from
  the star, wave reflection leads to an enhancement of the wave force
  relative to models in which all waves are presumed to be outwardly
  propagating. This tendency, when combined with the previously noted
  reduction in mass flux, produces winds with higher asymptotic flow
  speeds than those driven by high-frequency, short-wavelength Alfvén
  waves. Given that the challenge of modeling winds from cool evolved
  stars is to produce winds with high mass fluxes and low asymptotic
  flow speeds, we argue that Alfvén waves provide an acceptable driving
  mechanism only if their wavelengths are sufficiently short that minimal
  reflection occurs near the base of the flow. For stellar parameters
  characteristic of a supergiant star with spectral type ∼K5, this
  translates into an upper bound on Alfvén wave periods of ∼1 day.

---------------------------------------------------------
Title: On the evolution of rotational velocity distributions for
    solar-type stars.
Authors: Keppens, R.; MacGregor, K. B.; Charbonneau, P.
1995A&A...294..469K    Altcode:
  We investigate how the distribution of rotational velocities for
  late-type stars of a given mass evolves with age, both before
  and during residence on the main sequence. Starting from an age
  ~10^6^years, an assumed pre-main sequence rotational velocity/period
  distribution is evolved forward in time using the model described by
  MacGregor &amp; Brenner (1991) to trace the rotational histories of
  single, constituent stars. This model treats: (i) stellar angular
  momentum loss as a result of the torque applied to the convection
  zone by a magnetically coupled wind; (ii) angular momentum transport
  from the radiative interior to the convective envelope in response
  to the rotational deceleration of the stellar surface layers; and
  (iii), angular momentum redistribution associated with changes in
  internal structure during the process of contraction to the main
  sequence. We ascertain how the evolution of a specified, initial
  rotational velocity/period distribution is affected by such things as:
  (i) the dependence of the coronal magnetic field strength on rotation
  rate through a prescribed, phenomenological dynamo relation; (ii) the
  magnitude of the timescale τ_c_ characterizing the transfer of angular
  momentum from the core to the envelope; (ii) differences in the details
  and duration of pre-main sequence structural evolution for stars with
  masses in the range 0.8&lt;=M_*_/M<SUB>sun</SUB>_&lt;=1.0 and (iv),
  the exchange of angular momentum between a star and a surrounding,
  magnetized accretion disk during the first few million years of
  pre-main sequence evolution following the development of a radiative
  core. The results of this extensive parameter study are compared with
  the distributions derived from measurements of rotational velocities
  of solar-type stars in open clusters with known ages. Starting from an
  initial distribution compiled from observations of rotation among T
  Tauri stars, we find that reasonable agreement with the distribution
  evolution inferred from cluster observations is obtained for: (i)
  a dynamo law in which the strength of the coronal field increases
  linearly with surface angular velocity for rotation rates &lt;=20
  times the present solar rate, and becomes saturated for more rapid
  rotation; (ii) a coupling timescale ~10^7^years; (iii) a mix of stellar
  masses consisting of roughly equal numbers of 0.8M<SUB>sun</SUB>_ and
  1.0M<SUB>sun</SUB>_ stars; and (iv), disk regulation of the surface
  rotation up to an age ~6x10^6^years for stars with initial rotation
  periods longer than 5days. A number of discrepancies remain, however:
  even with the most favorable choice of model parameters, the present
  calculations fail to produce a sufficiently large proportion of slow
  (equatorial velocities less than 10km/s) rotators on the Zero-Age
  Main Sequence.

---------------------------------------------------------
Title: Stellar Winds with Non-WKB Alfven Waves. I. Wind Models for
    Solar Coronal Conditions
Authors: MacGregor, K. B.; Charbonneau, P.
1994ApJ...430..387M    Altcode:
  We have constructed numerical models for stationary, wind-type
  outflows that include treatment of the force produced by propagating
  Alfven waves. We make no assumptions regarding the relative sizes
  of the wavelengths of such disturbances and the scale lengths that
  characterize the variation of the physical properties of the expanding
  stellar atmosphere. Consequently, our models take account the process of
  Alfven wave reflection, and provide for dynamical effects arising from
  the simultaneous presence of outward and inward traveling waves in the
  wind. For physical conditions like those prevailing in the outer solar
  corona and wind, we find that even relatively high frequency, short
  wavelength waves can suffer some reflection from the gradient in Alfven
  speed at the vase of the flow. Among the consequences of the interaction
  between outward and inward directed perturbations in the sub-Alfvenic
  portion of the wind is a reduction in the magnitude of the time-averaged
  wave force relative to its value in the Wentzel-Kramer-Brillouin (WKB)
  (i.e., short-wavelenght) limit. As a result, the flow velocities of
  our models interior to the Alfven radius are smaller than those of
  corresponding WKB models. For models containing very low frequency,
  long wavelength waves, a substantial amount of wave reflection can also
  take place in the super-Alvenic portion of the wind. The resulting
  modifications to the spatial dependences of the eave magnetic and
  velocity amplitudes can lead to a wave force whose magnitude at large
  distances exceeds that of an equivalent WKB solution.

