Author name code: crivellari ADS astronomy entries on 2022-09-14 author:"Crivellari, Lucio" ------------------------------------------------------------------------ Title: AGB Stars and Their Circumstellar Envelopes: An Operative Approach to Computing Their Atmospheres Authors: Crivellari, Lucio; Cristallo, Sergio; Piersanti, Luciano Bibcode: 2021Univ....7..340C Altcode: No abstract at ADS Title: Alternative Strategies to Solve the Stellar Atmosphere Problem Authors: Crivellari, L. Bibcode: 2021POBeo.100...15C Altcode: At the heart of the computation of model atmospheres there is the so-called Stellar Atmosphere Problem, which consists of the self-consistent solution of the radiative transfer equations under specific constraints. The amazing progresses achieved in the field since the 1970s are due to both the dramatic increase of the computational facilities and the development of effective numerical algorithms. The purpose of this review is to draw attention to some methods, alternative to those that are mostly used nowadays such as the ALI methods. The improvement of the latter has been brought about by mathematical refinement, whereas the former are the result of a careful analysis of the physics of the problem. Rather than attempting an exhaustive presentation of these novel methods, which would be out of place here, the prime aim of this article is to sketch the main guidelines and to stress that it is always the physics itself that dictated the most effective algorithm. Title: AGB Stars and Their Circumstellar Envelopes. I. the VULCAN Code Authors: Cristallo, Sergio; Piersanti, Luciano; Gobrecht, David; Crivellari, Lucio; Nanni, Ambra Bibcode: 2021Univ....7...80C Altcode: No abstract at ADS Title: Radiative Transfer in Stellar and Planetary Atmospheres Authors: Crivellari, Lucio; Simón-Díaz, Sergio; Arévalo, María Jesús Bibcode: 2019rtsp.book.....C Altcode: No abstract at ADS Title: The Stellar Atmosphere Physical System II. An Operative Sequential Algorithm to Solve the Stellar Atmosphere Problem Authors: Crivellari, L. Bibcode: 2019SerAJ.198....1C Altcode: 2019SerAJ.tmp....1C In this paper, the second and the last of the series, we present a sequential algorithm to solve the stellar atmosphere problem that may serve as a paradigm for the solution of more general non-linear and non-local problems. The Iteration Factors (IF) Method is applied to achieve a solution of the radiative transfer equations, consistent with the radiative equilibrium constraint. Title: The Stellar Atmosphere Physical System I. Phenomenological Definition and Representation of a Stellar Atmosphere Authors: Crivellari, Lucio Bibcode: 2018SerAJ.196....1C Altcode: This paper is the first in a series of two that deals with the physical and numerical grounds of stellar atmospheres modelling. After a phenomenological definition of a star and a stellar atmosphere, the physics that shapes the stellar atmosphere physical system is discussed and three alternative pictures are considered for its representation. Title: An improved version of the implicit integral method to solving radiative transfer problems Authors: Simonneau, E.; Cardona, O.; Crivellari, L. Bibcode: 2012Ap.....55..110S Altcode: 2012Ap....tmp...12S; 2011arXiv1110.2018S Radiative transfer (RT) problems in which the source function includes a scattering-like integral are typical two-points boundary problems. Their solution via differential equations implies making hypotheses on the solution itself, namely the specific intensity I (τ; n) of the radiation field. On the contrary, integral methods require making hypotheses on the source function S(τ). It seems of course more reasonable to make hypotheses on the latter because one can expect that the run of S(τ) with depth is smoother than that of I (τ; n). In previous works we assumed a piecewise parabolic approximation for the source function, which warrants the continuity of S(τ) and its first derivative at each depth point. Here we impose the continuity of the second derivative S''(τ). In other words, we adopt a cubic spline representation to the source function, which highly stabilizes the numerical processes. Title: Thermodynamic Coefficients for Stellar Atmospheres and Plasma Spectroscopy Authors: Cardona, O.; Simonneau, E.; Crivellari, L. Bibcode: 2009ApJ...695..855C Altcode: A new method to obtain the thermodynamic coefficients in an analytic and exact form for applications in radiative and convective transport in thermodynamic equilibrium for stellar atmosphere and plasma spectroscopy is developed. The resulting exact expressions are formed by sums of the degrees of ionization of the components of the system. Therefore, they are easy to calculate and are numerically stable. The method is developed initially for two elements: hydrogen and helium, with constant partition functions, to show the simplicity of the procedure. The method is very easy to generalize to any number of elements and for partition functions dependent on temperature and pressure as well as for the first negative ions. The thermodynamic coefficients derived are the adiabatic gradient, the specific heats for constant volume and constant pressure, the dilatation coefficient, and the velocity of sound in the given medium. The derivation is based on the perfect gas and Saha ionization equations. Therefore, the results are valid for the regime where these equations are valid. These results will be of value to astrophysics in stellar structure and atmospheres, in geophysics, in planetary atmospheres, and in plasma physics in spectroscopic analysis and diagnostics. Title: A Novel Precise Method for Correcting the Temperature in Stellar Atmosphere Models Authors: Cardona, O.; Crivellari, L.; Simonneau, E. Bibcode: 2009ASSP....7..231C Altcode: 2009nqsa.conf..231C A mayor problem that arises in the computation of stellar atmosphere models is the self consistent determination of the temperature distribution via the constraint of energy conservation. The energy balance includes the gains due to the absorption of radiation: int χ(ν) J(ν) dν, and the losses due to emission: int χ(ν) S(ν) dν . It is well known that, within each one of the two above integrals, the part corresponding to spectral ranges whose opacity χ(ν) is huge can overcome by many orders of magnitude the part that corresponds to the remaining frequencies. On the other hand, at those frequencies where χ(ν) is very large, the mean intensity J(ν) of the radiation field shall be equal, up to many significant digits, to the source function S(ν) and consequently to the Planck function B(ν,T). Then their net share to the energy balance shall be null, albeit separately their contributions to the gain and loss integrals are the most important numerically. Thus, the spectral range whose physical contribution to the overall balance is null will dominate numerically both sides of the energy balance equation, and consequently the errors on the determination of J(ν) and S(ν) at these frequencies will falsify the balance. It is possible to circumvent the numerical problem brought about by the foregoing circumstances by solving the radiative transfer equation for the variable I(n,ν) - S(ν), instead of the customary intensity I(n,ν). We present here a novel iterative algorithm, based on iteration factors already employed by us with success, which makes it possible a fast correction of the temperature by computing directly the difference between the radiative losses and gains at each step of the iterations. Title: Method for Calculating the Opacity of the Atomic Lines in Stellar Atmospheres Authors: Cardona, O.; Simonneau, E.; Crivellari, L. Bibcode: 2009ApJ...690.1378C Altcode: A new approximate method for calculating the opacity of the atomic lines in the computation of stellar model atmospheres is presented. Transforming the sums in the equation of the opacity of the lines into integrals and considering the Kramers equation for the oscillator strengths for level transitions in the hydrogenic atoms as a continuous function of frequency, we can apply the mean value theorem for integrals in order to describe the variation of the global opacity of the lines of all the chemical elements. The high-lying levels in nonhydrogenic atoms as well as their hydrogen-like transitions can also benefit from this method. For the case of stellar atmospheres not in local thermodynamic equilibrium, we can apply our method directly. The compact analytic expressions obtained are easy to use. The application of the method to the hydrogen lines shows the classical line-blanketing effects. Title: A Novel Precise Method for Correcting the Temperature in Stellar Atmosphere Models Authors: Cardona, Octavio; Crivellari, Lucio; Simonneau, Eduardo Bibcode: 2007arXiv0711.0026C Altcode: A mayor problem that arises in the computation of stellar atmosphere models is the self consistent determination of the temperature distribution via the constraint of energy conservation. The energy balance includes the gains due to