---------------------------------------------------------
Title: Angular Momentum Evolution of Late-Type Stars: A Theoretical
    Perspective (Invited Review)
Authors: MacGregor, K. B.; Charbonneau, P.
1994ASPC...64..174M    Altcode: 1994csss....8..174M
  No abstract at ADS

---------------------------------------------------------
Title: Angular Momentum Loss from the Young Sun: Improved Wind and
    Dynamo Models
Authors: Keppens, R.; Charbonneau, P.; MacGregor, K. B.; Brandenburg,
   A.
1994ASPC...64..193K    Altcode: 1994csss....8..193K
  No abstract at ADS

---------------------------------------------------------
Title: Solar wind with non-WKB Alfvén waves
Authors: Charbonneau, P.; MacGregor, K. B.
1994smf..conf..405C    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Angular Momentum Transport in Magnetized Stellar Radiative
    Zones. II. The Solar Spin-down
Authors: Charbonneau, P.; MacGregor, K. B.
1993ApJ...417..762C    Altcode:
  We present a large set of numerical calculations describing the
  rotational evolution of a solar-type star, in response to the torque
  exerted on it by a magnetically coupled wind emanating from its
  surface. We consider a situation where the internal redistribution
  of angular momentum in the radiative part of the envelope is
  dominated by magnetic stresses arising from the shearing of a
  preexisting, large-scale, poloidal magnetic field. <P />By assuming
  a time-independent poloidal magnetic field, neglecting fluid motions
  in meridional planes, and restricting our attention to axisymmetric
  systems, we reduce the spin-down problem to solving the (coupled)
  ψ-components of the momentum and induction equations. Nevertheless, our
  computations remain dynamical, in that they take into account both the
  generation of a toroidal magnetic field by shearing of the preexisting
  poloidal field, and the back-reaction of the resulting Lorentz force
  on the differential rotation. It becomes possible to draw, for the
  first time, a reasonably realistic and quantitative picture of the
  effects of large-scale internal magnetic fields on the main-sequence
  rotational evolution of solar-type stars. <P />We perform spin-down
  calculations for a standard solar model, starting from the ZAMS and
  extending all the way to the solar age. The wind-induced surface torque
  is computed using the axisymmetric formulation of Weber &amp; Davis
  (1967). We consider a number of poloidal magnetic field configurations
  which differ both in the degree of magnetic coupling between the
  convective envelope and radiative core and in average strength. <P
  />The rotational evolution can be divided into three more or less
  distinct phases: an initial phase of toroidal field buildup in the
  radiative zone, lasting from a few times 10<SUP>4</SUP> to a few times
  10<SUP>6</SUP> yr; a second period in which oscillations set up in the
  radiative zone during the first phase are damped; and a third period,
  lasting from an age of about 10<SUP>7</SUP> yr onward, characterized
  by a state of dynamical balance between the total stresses (magnetic +
  viscous) at the core-envelope interface and the wind-induced surface
  torque, leading to a quasistatic internal magnetic and rotational
  evolution. <P />Our results also demonstrate (1) the existence of
  classes of large-scale internal magnetic fields that can accommodate
  rapid spin-down near the ZAMS and yield a weak internal differential
  rotation by the solar age, (2) the importance of phase mixing in
  efficiently damping large-scale toroidal oscillations pervading the
  radiative interior at early times, (3) the near-independence of the
  present solar surface angular velocity on the strength and geometry
  (past and present) of any internal large-scale magnetic field pervading
  the radiative interior, and (4) the greater dependence of the present
  solar internal differential rotation on the overall morphology (but
  not on the strength) of the internal magnetic field.

---------------------------------------------------------
Title: A Numerical Simulation of Two-dimensional Radiative Equilibrium
    in Magnetostatic Flux Tubes. II. Computational Results
Authors: Pizzo, V. J.; MacGregor, K. B.; Kunasz, P. B.
1993ApJ...413..764P    Altcode:
  We apply a recently developed numerical procedure for obtaining
  self-consistent radiative and mechanical equilibria of solar
  magnetostatic flux tubes. The tubes are modeled as localized but
  continuous field concentrations in a 2D slab geometry, and the radiation
  is treated in the LTE limit. The reduced opacities in the tube lead to
  the lateral intrusion of radiative flux into the tube, with subsequent
  modification of the internal atmosphere. Our calculations show that the
  effect is negligible, however, when the diameter of the tube exceeds the
  scale height of the atmosphere in the surface layers or if convective
  energy transport in the tube is itself sufficient to produce a thermal
  stratification close to that in the surrounding photosphere. That is,
  2D radiative effects are likely to be important only in small tubes
  in which the upward convective energy flux is strongly suppressed. The
  treatment of the underlying convective layers appears to be of paramount
  importance in determining the computed surface properties.

---------------------------------------------------------
Title: The Evolution of Angular Momentum among Zero-Age Main-Sequence
    Solar-Type Stars
Authors: Soderblom, David R.; Stauffer, John R.; MacGregor, Keith B.;
   Jones, Burton F.
1993ApJ...409..624S    Altcode:
  We consider a survey of rotation among F, G, and K dwarfs of the
  Pleiades in the context of other young clusters (Alpha Persei and the
  Hyades) and pre-main-sequence (PMS) stars (in Taurus-Auriga and Orion)
  in order to examine how the angular momentum of a star like the sun
  evolves during its early life on the main sequence. The rotation of PMS
  stars can be evolved into distributions like those seen in the young
  clusters if there is only modest, rotation-independent angular momentum
  loss prior to the ZAMS. Even then, the ultrafast rotators (UFRs,
  or ZAMS G and K dwarfs with v sin i equal to or greater than 30 km/s)
  must owe their extra angular momentum to their conditions of formation
  and to different angular momentum loss rates above a threshold velocity,
  for it is unlikely that these stars had angular momentum added as they
  neared the ZAMS, nor can a spread in ages within a cluster account for
  the range of rotation seen. Only a fraction of solar-type stars are
  thus capable of becoming UFRs, and it is not a phase that all stars
  experience. Simple scaling relations (like the Skumanich relation)
  applied to the observed surface rotation rates of young solar-type
  stars cannot reproduce the way in which the Pleiades evolve into the
  Hyades. We argue that invoking internal differential rotation in these
  ZAMS stars can explain several aspects of the observations and thus
  can provide a consistent picture of ZAMS angular momentum evolution.

---------------------------------------------------------
Title: Solar Spin-down with Internal Magnetic Fields: Erratum
Authors: Charbonneau, P.; MacGregor, K. B.
1993ApJ...403L..87C    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: A Numerical Simulation of Two-dimensional Radiative Equilibrium
    in Magnetostatic Flux Tubes. I. The Model
Authors: Pizzo, V. J.; MacGregor, K. B.; Kunasz, P. B.
1993ApJ...404..788P    Altcode:
  We describe an efficient numerical procedure for obtaining
  self-consistent radiative and mechanical equilibria of solar
  magnetostatic flux tubes. The flux tube is treated as a localized but
  continuous concentration of magnetic flux in a two-dimensional slab
  geometry. Starting from some given initial atmosphere, we compute a
  preliminary magnetostatic equilibrium state by a nonlinear iterative
  technique, subject to certain boundary conditions on the magnetic
  field. Given the resulting spatial distribution of opacities (derived
  from the initial run of temperature and density along each field line),
  and with knowledge of a distant radiation field incident upon the
  magnetic region, the radiative state of the atmosphere is computed by
  the short-characteristics method assuming gray, LTE conditions. The new
  temperature derived from the angle-averaged radiation field is used to
  update the hydrostatic atmosphere for the magnetostatic calculation,
  and so on, until both mechanical and radiative balance is attained.

---------------------------------------------------------
Title: Spin down of solar-type stars with internal magnetic fields
Authors: Charbonneau, P.; MacGregor, K. B.
1993ASPC...40..464C    Altcode: 1993IAUCo.137..464C; 1993ist..proc..464C
  No abstract at ADS

---------------------------------------------------------
Title: Winds.
Authors: MacGregor, K. B.
1993ASPC...40..620M    Altcode: 1993IAUCo.137..620M; 1993ist..proc..620M
  It is by now well known that most main sequence stars continuously lose
  mass as a consequence of the winds they emit. In addition to affecting
  the thermal and dynamical state of the stellar atmosphere, such mass
  loss can also induce changes in the interiors of stars. In the present
  review, the author considers a few of the ways in which sustained,
  wind-like mass loss can alter the physical state of main sequence
  stellar interiors by examining the differences in internal structure,
  composition, and rotation between mass-losing and conservatively
  evolving stars.

---------------------------------------------------------
Title: On the Interaction between Dust and Gas in Late-Type Stellar
    Atmospheres and Winds
Authors: MacGregor, K. B.; Stencel, R. E.
1992ApJ...397..644M    Altcode:
  An assumption inherent to most models of dust-driven winds from cool,
  evolved stars is that the radiative and collisional drag forces acting
  on an individual dust grain are in balance throughout the flow. We have
  checked the validity of this supposition of 'complete momentum coupling'
  by comparing the grain motion obtained from such a model with that
  derived from solution of the full grain equation of motion. For physical
  conditions typical of the circumstellar envelopes of oxygen-rich red
  giants, we find that silicate grains with initial radii smaller than
  about 5 x 10 exp -6 cm decouple from the ambient gas near the base of
  the outflow. The implications of these results for models of dust-driven
  mass loss from late-type giants and supergiants are discussed.

---------------------------------------------------------
Title: Solar Spin-down with Internal Magnetic Fields
Authors: Charbonneau, P.; MacGregor, K. B.
1992ApJ...397L..63C    Altcode:
  We investigate the rotational evolution of a solar-type star containing
  a large-scale poloidal magnetic field in its radiative core, in response
  to the torque applied to it by magnetically coupled wind. Our model
  takes into account both the generation of a toroidal magnetic component
  via shearing of the existing poloidal component by differential
  rotation, as well as the back-reaction on the differential rotation due
  to Lorentz forces associated with the toroidal field. Our computations
  demonstrate the existence of classes of large-scale poloidal magnetic
  fields allowing rapid spin-down of the surface layers shortly after the
  arrival on the zero-age main sequence, while producing weak internal
  differential rotation in the radiative core by the solar age. This
  indicates that the constraints brought about by rotational evolution
  of solar analogs in young clusters and by helioseismology are not
  incompatible with the existence of large-scale magnetic fields in
  stellar radiative interiors. The present surface solar rotation rate
  is also shown to be a poor indicator of the strength and geometry of
  hypothetical poloidal magnetic fields pervading the solar radiative
  interior.

---------------------------------------------------------
Title: Angular Momentum Transport in Magnetized Stellar Radiative
    Zones. I. Numerical Solutions to the Core Spin-up Model Problem
Authors: Charbonneau, P.; MacGregor, K. B.
1992ApJ...387..639C    Altcode:
  The present paper investigates the time evolution of the angular
  momentum and induced toroidal magnetic field distribution in
  an initially nonrotating radiative stellar envelope containing a
  large-scale poloidal magnetic field, following the impulsive spin-up of
  the underlying core. A large set of numerical calculations pertaining
  to monopolar, dipolar, and quadrupolar magnetic configurations, with
  and without density gradients across the envelope, as well as a set
  of solutions for which the poloidal field is only partially anchored
  on the core is presented. It is demonstrated that in moderate to
  high Reynolds-number systems, any global magnetic dissipation time
  scale constructed using length scales of order of the stellar radius
  greatly overestimates the true dissipation time scale of the toroidal
  magnetic component.

---------------------------------------------------------
Title: Winds from rotating, magnetic, hot stars: consequences for
    the rotational evolution of O and B stars.
Authors: MacGregor, K. B.; Friend, D. B.; Gilliland, R. L.
1992A&A...256..141M    Altcode:
  In an effort to obtain estimates of magnetic field strengths in hot
  stars, the authors have computed the evolution of rigidly rotating
  15 and 30 M<SUB><SUB>sun</SUB></SUB> stars, including the effects of
  magnetically coupled, line-driven mass loss as described by the model of
  Friend and Mac-Gregor (1984). Using mean rotational velocities (i.e. v
  sin i values) derived from observations of main-sequence stars of these
  masses to specify the initial state, they followed the variations in
  time of the surface rotation rate due to internal structural changes
  and wind-related breaking. In each case the initial magnetic field
  strength was varied until the calculated rotational velocity at the
  onset of the blue supergiant phase was in resonable agreement with the
  corresponding mean v sin i value derived from observations. For both
  evolutionary models, it is found that the computed rotational velocity
  decrease in the presence of a magnetic field of even modest strength
  (⪉100 G) exceeds the limits set by observations. The limitations
  of this study and its implications for both the magnetic fields and
  winds of hot stars are discussed.