the absorption of radiation and the losses due to emission. It is well known that within each one of the two above integrals the part corresponding to spectral ranges whose opacity X(nu) is huge can overcome by many orders of magnitude the part that corresponds to the remaining frequencies. On the other hand, at those frequencies where X(nu) is very large, the mean intensity J(nu) of the radiation field shall be equal, up to many significant digits, to the source function S(nu) and consequently to the Planck function B(nu,T). Then their net share to the energy balance shall be null, albeit separately their contributions to the gain and loss integrals are the most important numerically. Thus the spectral range whose physical contribution to the overall balance is null will dominate numerically both sides of the energy balance equation, and consequently the errors on the determination of J(nu) and S(nu) at these frequencies will falsify the balance. It is possible to circumvent the numerical problem brought about by the foregoing circumstances by solving the radiative transfer equation for the variable I(n,nu) - S(nu), instead of the customary intensity I(n,nu). We present here a novel iterative algorithm, based on iteration factors already employed by us with success, which makes it possible a fast correction of the temperature by computing directly the difference between the radiative losses and gains at each step of the iterations. Title: Physical requirements for modeling stellar atmospheres according to the different spectral features observed Authors: Crivellari, L.; Cardona, O.; Simonneau, E. Bibcode: 2007IAUS..241...91C Altcode: No abstract at ADS Title: Método integral implícito para resolver problemas de transporte radiativo en condiciones típicas de las atmósferas estelares Title: Método integral implícito para resolver problemas de transporte radiativo en condiciones típicas de las atmósferas estelares Title: An implicit integral method to solve radiative transfer problems under conditions typcal of stellar atmospheres; Authors: Crivellari, Lucio Bibcode: 2004PhDT.......297C Altcode: No abstract at ADS Title: Structure and components of an LTE stellar atmosphere model Authors: Crivellari, L.; Simonneau, E.; Cardona, O. Bibcode: 2004MmSAI..75..154C Altcode: We summarize in this paper the principles for the algorithmic representation of the structure of a stellar atmosphere. The physical analysis of the problem leads to the classification of the elementary blocks that constitute a stellar atmosphere model and suggest the numerical algorithms required for its computation. Compatibly with the space allotted, we review our progress in the treatment of the basic components of an LTE stellar atmosphere model, and show a significant preliminary result. Title: A numerical laboratory for the diagnostics of stellar properties Authors: Crivellari, L.; Simonneau, E.; Cardona, O. Bibcode: 2003MSAIS...3...97C Altcode: In a recent paper (Crivellari et al., 2003) we summarized the principles of the algorithmic representation of the structure of a stellar atmosphere. We mentioned there our recent progresses in the numerical treatment of both the global method and the basic components of the corresponding models. As the natural sequel to that paper, we wish to outline here the iterative sequential procedure that we have designed for the computation of such models, and briefly comment about the possibilities that it offers. Title: Multilevel Line Transfer with the Implicit Integral Method Authors: Crivellari, L.; Cardona, O.; Simonneau, E. Bibcode: 2002Ap.....45..480C Altcode: Once the need for an iterative procedure in order to solve the problem of the formation of spectral lines in the case of a model atom with many energy levels, the sequel is to seek for the most effective form of such an iterative scheme. It is an almost universal is assumed within all the iterative methods for the solution of those radiative transfer problems, in which the transfer equations are coupled to the state of the matter, to take as the input of each step of iterations the values of the opacity coefficients obtained as a result of the previous one. This is, for instance, the procedure used to correct the temperature in the computation of stellar atmosphere models, or that to build the -operator (either the exact or the approximated one) within the Accelerated Lambda Iteration methods. Yet, if we assume, in order to solve the multilevel line transfer problem, that at each step of iterations the line opacities are known, we can express via the statistical equilibrium equations the populations of the energy levels - and consequently the source functions of the relevant spectral lines - as a linear function of the full set of the corresponding mean intensities of the radiation field. Once such linear forms for the source functions, we are able to solve without the need of any further approximation the radiative transfer equations for are obtained lines, now linearly coupled through the above linear forms of the statistical equilibrium equations. This is achieved by means of the Implicit Integral Method that we already presented in a series of previous papers. Title: Some Iterative Methods for Radiative Transfer Problems Authors: Simonneau, E.; Crivellari, L. Bibcode: 2002EAS.....5...31S Altcode: In the mathematical computation of many physical problems we have to work with the numerical solution of non-linear problems. Even in case that a direct solution does in principle exist, its feasibility is however actually restricted to a very limited number of instances. Therefore, in this study we will disregard this possibility. Consequently, we assume that an iterative procedure shall be necessary in order to tackle, in practice, all kind of non-linear problems. Title: A precise new method to correcting temperature in stellar atmospheres Authors: Cardona, O.; Crivellari, L.; Simonnneau, E. Bibcode: 2002ASSL..274...29C Altcode: Calculations of energy balance in model stellar atmospheres for cold and moderately cold stars show that the overall energy gained or lost at the Lyman frequencies (lines plus continuum) can be up to 20 orders of magnitude higher than in the rest of the spectrum. Due to the great optical depth at those frequencies, however, monochromatic mean intensity of the radiation and the corresponding source function are numerically equal with many significant figures, and give therefore a vanishing net contribution to the energy balance. As all methods of temperature correction compute both integrals independently, this can lead to a biased temperature correction to the model output. We present here a method to calculate the correction of the temperature in stellar atmospheres that overcomes the aforesaid difficulty. Title: On the temperature determination in theoretical modelling of stellar atmospheres Authors: Giammanco, C.; Crivellari, L.; Caccin, B. Bibcode: 1998MmSAI..69..663G Altcode: No abstract at ADS Title: Angular resolution of the classical redistribution functions Authors: Crivellari, L. Bibcode: 1998MmSAI..69..643C Altcode: No abstract at ADS Title: An Implicit Integral Method to Solve Selected Radiative Transfer Problems. IV. The Case of Spherical Geometry Authors: Gros, M.; Crivellari, L.; Simonneau, E. Bibcode: 1997ApJ...489..331G Altcode: In the previous papers of this series, we introduced the implicit integral method (IIM) to solve those radiative transfer (RT) problems in which the source function depends on an integral of the specific intensity of the radiation field over directions and frequencies. The IIM rests upon a forward-elimination, back-substitution scheme naturally based on the physics of the RT process, and does not require any matricial algorithm.

Customary methods to solve RT problems, in which the source function depends on the aforesaid integral, rest upon matrix algorithms. In spherical geometry, due to the strong anisotropy of the radiation field brought about by the limb curvature, the so-called peaking effect, the number of directions necessary to describe this anisotropy is exceedingly high, and consequently the relevant matrices are hard to handle.

The present paper deals with the application of the IIM to RT problems in spherical geometry, where the distinctive nonmatricial character of the method can be fully exploited, given the intrinsic high dimensionality of the problem. Title: A Forth-and-Back Implicit Λ-Iteration Authors: Atanacković-Vukmanović, O.; Crivellari, L.; Simonneau, E. Bibcode: 1997ApJ...487..735A Altcode: In many radiative transfer (RT) problems, the sources contain a scattering term that couples all the specific RT equations, one for each frequency and direction, so that solving the problem means solving the system formed by these equations. Each of them is a first-order linear differential equation with its own initial condition assigned at a different point of the medium, which makes the solution of the system extraordinarily difficult.