---------------------------------------------------------
Title: Wind-Driven Dust: The Interaction Between Dust; Gas in
    Late-Type Stellar Atmospheres; Winds
Authors: MacGregor, K. B.; Stencel, R. E.
1992ASPC...26..484M    Altcode: 1992csss....7..484M
  No abstract at ADS

---------------------------------------------------------
Title: Rotational Evolution of Solar-Type Stars. I. Main-Sequence
    Evolution
Authors: MacGregor, K. B.; Brenner, M.
1991ApJ...376..204M    Altcode:
  A simple, parameterized model for the redistribution of angular momentum
  within the interiors of solar-type stars is presented. By incorporating
  it with a description of angular momentum loss through the action
  of a magnetically coupled wind, tracing the rotational histories
  of low-mass dwarf stars for a variety of initial conditions and
  parameter specifications is accomplished. The results of calculations
  are discussed for the rotational evolution of a 1 solar mass star, from
  the time of its arrival on the zero-age main sequence to the age of the
  present-day sun. Best agreement is with observational constraints for
  (1) a surface magnetic field strength which is largely independent of
  surface angular velocity omega for rapid rotation, and approximately
  linearly dependent on omega for approximately solar values, and (2), a
  time scale for angular momentum transfer from the core to the convection
  zone which remains essentially constant throughout the evolution, with
  magnitude of a about 10 million yr. This value is approximately equal
  to the initial wind braking time. These results are discussed in the
  context of a qualitative description of angluar momentum redistribution
  by magnetic fields in the radiative interiors of solar-type stars.

---------------------------------------------------------
Title: A Numerical Simulation of Two-Dimensional Radiative Equilibrium
    in Magnetostatic Flux Tubes
Authors: Pizzo, V. J.; MacGregor, K. B.; Kunasz, P.
1991BAAS...23Q1051P    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Pre-Main Sequence and Main Sequence Rotational Evolution -
    Constraints on Models Derived from Observations
Authors: MacGregor, K. B.
1991ASIC..340..315M    Altcode: 1991amey.conf..315M
  No abstract at ADS

---------------------------------------------------------
Title: A Numerical Simulation of Two-Dimensional Radiative Equilibrium
    in Magnetostatic Flux Tubes
Authors: Pizzo, V. J.; MacGregor, K. B.; Kunasz, P.
1990BAAS...22R1198P    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Pre-Main Sequence Rotational Evolution of Solar-Type Stars
Authors: MacGregor, K. B.; Cohen, P.
1990BAAS...22.1207M    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Lower Solar Chromosphere-Corona Transition Region. II. Wave
    Pressure Effects for a Specific Form of the Heating Function
Authors: Woods, D. Tod; Holzer, Thomas E.; MacGregor, Keith B.
1990ApJS...73..489W    Altcode:
  Lower transition region models with a balance between mechanical heating
  and radiative losses are expanded to include wave pressure effects. The
  models are used to study the simple damping length form of the heating
  function. The results are compared to the results obtained by Woods et
  al. (1990) for solutions in the lower transition region. The results
  suggest that a mixture of fast-mode and slow-mode waves may provide
  the appropriate heating mechanism in the lower transition region, with
  the decline in effective vertical wave speed caused by the refraction
  and eventual total reflection of the fast-mode wave resulting from
  the decreasing atmospheric density.

---------------------------------------------------------
Title: Lower Solar Chromosphere-Corona Transition
    Region. I. Theoretical Models with Small Temperature Gradients
Authors: Woods, D. Tod; Holzer, Thomas E.; MacGregor, Keith B.
1990ApJ...355..295W    Altcode:
  A study of transition region models including the effects of classical
  thermal conduction, heating, and radiative cooling is carried out
  with attention directed toward the problem of understanding the
  observed emission in the lower transition region. It is found that the
  observationally inferred emission measure curve implies a near-balance
  between heating and radiative cooling in the lower transition region,
  and that the presence of strong hydrogen Ly-alpha cooling leads to
  the existence of singularities in the solutions of the force balance
  and energy balance equations when such a near-balance between heating
  and cooling is assumed. These singularities place strong constraints
  on the nature of viable models of the lower transition region and must
  be considered when Ly-alpha cooling is important. Previously suggested
  explanations of the observed emission from the lower transition region
  are considered in the context of the results of the present study,
  and conditions for the applicability of these suggested explanations
  are discussed.

---------------------------------------------------------
Title: Lower Solar Chromosphere-Corona Transition
    Region. III. Implications of the Observed Quiet-Sun Emission Measure
    Including Wave Pressure Effects
Authors: Woods, D. Tod; Holzer, Thomas E.; MacGregor, Keith B.
1990ApJ...355..309W    Altcode:
  The observed form of the emission measure (EM) is used as a function
  of temperature to infer the wave energy flux density and pressure
  throughout the lower transition region (TR). This procedure eliminates
  the need for specifying how the wave energy flux density is damped and
  addresses the question of whether there is any form of the mechanical
  heating associated with the degradation of an upward traveling wave
  energy flux density which is consistent with the observed EM and other
  observational constraints for the quiet sun. It is found that the
  observed form of the EM curve is incompatible with waves traveling
  vertically at the sound speed, regardless of any filling factor
  arguments. The same conclusion also applies to waves traveling at
  the Alfven speed, unless it is assumed that the emission in lower TR
  lines originates solely from small, spatially unresolved regions of
  large magnetic field strength (100 G), which cover a small fraction
  (filling factors of 1 percent) of the solar surface.