One simple way to achieve a solution is with the so-called Λ-iteration: sources and sinks given as a first approximation --> computation of the specific intensities from their own RT equations --> computation of the scattering terms --> recomputation of the sources and sinks. This scheme is straightforward, but unfortunately in practice its convergence rate is too slow to be of value in the case of optically thick systems.

The aim of this paper is to show that a forth-and-back approach (the natural approach to describing sequentially the two intensities propagating along the two directions of a straight line), together with an implicit representation of the source function in the computation of the intensities within the above iterative scheme, can dramatically accelerate the convergence of the iterative process while retaining the straightforwardness of ordinary Λ-iteration. Title: Iteration Profiles in Radiative Transfer Problems. I. From Vectorial to Scalar Coupling Authors: Crivellari, L.; Simonneau, E. Bibcode: 1995ApJ...451..328C Altcode: We have recently introduced a new algorithm, the implicit integral method (IIM), for solving radiative transfer problems in which the specific source functions (for each frequency and direction) depend linearly on the radiation field via a single quantity which is independent of both frequency and direction. We define this kind of relationship as scalar coupling. The fact that our method turned out to be fast, robust, and highly reliable leads us to seek its extension to include those problems where the above, necessary condition is not fulfilled. In these problems, the specific source functions depend on the radiation field through a nonfactorable redistribution operator. In our definition, these are cases of vectorial coupling.

In this paper we present the successful application of the IIM, through an iterative procedure, to two specific instances of vectorial coupling. The first is the determination of the temperature distribution, self-consistent with the energy conservation constraint, within a LTE stellar atmosphere model. Here the physical processes other than radiative transfer require an iterative procedure for the global solution of the problem. Thus we take advantage of this circumstance to solve iteratively the radiative transfer part as well.

The second is the case of the non-LTE two-level-atom line formation problem in which partial redistribution is taken into account in the presence of a background continuum. This problem allows a direct solution, but at the cost of using algorithms that necessarily require the storage and inversion of very high order matrices. On the contrary, we show that a solution based on the iterative application of the IIM, thanks to the outstanding features of the latter, is not only fast, but above all much more reliable in numerical terms. Title: An Implicit Integral Method to Solve Selected Radiative Transfer Problems. II. LTE Stellar Atmosphere Models Authors: Crivellari, L.; Simonneau, E. Bibcode: 1994ApJ...429..331C Altcode: The implicit intergral method that we have already introduced in a previous paper, together with its application to the non-Local Thermal Equilibrium (non-LTE) line formation problem, is considered here for another paradigm problem, namely, the self-consistent temperature correction when computing Local Thermal Equilibrium (LTE) stellar atmosphere models. The specific form of the source function, required by the algorithm, is obtained through a linear expansion of the Planck function around a trial temperature distribution. The correct solution is quickly obtained via an iterative correction procedure. The nonmatricial structure, which is the distinctive feature of the implicit intergral method, offers the possibility to take into account a very large number of frequency, angle, and depth discrete points, without any penalty in terms of numerical or computational problems. Title: An Implicit Integral Method to Solve Selected Radiative Transfer Problems. I. Non-LTE Line Formation Authors: Simonneau, E.; Crivellari, L. Bibcode: 1993ApJ...409..830S Altcode: In this paper we present a new method to solve those radiative transfer problems where the scattering term in the source function, i.e., the frequency-integrated mean intensity J(phi), is independent of both frequencies and directions. This particular form of the source function, together with an implicit description of the evolution of the specific intensities incoming to an individual layer from the neighboring ones, allows one to solve implicitly the radiative transfer equation layer by layer. Consequently, J(phi) can be expressed as an explicit function of the (as yet unknown) specific intensities without any need to solve numerically a system of equations or to invert matrices. In this way, the global problem is reduced to a series of one-layer two-point boundary problems. The resulting algorithm is the representation of the actual physical process. This, together with the fact that it does not require a matricial formalism, brings about self-evident advantages in terms of reliability and numerical accuracy, as well as computational time and memory storage. As an application, the instance of the spectral line formation in a two-level atomic model is considered. Title: Structural algorithms to solve radiative transfer problems Authors: Simonneau, Eduardo; Crivellari, Lucio Bibcode: 1993sats.book.....S Altcode: No abstract at ADS Title: Chromospheres of late-type active and quiscent dwarfs. III. Variability of CA II H emission profiles. Authors: Garcia Lopez, R. J.; Crivellari, L.; Beckman, J. E.; Rebolo, R. Bibcode: 1992A&A...262..195G Altcode: We have used high-resolution spectra of the Ca II H resonance line in late-type dwarfs, obtained with high S:N ratios, over a period of four years to widen our understanding of the dynamical behavior of the Ca II emission cores. All of the stars dealt with in this article, which are chromospherically active, show variability both in core emission flux and line width. They also show significant wavelength shifts with time of order hundreds of meters per second in the mean core wavelength, and with lower amplitude in the H3 self-absorption, compared to the photospheric rest wavelength of Ca II H. Comparing the emission core shifts with those observed in the H3 features, we find, for the first time, direct prima facie evidence for vertical chromospheric velocity fields, which show stability in sense over periods of years in a given star, with notable modulation in gradient, and which differ in gradient from star to star. We present evidence to show that the observed effects are almost certainly not due to projected rotational modulation, and offer new prospects, given spectral measurements closely sampled in time, for investigating the vertical velocity structures of chromospheres. Title: Book-Review - Stellar Atmospheres Beyond Classical Models Authors: Crivellari, L.; Hubeny, I.; Hummer, D. G. Bibcode: 1991JBAA..101..364C Altcode: No abstract at ADS Title: Red-shifted chromospheric emission in 70 OPH A Authors: Beckman, J. E.; Garcia Lopez, R. J.; Rebolo, R.; Crivellari, L. Bibcode: 1991Obs...111..314B Altcode: No abstract at ADS Title: The Use of Iteration Factors for Temperature Correction in a Stellar Atmosphere When Convective Transport Is Present Authors: Crivellari, L.; Simonneau, E. Bibcode: 1991ApJ...367..612C Altcode: In a previous work (Paper I of this series), we presented a procedure for computing the temperature distribution in a stellar atmosphere under the constraint of radiative equilibrium. This scheme, which answers the major requirements of simplicity and computational speed, is based on the choice of proper iteration factors that allow a quick convergence to the correct solution.

In this paper, we extended the method to accounting for a stellar atmosphere where convective transport is present. This is achieved by introducing a new iteration factor: the ratio of convective to radiative flux. The new iteration factor is introduced into a new loop of iterations, where the current temperature is doubly corrected by making use of both the convective and the radiative transport equations.

Thanks to the use of the radiative iteration factors, as defined in Paper I, also inside the new iteration loop, the computational time of each single iteration is kept small. The introduction of the new factor reduces the number of iterations within the loop to very small proportions. Title: Stellar atmospheres. Beyond classical models Authors: Crivellari, Lucio; Hubeny, I.; Hummer, D. G. Bibcode: 1991ASIC..341.....C Altcode: 1991QB809.N38......; 1991sabc.conf.....C No abstract at ADS Title: Ca II H high-resolution spectral monitoring of active late-type dwarfs Authors: Beckman, J. E.; Garcia López, R. J.; Rebolo, R.; Crivellari, L. Bibcode: 1991LNP...380..463B Altcode: 1991sacs.coll..463B; 1991IAUCo.130..463B We have monitored Ca II H with a spectral resolution of 8 x 104, in a sample of late-type MS stars over a four-year period. The high resolution enables us to add information on velocity fields to the usual flux monitoring. We detect changes in wavelength of different parts of the Ca II H feature, which can be interpreted as velocity fields in the lower chromosphere, with downflow and upflow of order 0.5 km s-1, depending on the star. Flux variations in Ca II H emission can be ascribed, via velocity tagging, to long-term change in plage cover rather than short-term modulation by (incompletely sampled) rotation cycles. Title: Chromospheric variability in late-type dwarfs from high-resolution spectra of Ca ii H Authors: García López, R. J.; Rebolo, R.; Crivellari, L.; Beckman, J. E. Bibcode: 1990Ap&SS.170..109G Altcode: We present an analysis of high-resolution (λ/δλ≈8×104) high signal-to-noise-ratio spectra of late-type chromospherically active dwarfs in Ca ii H (λ3968.47 Å), obtained with the Coudé Auxiliary Telescope plus the Coudé Echelle Spectrometer at ESO, La Silla. Observational parameters employed are the total emission flux attributable to the chromosphere, the asymmetry of the emission component and its bisector, the ratio of the intensities of the blue and red H2 emission peaks, and the absolute wavelengths of the H3 minimum and the mean emission feature. Given time-spaced sets of spectra for each object, which are as yet relatively sparse, we explore the potential use of these observational parameters to deepen our understanding of the chromospheric activity. Title: Diagnostics of chromospheric velocity fields Authors: Crivellari, Lucio Bibcode: 1990MmSAI..61..527C Altcode: This paper examines the possibility of recovering quantitative information on chromospheric velocity fields from high resolution (lambda/Delta lambda of about 100,000) spectrograms of the Ca II H and K resonance lines. A qualitative discussion of the observed features is given in terms of a schematic theory of line formation in the case of collisionally dominated resonance lines. Title: Chromospheres of late-type active and quiescent dwarfs. I. an atlas ofhigh resolution CA II H profiles. Authors: Rebolo, R.; Garcia Lopez, R.; Beckman, J. E.; Vladilo, G.; Foing, B. H.; Crivellari, L. Bibcode: 1989A&AS...80..135R Altcode: As a result of a major program of spectral observations using the CAT 1.4 m telescope and Coude Echelle Spectrograph at ESO, La Silla, 68 high resolution profiles of the 3968.5 A Ca H resonance line have been obtained for 16 late type stars (dwarfs and subgiants, including the sun) ranging from F9 to K5, selected to be representative of a wide span of chromospheric activity. The methods, and results of a set of reduction and analysis techniques applied to these spectra are presented. Each wavelength scale is converted to a frame at rest with respect to the photosphere of the star, and the flux scale is finally transformed into units of absolute flux. A major effort was applied to quantify the effects of the tilted baselines of the recorded spectra on these absolute flux calibrations. The calibrated profiles are used to extract and tabulate measured chromospheric H emission fluxes and the velocity shifts with respect to the photosphere of the emission cores and of the H3 self-reversals. Title: Chromospheres of late-type active and quiescent dwarfs. II. an activity index derived from profiles of the CA II lambda 8498 A and lambda 8542 A triplet lines. Authors: Foing, B. H.; Crivellari, L.; Vladilo, G.; Rebolo, R.; Beckman, J. E. Bibcode: 1989A&AS...80..189F Altcode: High resolution, high signal to noise spectral profiles of the 8498 and 8542 A lines of the Ca II infrared triplet (IRT) have been obtained for a sample of late type dwarfs, with a wide range of levels of chromospheric activity, in the spectral range F9 to K4/5. These profiles, and the difference profiles for selected active and quiescent objects of corresponding spectral type are presented. After absolute calibration in flux, advantage is taken of the high resolution and S/N ratios of the spectra to define several IRT indices of chromospheric activity, for a set of bandwidths between 0.2 A and 1 A about the line centers. Excellent correlations for active stars between the IRT and the Ca H indices and between the respective line widths are shown, which pertain despite the different depths of formation of the lines in the stellar chromospheres. Title: A spectroscopic analysis of the G8V star tau Cet. Authors: Arribas, S.; Crivellari, L. Bibcode: 1989A&A...210..211A Altcode: The physical parameters of Tau Cet are analyzed based on new and high-quality data. The effective temperature is determined to be 5250 + or - 50 K. The discrepancy of the abundance inferred from the Fe I lines with the lowest excitation cannot be explained by errors either in atomic or stellar parameters and implies the need for models incorporating non-LTE and/or a more reliable UV opacity. The obtained abundances for the alpha-particle elements exceed that of the Fe group by 0.2 dex, in complete agreement with previous results for metal-poor dwarfs of the Galactic disk population. The microturbulence value is found to be 1.1 + or - 0.1 km/s. The surface gravity value is determined to be 1.0 + or - 0.3 solar mass. Title: Abundances of light metals and NI in a sample of disc stars. Authors: Abia, C.; Rebolo, R.; Beckman, J. E.; Crivellari, L. Bibcode: 1988A&A...206..100A Altcode: High resolution, high S/N spectra obtained using RETICON and CCD detectors are used to determine the abundances of Al, Si, Ca, Ni and Fe in 23 disc stars in the metallicity range -1.2 ⪉ [Fe/H] ⪉ +0.3. The authors have found overabundances, with respect to iron for Al (≡0.4), Si (≡0.3), and Ca (≡0.2) in stars between -1.2 ⪉ [Fe/H] ⪉ -0.5. The results for Ca and Si (α-elements) are in excellent agreement with previous observational studies. The Al overabundances reported here show somewhat larger values close to [Fe/H] = -1.0 than those previously found in the literature. The abundance of Ni seems to follow that of Fe over the whole metallicity range studied. Title: On the Application of Iteration Factors for Temperature Correction in Stellar Atmospheres Authors: Simonneau, E.; Crivellari, L. Bibcode: 1988ApJ...330..415S Altcode: The authors present an improved iterative method for deriving temperature corrections in stellar atmospheres. At each step of iteration, the procedure computes, from the current values of the radiation field, a set of depth-dependent factors, averaged over angles and frequencies. Even a crude initial estimate of the radiation field leads to a fairly accurate determination of these factors on the first iteration: they are quasi-invariant in subsequent iterations. For a range of effective temperatures the procedure quickly converges to the correct solution of the radiative transfer equation. At each iteration, the temperature distribution is directly derived via the energy conservation constraint. Title: Improved Data Reduction Techniques for the ESO CES Plus RETICON Spectra Authors: Foing, B. H.; Crivellari, L. Bibcode: 1988IAUS..132..355F Altcode: The authors developed routines for intensity, equivalent width, and radial velocity measurements on CES plus Reticon spectra obtained with the ESO 1.4 m telescope. In order to achieve the optimal recovery of the signal, the noise has to be minimized by removing any parasitical effect. Special care has been devoted to the correction for remanence effects in the Reticon dark counts. Typical results are presented and