---------------------------------------------------------
Title: High-Latitude SPOT and Plage Activity on the Rapidly Rotating
    M Dwarf Star Gliese 890
Authors: Young, Arthur; Skumanich, Andrew; MacGregor, Keith B.;
   Temple, Scott
1990ApJ...349..608Y    Altcode:
  Simultaneous time-resolved observations in broadband photometry and
  in H-alpha and near-UV spectroscopy are presented of Gliese 890, a
  single M dwarf of the BY Draconis type with the shortest known period
  of rotation among field stars of its type. The continuum light curves
  exhibit modulations characteristic of the presence of localized dark
  spot regions, and the spectra show corresponding modulations of the
  intensity of H-alpha emission, with the latter being substantially in
  phase with the former. Secular changes in the continuum light curve
  indicate that the geometry of the spots changes appreciably over time
  scales of the order of 60 rotation periods. The overall chromospheric
  activity is not significantly greater than that found in similar stars
  which rotate much more slowly. The principal active region present on
  the visible disk of the star during the observations appear to be at
  a remarkably high latitude compared to the occurrence of such regions
  on the sun.

---------------------------------------------------------
Title: Rotational Evolution of Solar-Type Stars
Authors: MacGregor, K. B.; Brenner, M.
1989BAAS...21.1078M    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: On the Structure of Radiating Shocks
Authors: Halvorson, C. M.; MacGregor, K. B.
1989BAAS...21.1114H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Nonspherical Expansion of Radiation-driven Winds
Authors: MacGregor, K. B.
1988ApJ...327..794M    Altcode:
  The line-driven wind theory of Castor et al. (1975) is extended to
  include effects arising from nonspherical expansion. Specifically,
  isothermal flow along the central streamline of a flow tube whose
  cross-sectional area increases outward faster than r-squared near the
  stellar surface is considered. Using the area function of Kopp and
  Holzer (1976), it is found that rapid flow tube divergence causes the
  critical point to occur closer to the stellar photosphere. Even modest
  departures from spherically symmetric expansion can significantly
  enhance the radiative force, and lead to large increases in the
  asymptotic flow speed of the wind. For models in which the region
  of rapid geometrical divergence is located more than a few tenths of
  a stellar radius from the photosphere, the mass flux density at the
  base of the wind is essentially unchanged from that of the spherical
  case. These results may have implications for the occurrence and
  structure of streams and corotating interaction regions in the winds
  of hot stars.

---------------------------------------------------------
Title: Shallow Temperature Gradient Solutions and Wave Pressure
    Effects in the Lower Transition Region
Authors: Woods, D. T.; MacGregor, K. B.; Holzer, T. E.
1987BAAS...19Q.937W    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Winds from Rotating, Magnetic, Hot Stars. III. The Azimuthal
    Velocity and Magnetic Force
Authors: MacGregor, K. B.; Friend, D. B.
1987ApJ...312..659M    Altcode:
  The model of Friend and MacGregor (1984) is used to study the dynamical
  interplay between the azimuthal velocity and the Lorentz force in the
  line-driven wind from a rotating, magnetic, hot star. It is found
  that strong, outward acceleration by the radiative force can cause
  the azimuthal component of the magnetic force to reverse direction and
  act counter to the sense of the stellar rotation. One manifestation of
  this behavior is a faster than 1/r decrease in V(phi) over a limited
  spatial region near the base of the flow. By consideration of the wind
  momentum equation in the corotating reference frame, such a magnetic
  force reversal is shown to be a consequence of the required force
  balance perpendicular to the magnetic field.

---------------------------------------------------------
Title: Stars with Thin and Thick Convection Zones: M Giants (Invited
    review)
Authors: MacGregor, K. B.
1987LNP...291..250M    Altcode: 1987LNP87.291..250M; 1987csss....5..250M
  Observational results pertaining to the thermal and dynamical structure
  of the atmospheres of cool, evolved stars are summarized. It is
  noted that the inferred atmospheric properties of a given star of
  this type appear to depend on its position relative to the various
  dividing lines which have been identified in the HR diagram. Using
  computed evolutionary models, changes in the internal structure of an
  intermediate mass star are followed as its redward motion in the HR
  diagram carries it across the transition region dividing line. It is
  suggested that this boundary may reflect the onset of deep envelope
  convection during post main sequence evolution.

---------------------------------------------------------
Title: On the Size of Convective Cores in Rotating,
    Upper--Main-Sequence Stars
Authors: MacGregor, K. B.; Gilliland, Ronald L.
1986ApJ...310..273M    Altcode:
  The effects of rotation on the structure of upper main sequence
  stars are reviewed, and the effects resulting from alterations of the
  convective-radiative stability criterion due to changes in the radiative
  gradient are explicitly modeled and analyzed. It is found that rotation
  leads to a smaller convective core and reduced luminosity, the coupled
  effects of which lead to little change in the main sequence lifetime. It
  is noted that the rotational modification to the Schwarzschild criterion
  suggested by Sreenivasan and Wilson (1985) has in fact been included
  in the computations conducted by previous authors.