discussed. Title: A Method for Calibrating, in Absolute Flux Units, CA II H Profiles of Late Type Stars Observed at ESO Authors: Castelli, F.; Gouttebroze, P.; Beckman, J.; Crivellari, L.; Foing, B. Bibcode: 1988IAUS..132..153C Altcode: The authors have applied to the Sun a method for calibrating, in absolute flux units, Ca II H profiles of late-type stars. After comparing, in the region 3948 - 3882 Å, an LTE synthetic spectrum with the data of the solar flux atlas by Kurucz et al. (1984), they have defined the wavelength ranges where observations agree with computations, based on specific radiative equilibrium models and collisional broadening parameters. By fitting in these regions the spectrum of the moon observed at ESO with the corresponding synthetic spectrum, the authors derived a calibration factor that enables them to calibrate, in absolute flux units, the whole observed range. Title: Abundances of Light Metals in Field Stars with Metallicity Range - 1.2< [Fe/H]< + 0.3 Authors: Abia, C.; Rebolo, R.; Beckman, J. E.; Crivellari, L.; Vila, B. Bibcode: 1988IAUS..132..421A Altcode: High resolution, high S:N spectra are used to determine the abundances of Fe, Ni, Ca, Al and Si in 25 field dwarfs with -1.2 < [Fe/H] < +0.3. The authors find overabundances for Al, Ca and Si in stars with -1.2 < [Fe/H] < -0.5 and solar [Ni/Fe] over the whole studied range. Title: Search for Circumstellar Envelopes Around Late-Type Binary Systems with Lism-Free MGII Emission Lines Authors: Crivellari, L. Bibcode: 1988iue..prop.3271C Altcode: No abstract at ADS Title: Chromospheric MG II H and K emissions free of interstellar contamination : velocity structure in late-type dwarfs and giants. Authors: Vladilo, G.; Molaro, P.; Crivellari, L.; Foing, B. H.; Beckman, J. E.; Genova, R. Bibcode: 1987A&A...185..233V Altcode: The authors have used high resolution IUE spectra from their own studies and from the archive to examine the Mg II h and k chromospheric emission cores of a sample of late-type dwarfs and giants. Sharp photospheric absorptions were used to provide a velocity rest-frame with respect to each stellar photosphere with the IUE-limited precision of ±4 km s-1. The knowledge of the kinematics of the local interstellar medium (LISM) could then be used to identify cases where either the cores or the wings, or in best circumstances both features of the chromospheric lines were uncontaminated by LISM absorption. The authors derive, using only LISM-free emission wings, accurate Wilson-Bappu relations for both the h and k line, characterized by a slope higher than in previous determinations. Title: Detections of diffuse interstellar bands toward the SN 1987A in the Large Magellanic Cloud. Authors: Vladilo, G.; Crivellari, L.; Molaro, P.; Beckman, J. E. Bibcode: 1987A&A...182L..59V Altcode: The authors report the results of high resolution and high S/N observations of diffuse interstellar bands (DIBs) towards SN 1987A in the LMC. For the first time they clearly detect the LMC component of the 5780, 5797 and 6284 Å DIBs, and possibly that of 5778 and 6269 Å. Moreover, they observe the Galactic components of the 5778, 5780, 5797, 6283, 6376 and 6379 Å bands, along a line of sight with very low reddening [E(B-V) ≅ 0.07]. The LMC components are generally weaker than the Galactic ones, while the opposite is observed for ISNa I and K I. The band strengths are compared with the total color excess towards the supernova. Title: CA II H emission line cores of late-type dwarfs : variability measurements and velocity field diagnostics. Authors: Crivellari, L.; Beckman, J. E.; Foing, B. H.; Vladilo, G. Bibcode: 1987A&A...174..127C Altcode: High-resolution spectra of Alpha Cen B, Xi Boo A, 70 Oph A, and Epsilon Ind obtained using the Coude Echelle Spectrometer and the 1.4-m Coude Auxiliary Telescope of ESO are analyzed. Variations in the normalized fluxes for the four dwarfs are described. Position and intensity measurements of the Ca II H features are examined. The use of the spectra for chromospheric velocity field diagnostics is discussed. The data reveal that it is possible to separate the effects of plages from those due to vertical velocity fields; in some stars the H core samples net upflow and in other net downflow; the layer sampled by the H3 feature shows a velocity close to that defined by the photospheric rest frame; and the H2V/H2R asymmetry is useful as a linear measure of the chromospheric vertical velocity gradient. Title: Search for Circumstellar Envelopes Around Late Type Binary Systems with LISM-Free Mg II Emission Lines Authors: Crivellari, L. Bibcode: 1987iue..prop.2952C Altcode: No abstract at ADS Title: High Resolution Profiles of the Ca II Infrared Triplet Lines in Late Type Active and Quiescent Dwarfs Authors: Foing, B. H.; Crivellari, L.; Beckman, J. E.; Castelli, F.; Vladilo, G. Bibcode: 1987LNP...291..158F Altcode: 1987LNP87.291..158F; 1987csss....5..158F In the framework of a long term programme of chromospheric modelling of late-type dwarfs, high resolution high signal-to-noise spectral profiles of the 8498 and 8542 A lines of the Ca II infrared triplet (IRT) have been obtained for a sample of quiescent and active dwarfs from spectral types F8 to K5. Line core intensity indices are related to the stellar spectral type, effective temperature, gravity and rotation. The Ca II IRT spectra have been calibrated to estimate the excess chromospheric fluxes related to the activity. A relation is derived between the fluxes measured in the line cores and the Rossby number for the sample stars with known rotational periods. Line widths, core and wing intensities, and the differences between active and quiescent stars of the same spectral type are discussed. Title: Effects of interstellar obsorptions of MgII chromospheric emissions Authors: Vladilo, G.; Crivellari, L.; Molaro, P.; Beckman, J. E.; Genova, R. Bibcode: 1986ESASP.263..233V Altcode: 1986NIA86......233V; 1986niia.conf..233V The Mg II chromospheric emissions in cool stars were analyzed using high resolution IUE archive spectra. The ways in which interstellar (IS) absorptions can affect the shapes of the chromospheric profiles are discussed. It is possible to indicate regions of the sky where the Mg II IS absorption can be neglected, up to distances of a few tens of parsecs. When the absorption is present, radial velocity can be estimated, to predict the kind of distortion introduced onto the line profile. Once the IS contamination is taken into account, the chromospheric profiles of the stars tend to have a homogeneous behavior. Title: Lithium abundances and 7Li/6Li ratios in late-type population I field dwarfs. Authors: Rebolo, R.; Crivellari, L.; Castelli, F.; Foing, B.; Beckman, J. E. Bibcode: 1986A&A...166..195R Altcode: Using the Coudé Echelle Spectrograph fed by the 1.4 m Coudé Auxiliary Telescope at ESO, La Silla, the authors have obtained a sequence of fourteen high resolution (λ/Δλ = 105) spectra at high signal to noise ratio, of main sequence and near main sequence objects in the wavelength range containing the 6708 Å resonance doublet of lithium. Using Kurucz thermal equilibrium model atmospheres, and relying on the excellent signal to noise ratios in the spectra, they were able to assess not only the 7Li components, but also the weaker 6Li components of the blended doublet. Title: Book-Review - Progress in Stellar Spectral Line Formation Theory Authors: Beckman, J. E.; Crivellari, L.; Dworetsky, M. M. Bibcode: 1986Obs...106Q.123B Altcode: No abstract at ADS Title: Book-Review - Progress in Stellar Spectral Line Formation Theory Authors: Beckman, J. E.; Crivellari, L. Bibcode: 1986JBAA...96Q.187B Altcode: No abstract at ADS Title: High Resolution Observations of Chromospheric Lines in Late-Type Dwarfs Authors: Foing, B. H.; Beckman, J.; Crivellari, L.; Galleguillos, D. Bibcode: 1986RMxAA..12..214F Altcode: We obtained at the Coude Echelle Spectrometer (CES) attached to the 1.4m CAT telescope of the European Southern Observatory ,high resolution (R=1O0000),high signal to noise (S/N=30-20O) spectra of the Ca II H and k,H alpha and Ca II infrared triplet lines on a sample of southern late-type dwarfs also observed with lUE for the Mg II h and k lines .These profiles are compared for progressive spectral types from F8V to k5V,and for