---------------------------------------------------------
Title: Radiation-Driven Winds in Non-Spherical Geometries
Authors: MacGregor, K. B.
1986BAAS...18..954M    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The Effect of Extremely Rapid Rotation on Stellar Activity:
    The Case of Gliese 890
Authors: Young, Arthur; Skumanich, Andrew; MacGregor, Keith; Temple,
   Scott
1986LNP...254..127Y    Altcode: 1986csss....4..127Y
  We present an analysis of broad-band photometry and of the H-alpha
  emission line profile of the active, rapidly rotating (P = 0.43
  days) single M-dwarf star Gleise 890. Our analysis suggests that the
  (presumed dark) spots and the bright emission regions are confined
  to high latitudes on the star. We combine our results with published
  work on other rapidly rotating single stars which indicate that such
  activity is generally confined to the polar regions of such stars,
  and we contrast that to rapidly rotating active components in binary
  systems where the active regions do not appear to be confined to
  the polar latitudes. Noting that solar activity is confined to lower
  latitudes and seems unable to affect polar latitudes, we suggest that
  extremely rapid rotation may ater the convective turnover time in the
  equatorial latitudes, and that strong tidal coupling may restore it to
  normal values in spite of rapid rotation. The evidence suggests that
  even though radiative power losses due to activity may be comparable in
  single and binary stars, the driving mechanisms may differ appreciably.

---------------------------------------------------------
Title: Stellar chromospheres, coronae, and winds.
Authors: Cassinelli, J. P.; MacGregor, K. B.
1986psun....3...47C    Altcode:
  Contents: Late-type stars: observational evidence for the presence of
  chromospheres in late-type stellar atmospheres (spectral diagnostics and
  line formation, observational summary and location in the H-R diagram,
  the Wilson-Bappu effect), observational evidence for the presence of
  regions and coronae in late-type stellar atmospheres, chromospheric and
  coronal heating mechanisms, observational evidence for mass loss from
  late-type stars (main sequence stars, circumstellar absorption lines,
  chromospheric emission line asymmetries, circumstellar dust shells),
  mass loss mechanisms for late-type giants and supergiants. The winds and
  coronae of early-type stars: the velocity and mass loss rates derived
  from line and continuum observations (the formation of P Cygni profiles,
  the free-free continuum energy distribution of hot stars), coronal gas
  in early-type stars (superionization of the winds, X-ray observation
  of early-type stars), wind dynamics (radiation forces on line opacity:
  momentum deposition considerations, radiative acceleration, instability
  of line driven winds and the consequences, hybrid models with a base
  coronal zone, magnetically driven winds and magnetically dominated
  coronae).

---------------------------------------------------------
Title: Evolution of activity signatures during the main sequence phase
Authors: Skumanich, A.; MacGregor, K.
1986AdSpR...6h.151S    Altcode: 1986AdSpR...6Q.151S
  We review recent work on the decay of magnetic activity signatures,
  such as chromospheric/transition region/coronal emission as well as mean
  flare emission, with age for solar and later type stars. Both implicit
  and explicit evolutionary representations are discussed. In terms of
  magnetic flux, as measured by excess chromospheric CaII luminosity,
  we show that a simple dynamo-rotation relation that incorporates both
  a saturated state with its characteristic critical rotation (or scale
  factor) as well as an asymptotic linear power law, i.e. a scale free
  relation, fits the extant data that includes the dMe stars. Introducing
  the saturated dynamo state, as exemplified by the dMe stars, into
  activity power-power diagrams, allows us to not only specify the
  saturated state, but to define evolutionary tracks that represent
  the decay from the saturated state. Using the quiescent coronal X-ray
  power (luminosity) as a basic measure of magnetic activity, we find
  simple monomial relations for both the saturated state (linear) and
  for the evolutionary tracks governing both quiescent activity and mean
  flare activity. In particular, the coronal power loss is found to vary
  quadratically with the chromospheric power loss, hence with magnetic
  flux. <P />NCAR is sponsored by the National Science Foundation.

---------------------------------------------------------
Title: The Development of Thermally Driven Winds Around Low Mass
    Protostars
Authors: Terebey, S.; MacGregor, K. B.
1985BAAS...17..862T    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Mass Loss Mechanisms for Cool, Low-Gravity Stars (Review)
Authors: Holzer, T. E.; MacGregor, K. B.
1985ASSL..117..229H    Altcode: 1985mlrg.proc..229H
  A number of physical effects relevant to mass loss from cool,
  low-gravity stars is explored in order to provide a basis for
  understanding the various mechanisms proposed to explain such
  quasi-steady and non-steady mass loss. After considering a few general
  implications of the requirements of mass, momentum, and energy balance,
  four specific mass loss mechanisms are examined in some detail. Emphasis
  is placed on the basic physical constraints imposed uniformly on all
  these mechanisms by inferences drawn from observations of a broad
  range of stars. It is suggested that these physical constraints,
  rather than the techniques used to observe particular objects should
  serve as the dominant factor in organizing thinking about massive,
  low speed, late type stellar winds.

---------------------------------------------------------
Title: Rotational Evolution of Hot Stars
Authors: MacGregor, K. B.; Friend, D. B.; Gilliland, R. L.
1984BAAS...16..996M    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The Extremely Rapidly Rotating M-Dwarf Star Gliese 890
Authors: Young, A.; Skumanich, A.; MacGregor, K.; Temple, S.
1984BAAS...16.1014Y    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The Role of Diamagnetic Material in the Solar Atmosphere
Authors: Cargill, P. J.; MacGregor, K. B.; Pneuman, G. W.
1984BAAS...16.1005C    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: A Non-Radial Radiation Force in Hot Star Winds
Authors: Friend, D. B.; MacGregor, K. B.
1984BAAS...16..899F    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Winds from rotating, magnetic, hot stars. I. General model
    results.
Authors: Friend, D. B.; MacGregor, K. B.
1984ApJ...282..591F    Altcode:
  It is generally thought that the winds of hot stars are driven by
  the force arising from the absorption and scattering of photospheric
  continuum radiation by ultraviolet resonance lines of abundant ions in
  the flow. However, puzzling discrepancies were found in a comparison
  of observational data with the predictions of theoretical models for
  line-driven mass loss. It is pointed out that part of the apparent
  disagreement between theory and observation might result from the
  existence of additional, nonradiative forces which contribute to the
  acceleration of hot star winds, but which are omitted from the model
  considered by Castor et al. (1975). The present investigation takes,
  therefore, into account the dynamics of winds which are predominantly
  radiation driven, but which are also acted upon by the forces due to
  rapid rotation in the presence of a stellar magnetic field.

---------------------------------------------------------
Title: Theory of winds in late-type evolved and pre-main-sequence
    stars.
Authors: MacGregor, K. B.
1983NASCP.2280.241M    Altcode: 1983sowi.conf..241M
  Recent observational results confirm that many of the physical processes
  which are known to occur in the Sun also occur among late-type stars
  in general. One such process is the continuous loss of mass from a
  star in the form of a wind. There now exists an abundance of either
  direct or circumstantial evidence which suggests that most (if not all)
  stars in the cool portion of the HR diagram possess winds. An attempt
  is made to assess the current state of theoretical understanding of
  mass loss from two distinctly different classes of late-type stars: the
  post-main-sequence giant/supergiant stars and the pre-main-sequence T
  Tauri stars. Toward this end, the observationally inferred properties
  of the wind associated with each of the two stellar classes under
  consideration are summarized and compared against the predictions of
  existing theoretical models. Although considerable progress has been
  made in attempting to identify the mechanisms responsible for mass
  loss from cool stars, many fundamental problems remain to be solved.

---------------------------------------------------------
Title: Winds from Rotating, Magnetic, Hot Stars
Authors: Friend, D. B.; MacGregor, K. B.
1983BAAS...15..970F    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Mass loss from rotating magnetic stars - Weber and Davis
    re-revisited
Authors: MacGregor, K. B.; Pizzo, V. J.
1983ApJ...267..340M    Altcode:
  The criticisms by Barker and Marlborough (BM, 1982) concerning the
  model of Weber and Davis (WD, 1967) for the flow of a magnetically
  coupled wind from a rotating star are critically examined and the
  assumptions inherent to both approaches are compared. It is shown
  that when differences in the respective definitions of the stellar
  angular velocity are reconciled, the extended theory proposed by BM
  is equivalent to the original description of WD. In addition, the
  conditions under which the azimuthal velocity of a WD-type solar wind
  initial decreases outward from the surface of the star are determined
  and it is shown that such behavior is in no way contingent upon the
  modifications proposed by BM.

---------------------------------------------------------
Title: Future Prospects for the Theory of Solar-Stellar Winds
Authors: MacGregor, K. B.
1982BAAS...14..946M    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Protostellar mass and angular momentum loss
Authors: Hartmann, L.; MacGregor, K. B.
1982ApJ...259..180H    Altcode:
  Recent radio observations have indicated that appreciable, continuing
  mass loss is occurring from regions of star formation. The conditions
  under which massive protostellar objects may exhibit strong,
  rotationally driven winds are investigated. For plausible rotational
  velocities and magnetic field strengths, ejection of about 10,000 solar
  masses a year at speeds of 10-100 km/s can be maintained for time scales
  of about 0.0001 yr; these values are insensitive to the protostellar
  luminosity. An evolutionary scenario is sketched which suggests that
  protostellar clouds may be expected to be rapidly rotating and to
  possess substantial magnetic fields when the clouds have contracted
  to radii of about 10 to the 14th cm. The centrifugally driven winds
  proposed present an alternative to fragmentation for providing the
  angular momentum loss required in the phase of rapid cloud contraction.

---------------------------------------------------------
Title: Stellar winds in binary X-ray systems.
Authors: MacGregor, K. B.; Vitello, P. A. J.
1982ApJ...259..267M    Altcode:
  It is thought that accretion from a strong stellar wind by a compact
  object may be responsible for the X-ray emission from binary systems
  containing a massive early-type primary. To investigate the effect
  of X-ray heating and ionization on the mass transfer process in
  systems of this type, an idealized model is constructed for the flow
  of a radiation-driven wind in the presence of an X-ray source of
  specified luminosity, L<SUB>x.</SUB> It is noted that for low values
  of L<SUB>x,</SUB> X-ray photoionization gives rise to additional ions
  having spectral lines with wavelengths situated near the peak of the
  primary continuum flux distribution. As a consequence, the radiation
  force acting on the gas increases in relation to its value in the
  absence of X-rays, and the wind is accelerated to higher velocities. As
  L<SUB>x</SUB> is increased, the degree of ionization of the wind
  increases, and the magnitude of the radiation force is diminished in
  comparison with the case in which L<SUB>x</SUB> = 0. This reduction
  leads at first to a decrease in the wind velocity and ultimately (for
  L<SUB>x</SUB> sufficiently large) to the termination of radiatively
  driven mass loss.

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Title: Wave-driven winds from cool stars. I - Some effects of magnetic
    field geometry
Authors: Hartmann, L.; MacGregor, K. B.
1982ApJ...257..264H    Altcode:
  The wave-driven wind theory of Hartmann and MacGregor (1980) is
  extended to include effects due to non-radial divergence of the
  flow. Specifically, isothermal expansion within a flow tube whose
  cross-sectional area increases outward faster than the square of
  the radius near the stellar surface is considered. It is found that
  the qualitative conclusions of Hartmann and MacGregor concerning
  the physical properties of Alfven wave-driven winds are largely
  unaffected. In particular, mass fluxes of similar magnitude are
  obtained, and wave dissipation is still necessary to produce
  acceptably small terminal velocities. Increasingly divergent flow
  geometries generally lead to higher initial wind speeds and slightly
  lower terminal velocities. For some cases of extremely rapid flow tube
  divergence, steady supersonic wind solutions which extend to infinity
  with vanishing gas pressure cannot be obtained. In addition, departures
  from spherical symmetry can cause the relative Alfven wave amplitude
  delta-B/B to become approximately greater than 1 within several stellar
  radii of the base of the wind, suggesting that nonlinear processes
  may contribute to the wave dissipation required by the theory.