pairs of active-low active stars of the same type and we show the variety of the core emissions and asymmetries. The differential emission in the cores of is discussed in terms of chromospheric heating and of coverage of these stars with active regions.The spectral characteristics of the activity signature are compared with observations and modelling of the profiles for solar "plages" and active components.Some activity indicators obtained from the Ca infrared triplet and H alpha lines are plotted versus the spectral type,showing a lower envelope in the activity corresponding to the quiescent stars,and the range of variation of the activity at a given spectral type. These high resolution profiles are used as a constraint on multi-component models of the stellar chromospheres,which are being developed to estimate the temperature structure,the radiative losses and the heating processes in these late-type dwarfs. Title: Circumstellar Envelopes Around Late-Type Binary Systems Authors: Crivellari, L. Bibcode: 1986iue..prop.2705C Altcode: No abstract at ADS Title: Spectroscopic Variability Associated with Chromospheric Activity: The α Cen System Authors: Foing, B.; Beckman, J.; Crivellari, L.; Vladilo, G.; Char, S. Bibcode: 1986LNP...254..488F Altcode: 1986csss....4..488F No abstract at ADS Title: Book-Review - Progress in Stellar Spectral Line Formation Theory Authors: Beckman, J. E.; Crivellari, L.; Rutten, R. J. Bibcode: 1986SSRv...43Q.384B Altcode: No abstract at ADS Title: Book-Review - Progress in Stellar Spectral Line Formation Theory Authors: Beckman, J. E.; Crivellari, L. Bibcode: 1985Sci...230..835B Altcode: No abstract at ADS Title: Book-Review - Progress in Stellar Spectral Line Formation Theory Authors: Beckman, J. E.; Crivellari, L. Bibcode: 1985JBAA...95..277B Altcode: No abstract at ADS Title: Chromospheric modelling in late-type dwarfs. 2. CES (Coudé Echelle Spectrograph) observations of active and quiescent stars. Authors: Foing, B. H.; Beckman, J.; Crivellari, L.; Galleguillos, D. Bibcode: 1985Msngr..41...18F Altcode: The characteristics of the chromospheres of late-type dwarf stars were studied using the CoudeEchelle Spectrograph (CES) at ESO and the IUE. The study focused on the h and k Mg II lines of F and G dwarfs. The stability of the IUE spectra permitted calculations of averaged absorption features and the subtraction of the Mg II lines to leave intrinsic chromospheric and photospheric line characteristics. Stras with Mg emission cores are very similar in their chromospheres, while stars with Ca II cores vary greatly and exhibit more intense emissions. The higher activity has been linked to magnetic forces which produce spicules with diameters in the range of 1000 km. CES spectra on the Ca II core H lines in Epsilon Eri and Alpha Cen B, active and passive stars, respectively, show a 40 percent lower intensity in the quiescnet star than in the active star, which has an asymmetric core emission and a sharp central absorption with Ca IR triplet lines. The emission features in the active stars are similar to those of the active regions, photospheric spots, chromospheric plages and coronal structures observed in the sun. Title: The distribution of the local interstellar medium derived from MG II column densities towards seven cool stars. Authors: Vladilo, G.; Beckman, J. E.; Crivellari, L.; Franco, M. L.; Molaro, P. Bibcode: 1985A&A...144...81V Altcode: High resolution spectra containing the Mg II h and k resonance doublet have been obtained using the long wavelength spectrometer of IUE, for 7 late-type stars (F0 to G8). In one of these stars, β Hyi, two sharp absorption features were detected against the chromospheric emission cores. Using the study of Crutcher (1982) the authors can employ the velocity of one of these features to identify it as arising from the local interstellar medium (LISM), while the other feature appears due to chromospheric self-absorption. Although the other stars do not show separate intrinsic and LISM features it is possible to use evidence both from wavelength shifts and line half-widths to identify the interstellar absorption components. This gives the Mg II column densities along the lines of sight to five of these stars and upper limits towards the other two. Title: Progress in stellar spectral line formation theory Authors: Beckman, J. E.; Crivellari, L. Bibcode: 1985ASIC..152.....B Altcode: 1985pssl.proc.....B Papers on stellar spectral line formation theory are presented. The general topics considered include: frequency redistribution problems in line formation theory, methods in line radiative transfer, and observational and theoretical aspects of spectral line formation in astrophysical and laboratory environments. Some individual subjects discussed include: kinetic aspects of redistribution in spectral lines, computational methods in redistribution functions, numerical methods in radiative transfer, partial versus complete linearization, line formation in laboratory plasmas, observational problems in spectral line formation, line transfer in expanding atmospheres, partial redistribution in the wind of red giants, transfer of Lyman-alpha radiation in solar coronal loops, pressure broadening and the solar limb effect, and line formation in molecular clouds. Title: The spectra of late-type dwarfs and sub-dwarfs in the near ultraviolet. III. an atlas of MG II H and K profiles. Authors: Franco, M. L.; Crivellari, L.; Molaro, P.; Vladilo, G.; Ramella, M.; Morossi, C.; Allocchio, C.; Beckman, J. E. Bibcode: 1984A&AS...58..693F Altcode: Using a radically new suite of data extraction routines, IUEARM, high resolution IUE spectra of six F and G stars, taken at regular intervals since 1978 are analysed. The salient features of the new routines are: reduction of interorder scattering via two-dimensional deconvolution of the echelle spectrograph image plane, with high frequency filtering, to reduce the effects of the instrument function; enhanced spectral resolution by data extraction using a small pseudo-slit; improved "ripple correction" yielding mutually consistent inter-order overlap regions; and an improved absolute calibration. Rigorous limits, twice as precise as any previously available, are set on weak spectral features, and on possible variability of the h and k chromospheric emission cores in the G stars. The authors show how the interstellar Mg II components of the stars can be abstracted and used to examine a sector of the local interstellar medium. Title: Chromospheric Modelling in Late-Type Dwarfs - Part One - Quiescent Objects Authors: Beckman, J.; Crivellari, L.; Foing, B. Bibcode: 1984Msngr..38...24B Altcode: Attention is given to problems which arise when an attempt is made to use observational material to clarify the mechanisms responsible for the heating of the chromosphere in a late-type star, in order to derive a clearer physical view of the nature of a chromosphere, its relation to the underlying photosphere, and the overlying corona. Problems arise in the prediction of line shapes by means of simple models, in chromospheric inhomogeneities, in two-stream modeling, and in the coexistence of two temperature structures. It is deemed necessary to take the dominant role played by inhomogeneities into account, and to ensure that any interstellar effects in the line profile data are well and truly eliminated. Title: MG II Spectra of Late Type Stars Used to Probe the LISM Authors: Beckman, J. E.; Crivellari, L.; Franco, M.; Molaro, P.; Vladilo, G. Bibcode: 1984NASCP2345...67B Altcode: 1984lism.rept...67B; 1984IAUCo..81...67B IUE spectra of Mg II h and k in late type dwarfs and giants were used to detect and measure absorption components due to the LISM. This technique gives a method of probing the awkward range from d = 3 pc to d = 80 pc from the Sun. In spite of interpretational uncertainties the HI component of the LISM can be plotted well enough to confirm it as a cloud some 20 to 30 pc in extent, peaking sharply in density towards l(II)-25 deg., moving towards the Sun from l(II)-25 deg, b(II) = + 10 deg., at 28 Km/sec. The hole towards l(II) = 150 deg is confirmed, suggesting a solar position close to the cloud's edge in this direction. Title: Use of high resolution IUE spectra of Mg II in cool stars to probe the local interstellar