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Title: Momentum and energy balance in late-type stellar winds.
Authors: MacGregor, K. B.
1982SAOSR.392A..83M    Altcode: 1982csss....2...83M
  Observations at ultraviolet and X-ray wavelengths indicate that
  the classical picture of a static stellar atmosphere containing a
  radiative equilibrium temperature distribution is inapplicable to the
  majority of late type stars. Mass loss and the presence of atmospheric
  regions characterized by gas temperatures in excess of the stellar
  effective temperature appear to be almost ubiquitous throughout the
  HR diagram. Evidence pertaining to the thermal and dynamical structure
  of the outer envelopes of cool stars is summarized. These results are
  compared with the predictions of several theoretical models which were
  proposed to account for mass loss from latetype stars. Models in which
  the outflow is thermally radiatively, or wave driven are considered
  for identification of the physical processes responsible for the
  observed wind properties. The observed variation of both the wind,
  thermal and dynamical structure as one proceeds from the supergiant
  branch toward the main sequence in the cool portion of the HR diagram
  give consideration to potential mechanisms for heating and cooling
  the flow from low gravity stars.

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Title: Protostellar Mass and Angular Momentum Loss
Authors: MacGregor, K. B.; Hartmann, L.
1981BAAS...13R.855M    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Momentum and energy deposition in late-type stellar atmospheres
    and winds.
Authors: Hartmann, L.; MacGregor, K. B.
1980ApJ...242..260H    Altcode:
  The present study calculates the response of the outer atmospheres
  of cool low-gravity stars to the passage of the mechanical energy
  fluxes of solar magnitude in the form of acoustic waves and Alfven
  waves. It is shown that Alfven waves are efficient in generating
  outflow, and can account for the order of magnitude of observed mass
  loss in late-type luminous stars. However, unless these magnetic waves
  undergo some dissipation within several stellar radii of the surface,
  the predicted terminal velocities of the resulting stellar winds are
  far too high. Alfven wave dissipation should give rise to extended
  warm chromospheres in low-gravity late-type stars, a prediction which
  can be observationally tested.

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Title: Radiative amplification of sound waves in the winds of O and
    B stars.
Authors: MacGregor, K. B.; Hartmann, L.; Raymond, J. C.
1979ApJ...231..514M    Altcode:
  The velocity perturbation associated with an outwardly propagating sound
  wave in a radiation-driven stellar wind gives rise to a periodic Doppler
  shifting of absorption lines formed in the flow. A linearized theory
  applicable to optically thin waves is used to show that the resulting
  fluctuation in the absorption-line force can cause the wave amplitude to
  grow. Detailed calculations of the acceleration due to a large number of
  lines indicate that significant amplification can occur throughout the
  high-velocity portion of winds in which the dominant force-producing
  lines have appreciable optical depths. In the particular case of the
  wind of Zeta Pup (O4f), it is found that the e-folding distance for
  wave growth is considerably shorter than the scale lengths over which
  the physical properties of the flow vary. A qualitative estimate of
  the rate at which mechanical energy due to nonlinear waves can be
  dissipated suggests that this mechanism may be important in heating
  the supersonic portion of winds of early-type stars.

---------------------------------------------------------
Title: Model Winds and Chromospheres for Late-Type Giants and
    Supergiants
Authors: MacGregor, K. B.; Hartmann, L.
1979BAAS...11..448M    Altcode:
  No abstract at ADS

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Title: Radiation-Driven Winds in Binary X-Ray Source Systems
Authors: MacGregor, K.; Vitello, P. A. J.
1979xras.proc..267M    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Radiation-driven winds in binary X-ray source systems
Authors: MacGregor, K.; Vitello, P. A. J.
1978cosp.meetS....M    Altcode:
  The hydrodynamics of line-driven stellar winds in binary systems
  containing a weak X-ray source is investigated. The gravitational,
  ionization, and heating effects due to the secondary on the acceleration
  of the wind are taken into account. It is found that the changes in
  the acceleration which result from the presence of even a weak X-ray
  source can profoundly alter the wind velocity profile.

---------------------------------------------------------
Title: Coronal Regions in the Winds of Early-type Stars.
Authors: MacGregor, K. B.
1978BAAS...10..412M    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The Effects of Centrifugal, Magnetic, and Radiative Forces
    on the Dynamics of Stellar Wind Flow.
Authors: MacGregor, Keith Bruce
1977PhDT.......129M    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Magnetic acceleration of winds from solar-type stars.
Authors: Belcher, J. W.; MacGregor, K. B.
1976ApJ...210..498B    Altcode:
  The spin-down of solar type stars (F5 V to G3 V) is generally ascribed
  to the outflow of magnetized plasma in the form of a wind. Magnetically
  coupled stellar winds are thought to provide the dominant mechanism for
  angular momentum loss over the entire main-sequence lifetime of stars
  possessing hydrogen convective zones. The associated loss in rotational
  kinetic energy can strongly affect the energetics of winds emanating
  from such stars, for sufficiently high rotation rates and magnetic
  field strengths. In the present paper, an attempt is made to describe
  qualitatively how MHD plasma outflow from a rotating star adjusts itself
  to a broad range of stellar conditions, including fast, intermediate,
  and slow magnetic rotator configurations. Using the Weber and Davis
  (1967) model of MHD winds, it is shown that the magnetic deceleration
  of an MHD wind is of importance when the loss of rotational kinetic
  energy due to magnetic braking exceeds the energy flux due to thermal
  processes alone.

---------------------------------------------------------
Title: Magnetic Acceleration of Winds from Solar-Type Stars
Authors: Belcher, J. W.; MacGregor, K. B.
1976BAAS....8..307B    Altcode:
  No abstract at ADS