medium. Authors: Molaro, P.; Beckman, J. E.; Crivellari, L.; Franco, M. L.; Vladilo, G. Bibcode: 1984ESASP.218..139M Altcode: 1984iue..conf..139M It is shown that, given suitable radial velocity values for the stars and the local interstellar medium (LISM) interstellar absorption can be distinguished from chromospheric self absorption within the MgII emissions from late type stars. Beta Hyi (G2IV) offers a clear example. Zeta Tuc (G2V) and delta Pav (G8V) which are very close to beta Hyi show similar narrow MgII absorption features, which are also interpretable as interstellar. Narrow MgII absorptions in the rapidly rotating F-type stars beta TrA and alpha Hyi are reported. The absence of IS Mg II in the stars tau Cet and 24UMa confirms the presence of a hole in the LISM near galactic longitude 150 deg. Title: The spectra of late type dwarfs and sub-dwarfs in the near ultraviolet II. Limits to variability in MgII emission from IUE spectrophotometry. Authors: Crivellari, L.; Franco, M. L.; Molaro, P.; Vladilo, G.; Beckman, J. E. Bibcode: 1983A&AS...52..135C Altcode: The chromospheric h and k emission cores of the G dwarfs and subdwarfs Beta Hyi (14), Zeta Tuc (5), Delta Pav (5), and Tau Cet (5) have been monitored over a period of four years using the IUE satellite long wavelength camera. A detailed statistical examination of the data has shown that no variation in h and k fluxes of less than 15 percent could have been detected by means of this technique. This limit applies, even after removing the effects of saturation, variable gain, and scattered light between echelle orders, for any astrophysical use of comparable IUE data. No flux excursions larger than this value were detected in any of the objects presently considered. Title: Observed and computed UV spectral distribution of A and F stars. Authors: Malagnini, M. L.; Faraggiana, R.; Morossi, C.; Crivellari, L. Bibcode: 1982A&A...114..170M Altcode: An automatic and fast procedure was implemented to determine Te and log g from the comparison of the UV S 2/68 spectra with the Kurucz models. The method has been applied to all A and F stars with (B-V)0 greater than or equal to 0 and luminosity classes III, IV, IV-V, and V, included in the Ultraviolet Bright-Star Spectrophotometric Catalogue. From the analysis, it appears that the models match the observed fluxes fairly well and that the effective temperatures derived from the UV data agree with those derived from Stromgren photometric indices within 200 K. Title: On the MG II and FE II resonance lines in Herbig AE stars : preliminary results. Authors: Talavera, A.; Catala, C.; Crivellari, L.; Czarny, J.; Felenbok, P.; Praderie, F. Bibcode: 1982ESASP.176...99T Altcode: 1982iue..conf...99T; 1982IUE3r.......99T The IUE observed AB Aur, HD250550, and BD + 46 deg 3471 at high resolution in the range 2000 to 32000 A. The MgII resonance line profiles all show a P Cygni structure, with an intense emission, and a shortward wing velocity in the absorption component of 300 to 600 km/sec. In HD 250550, the Fe II line spectrum exhibits a shell phenomenon: all lines are blue displaced by - 290 km/sec; other lines of singly ionized species show the same effect. The mass loss rate in AB Aur estimated from an analysis of both MgII resonance lines and unsaturated Fe II multiplet 1 lines is smaller than previously reported. Title: A search for MG II and K emission core variability in six main sequence stars. Authors: Beckman, J. E.; Franco, M. L.; Molaro, P.; Uladilo, G.; Crivellari, L. Bibcode: 1982ESASP.176..149B Altcode: 1982iue..conf..149B; 1982IUE3r......149B Spectra of six main sequence stars with spectral types from F0 to G8 were obtained with the IUE long wavelength spectrograph at high resolution. Four or more spectra of each object were taken over a period of 3.5 years, supplemented by archive spectra from IUE of the same type. Quantitative analysis of the Mg II h and k emission cores from these spectra was performed to search for sporadic or periodic variability. No variations the + or - 10% level set by the statistical fluctuations within each set of spectra occur for any object. Even the limited photometric accuracy claimed here is not guaranteed among spectra which vary in exposure time by more than 15%. This situation for all IUE LWR spectra can be improved only by solving the problem of background subtraction. Title: On the search for transition zone lines in late A type stars. Authors: Crivellari, L.; Praderie, F. Bibcode: 1982A&A...107...75C Altcode: Observations of the 1400-1800 A region in late A type stars are presented, which were obtained using the IUE satellite at low and high resolution. The calibrated energy distribution is derived and presents a large variation with wavelength in the spectral range due to Si I photoionization discontinuities. Emission lines formed in the transition region do not appear, although upper limits on the emission in these lines are established. A schematic model is proposed for the outer layers in A type stars, which suggests that the chromosphere in solar type stars is likely absent in A stars, while the NLTE photosphere could be immediately connected to the transition region, joining the upper photosphere to the corona. In addition, the amount of matter in the transition zone is found to be small. Title: An alternative procedure for extracting IUE low resolution spectra. Authors: Crivellari, L.; Morossi, C. Bibcode: 1982A&A...106..332C Altcode: This paper presents an alternative procedure which extracts spectra from IUE low resolution images. The -by-line image is scanned perpendicularly to the direction of dispersion . The resulting cross-cuts are then fitted with a known-form analytical function, which consists of a linear base-line describing the background plus as many bands as there are spectra in the image under study. At each wavelength point the value of the spectrum is assumed to be the area of the corresponding band. A proper choice of the band profile allows us to recover the light scattered perpendicularly to the direction of dispersion, due to the "halation" effect in the on-board UV-to-visible image converter. In such a way a more correct extracting procedure is achieved. It furnishes noticeably better results than the standard VILSPA extracting routine in the case of highly exposed images. As a by-product, some properties of the Point Spread Function perpendicular to the direction of dispersion are determined. Title: The spectra of late-type dwarfs in the near ultraviolet. I. Line identifications. Authors: Beckman, J. E.; Crivellari, L.; Selvelli, P. L. Bibcode: 1982A&AS...47..295B Altcode: Spectra from IUE of 6 main sequence and near main sequence stars from classes A5 to G8, between 2700 A and 2900 A wavelength are described. In toto, some 250 absorption lines are identified. Observed differences in those atomic and ionic multiplets present in each spectrum are shown to be due either to the regular progression in spectral class, or to the smearing effect of stellar rotation. Title: The Visible Spectra of Three B-Stars - Pi-Ceti 36-LYNCIS 134-TAURII Authors: Crivellari, L.; Flora, U.; Mercanti, M.; Morossi, C.; Rusconi, L.; Sedmak, G. Bibcode: 1981Ap&SS..80..425C Altcode: We present an identification list for the visible spectra of three B stars: π Cet (B7V), 36 Lyn B8 IIIp (?)),and 134 Tau (B9 IV). Equivalent widths have been measured on medium dispersion plates (7.0 and 12.4 Å mm-1) taken with the 152 cm coudé reflector at the Observatoire de Haute Provence (France). These results are also presented. The photographic plates were digitized by using a PDS 1010A microdensitometer. The spectroscopic data were reduced by means of a dedicated software package and an attempt was made to compute equivalent widths in a homogeneous way. T eff and logg parameters were estimated by using the computer to compare automatically the spectroscopic data with the value of theoretical models. Title: A statistical analysis of Mg II h and k emission cores for variability in beta Hydri (G2 IV). Authors: Beckman, J. E.; Crivellari, L.; Morossi, C.; Ramella, M.; Vladilo, G. Bibcode: 1980idr..conf..207B Altcode: No abstract at ADS Title: A Theoretical Approach to the Estimate of Scattered Light into IUE Low Resolution Short Wavelength Spectra Authors: Crivellari, L.; Morossi, C.; Ramella, M. Bibcode: 1980idr..conf..185C Altcode: No abstract at ADS Title: An automatic procedure for a determination of the effective temperature and gravity. Application to 100 O-type stars. Authors: Morossi, C.; Crivellari, L. Bibcode: 1980A&AS...41..299M Altcode: We have developed an automatic procedure to determine Teff and log g from the comparison of observational data with their theoretical values. After a description of the method we compare our results for a sample of 100 O-type stars with the classifications of Conti and Walborn and with the determinations of Morrison by photometric indices. The agreement is fair and we present a HR diagram for O-type stars based on our data. Title: Parasite light effects in International Ultraviolet Explorer (IUE) low resolution spectra Authors: Crivellari, L.; Morossi, C. Bibcode: 1980ESASP.157...99C Altcode: 1980iue..conf...99C Procedures were developed for extracting low resolution spectra while correcting for halation effects and scattered light within the instrument. To correct for halation, the spectra were extracted from the line-by-line images. Perpendicular scans of the image were fitted using an iterative least squares procedure with a known form function. The fitting function was a linear combination of a Gaussian with a Lorentz function. Correction for light scattered from the gratings employed a guess and test method. A priori choices of schematic spectral distributions were used to compute different sets of scattered light coefficients. Satisfactory results are shown for both methods. Title: Correction of Halation Effects in Low-Resolution IUE Spectra Authors: Crivellari, L. Bibcode: 1980IUEEN...5...34C Altcode: No abstract at ADS Title: Outer Layers of Dwarf A Type Stars. Authors: Praderie, F.; Crivellari, L. Bibcode: 1979BAAS...11..776P Altcode: No abstract at ADS Title: The visible spectrum of beta Ori, B8 Ia. Authors: Crivellari, L.; Flora, U.; Rusconi, L.; Sedmak, G. Bibcode: 1979A&AS...36...73C Altcode: An exhaustive description of the visible spectrum of Beta Ori from the Balmer discontinuity to about 9000 A is reported. Equivalent widths and radial-velocity values for the most significant lines in the spectral range studied are presented. Moreover, an automatic data processing technique is described. Title: A criterion for the measurement of equivalent widths. Authors: Crivellari, L.; Mardirossian, F.; Morossi, C. Bibcode: 1979A&A....72..256C Altcode: A simple standard criterion for computer measurement of the equivalent widths of digitized blended spectral lines and for drawing a local continuum as defined by Ardeberg (1975) is outlined. A procedure is developed with which the user need only select the spectral interval enclosing the line blend and the spectral features themselves determine the position of the local continuum as well as the parameters of the mathematical function that fits the lines. The errors associated with the procedure are evaluated. Some examples involving the Si III, C I, and S III features at 1192-1194 A in the spectra of Alpha Cam, Delta Ori A, and Iota Ori are provided. It is noted that the program used to solve the line blends is particularly suitable for interactive small computers. Title: Analysis of UV spectrophotometric observations for O-type stars. Authors: Morossi, C.; Stalio, R.; Crivellari, L. Bibcode: 1979IAUS...83..109M Altcode: Spectral indices (delta m2100 and delta m1500) for 32 O-type stars with known spectral class and reddening are plotted versus log of effective temperature. The delta m2100 indices are considered to be almost luminosity-independent, while evidence of a well-established relationship between observed spectral indices and empirical temperatures appears. Also graphed are equivalent widths of selected photospheric lines versus temperature. Theoretical line strengths computed with a code that solves the LTE line transfer problem from the model atmosphere input of Kurucz, et al. (1974) indicate that the general behavior of the silicon lines, but not the P V resonance line, appears consistent with the computations. Title: Lo spettro visibilie di beta Ori (B8 Ia). Authors: Crivellari, L.; Flora, U. Bibcode: 1979RSAI...22..194C Altcode: No abstract at ADS Title: A software package for the presentation and evaluation of digitized images Authors: Crivellari, L.; Giuricin, G.; Mardirossian, F.; Mezzetti, M.; Pucillo, M.; Santin, P. Bibcode: 1978MmSAI..49..741C Altcode: A brief description is given of the IMAGE-1 software package for the presentation and evaluation of digitized images at the Osservatorio Astronomico di Trieste. The package makes it possible to draw isophotes in an interactive way and to have an assonometric display of the image studied. Title: Processing of stellar spectrograms digitized by means of the PDS 1010A microdensitometer - The ELSPEC/1 software package Authors: Crivellari, L.; Santin, P.; Sedmak, G.; Rusconi, L. Bibcode: 1978MmSAI..49..717C Altcode: The software package ELSPEC/1 written at Trieste Astronomical Observatory for the automatic reduction of spectrograms digitized by a PDS 1010A microdensitometer is described and an example of the results obtained is shown. The package accepts spectrograms in the format of the Observatoire de Hauts Provence and operates on a HP 2100A minicomputer. Title: a Criterion for the Measurement of Equivalent Widths Authors: Crivellari, L.; Mardirossian, F.; Morossi, C. Bibcode: 1978hrs..conf..625C Altcode: No abstract at ADS Title: Spectral energy distribution and line blanketing in beta Ori. Authors: Crivellari, L.; Stalio, R. Bibcode: 1977MmSAI..48..773C Altcode: No abstract at ADS Title: Line blocking and reddening of beta Orionis. A new determination of the empirical effective temperature. Authors: Stalio, R.; Selvelli, P. L.; Crivellari, L. Bibcode: 1977A&A....60..109S Altcode: Summary. The ultraviolet spectral energy distribution of the B8 Ia star fi Orionis, derived from OAO-2 observations, has been corrected for the line-blocking fraction (LBF) measured from Copernicus scans with 0.4 A resolution. The resulting continuum appears to be affected by a not negligible color excess; from the size of the characteristic interstellar extinction bump at 2175 one may estimate a value of EB - V =0.04. The empirical effective temperature determined from the angular diameter and from the total absolute flux, corrected for reddening, is 12070 0K. This value is in good agreement with the temperature obtained by comparing the observed continuum flux, corrected for LBF and reddening, with LTE, plane-parallel model atmospheres predictions in the region A. At shorter wavelengths, however, the continuum flux appears deficient, thus indicating a lower effective temperature, while in the visual it is in excess. This effect may be ascribed to line- blanketing in the UV which forces a flux redistribution and maintains the region between 2000 and 3000 A invariant as a result of a balance between the fluxes in the two neighbouring regions. There are indications of a higher value ofthe effective temperature from the line spectrum. The LBF curve in the region A, compared with theoretical LBFs, indicates a Teff 13000 0K, while the ionization ratios of Fe and Si indicate an even greater value. Key words: Orionis - stellar atmospheres effective temperature - line blocking reddening Title: The ultraviolet spectrum of beta Ori, B8 Ia. Authors: Selvelli, P. L.; Crivellari, L.; Stalio, R. Bibcode: 1977A&AS...27....1S Altcode: A Copernicus spectrum of the early-type supergiant Beta Ori, extending from 1010 to 1440 A with 0.2-A resolution and from 1710 to 3215 A with 0.4-A resolution, is described. Some 1137 absorption lines are identified, and equivalent widths and profiles are computed for selected features. It is found that: (1) the Ly-alpha line is the major feature over the entire spectrum; (2) 'metals' are mostly in the first and second state of ionization; and (3) only the resonance lines of Si III and Si IV are markedly blue-shifted by about 180 km/s, but there is no evidence of P Cyg profiles. Title: ELSPEC/1. Un software package per l'elaborazione automatizzata di spettrogrammi stellari. I. Struttura. Authors: Rusconi, L.; Sedmak, G.; Crivellari, L. Bibcode: 1977TriP..544.....R Altcode: No abstract at ADS