Author name code: kupka ADS astronomy entries on 2022-09-14 author:"Kupka, Friedrich" ------------------------------------------------------------------------ Title: Stellar evolution models with overshooting based on 3-equation non-local theories, II. Main-sequence models of A- and B-type stars Authors: Ahlborn, Felix; Kupka, Friedrich; Weiss, Achim; Flaskamp, Martin Bibcode: 2022arXiv220712512A Altcode: Convective overshoot mixing is a critical ingredient of stellar structure models, but is treated in most cases by ad hoc extensions of the mixing-length theory for convection. Advanced theories which are both more physical and numerically treatable are needed. Convective flows in stellar interiors are highly turbulent. This poses a number of numerical challenges for the modelling of convection in stellar interiors. We include an effective turbulence model into a 1D stellar evolution code in order to treat non-local effects within the same theory. We use a turbulent convection model which relies on the solution of second order moment equations. We implement this into a state of the art 1D stellar evolution code. To overcome a deficit in the original form of the model, we take the dissipation due to buoyancy waves in the overshooting zone into account. We compute stellar models of intermediate mass main-sequence stars between 1.5 and 8 $M_\odot$. Overshoot mixing from the convective core and modified temperature gradients within and above it emerge naturally as a solution of the turbulent convection model equations. For a given set of model parameters the overshooting extent determined from the turbulent convection model is comparable to other overshooting descriptions, the free parameters of which had been adjusted to match observations. The relative size of the mixed cores decreases with decreasing stellar mass without additional adjustments. We find that the dissipation by buoyancy waves constitutes a necessary and relevant extension of the turbulent convection model in use. Title: Stellar evolution models with overshooting based on 3-equation non-local theories I. Physical basis and the computation of the dissipation rate Authors: Kupka, F.; Ahlborn, F.; Weiss, A. Bibcode: 2022arXiv220712296K Altcode: Context. Mixing by convective overshooting has long been suggested to play an important role for the amount of hydrogen available to nuclear burning in convective cores of stars. The best way to model this effect is still debated. Aims. We suggest an improved model for the computation of the dissipation rate of turbulent kinetic energy which can be used in non-local models of stellar convection and can readily be implemented and self-consistently used in 1D stellar evolution calculations. Methods. We review the physics underlying various models to compute the dissipation rate of turbulent kinetic energy, {\epsilon}, in local and particularly in non-local models of convection in stellar astrophysics. The different contributions to the dissipation rate and their dependence on local stratification and on non-local transport are analysed and a new method to account for at least some of these physical mechanisms is suggested. Results. We show how the new approach influences predictions of stellar models of intermediate-mass main-sequence stars and how these changes differ from other modifications of the non-local convection model that focus on the ratio of horizontal to vertical (turbulent) kinetic energy. Conclusions. The new model is shown to allow for a physically more complete description of convective overshooting and mixing in massive stars. Dissipation by buoyancy waves is found to be a key ingredient which has to be accounted for in non-local models of turbulent convection. Title: On the Potential of the Reynolds Stress Approach to Model Convective Overshooting in Grids of Stellar Evolution Models Authors: Kupka, Friedrich Bibcode: 2021plat.confE..88K Altcode: Convection is one of the main physical processes probed by means of asteroseismology these days and it is a key topic of several workpackages within the PLATO mission. A lot of attention in this field is currently given to the parameter calibration of fairly simple models by means of 3D RHD numerical simulations or the direct use of the latter in asteroseismological analyses. However, this approach is not available to all situations of interest where convection plays a role in stellar mixing and in the evolution of the thermal structure of a star, particularly not for overshooting and mixing when they take place deeply inside a star. For such cases Reynolds stress models provide an interesting alternative. In this talk I will report on the potential of this method, also for future calculations of model grids for asteroseismology, and summarize earlier results and recent progress made with this approach. Title: Stellar evolution models with entropy-calibrated mixing-length parameter: application to red giants Authors: Spada, Federico; Demarque, Pierre; Kupka, Friedrich Bibcode: 2021MNRAS.504.3128S Altcode: 2021arXiv210408067S; 2021MNRAS.tmp.1089S We present evolutionary models for solar-like stars with an improved treatment of convection that results in a more accurate estimate of the radius and effective temperature. This is achieved by improving the calibration of the mixing-length parameter, which sets the length scale in the 1D convection model implemented in the stellar evolution code. Our calibration relies on the results of 2D and 3D radiation hydrodynamics simulations of convection to specify the value of the adiabatic specific entropy at the bottom of the convective envelope in stars as a function of their effective temperature, surface gravity, and metallicity. For the first time, this calibration is fully integrated within the flow of a stellar evolution code, with the mixing-length parameter being continuously updated at run-time. This approach replaces the more common, but questionable, procedure of calibrating the length scale parameter on the Sun, and then applying the solar-calibrated value in modelling other stars, regardless of their mass, composition, and evolutionary status. The internal consistency of our current implementation makes it suitable for application to evolved stars, in particular to red giants. We show that the entropy calibrated models yield a revised position of the red giant branch that is in better agreement with observational constraints than that of standard models. Title: Accurate Short-Characteristics Radiative Transfer in A Numerical Tool for Astrophysical RESearch (ANTARES) Authors: Kostogryz, Nadiia M.; Kupka, Friedrich; Piskunov, Nikolai; Fabbian, Damian; Krüger, Daniel; Gizon, Laurent Bibcode: 2021SoPh..296...46K Altcode: We aim to improve the accuracy of radiative energy transport in three-dimensional radiation hydrodynamical simulations in ANTARES (A Numerical Tool for Astrophysical RESearch). We implement in the ANTARES short-characteristics numerical schemes a modification of the Bézier interpolant solver. This method yields a smoother surface structure in simulations of solar convection and reduces the artifacts appearing due to the limited number of rays along which the integration is done. Reducing such artifacts leads to increased stability of the code. We show that our new implementation achieves a better agreement of the temperature structure and its gradient with a semi-empirical model derived from observations, as well as of synthetic spectral-line profiles with the observed solar spectrum. Title: Surface effects and turbulent pressure. Assessing the Gas-Γ1 and Reduced-Γ1 empirical models Authors: Belkacem, K.; Kupka, F.; Philidet, J.; Samadi, R. Bibcode: 2021A&A...646L...5B Altcode: 2021arXiv210106065B The application of the full potential of stellar seismology is made difficult by the improper modelling of the upper-most layers of solar-like stars and their influence on the modelled frequencies. Our knowledge of these so-called `surface effects' has improved thanks to the use of 3D hydrodynamical simulations, however, the calculation of eigenfrequencies relies on empirical models for the description of the Lagrangian perturbation of turbulent pressure, namely: the reduced-Γ1 model (RGM) and the gas-Γ1 model (GGM). Starting from the fully compressible turbulence equations, we derived both the GGM and RGM models by using a closure to model the flux of turbulent kinetic energy. We find that both models originate from two terms: the source of turbulent pressure due to compression produced by the oscillations and the divergence of the flux of turbulent pressure. We also demonstrate that they are both compatible with the adiabatic approximation and, additionally, that they imply a number of questionable assumptions, mainly with respect to mode physics. Among other hypotheses, it is necessary to neglect the Lagrangian perturbation of the dissipation of turbulent kinetic energy into heat and the Lagrangian perturbation of buoyancy work. Title: The ANTARES code: recent developments and applications Authors: Kupka, Friedrich; Zaussinger, Florian; Fabbian, Damian; Krüger, Daniel Bibcode: 2020JPhCS1623a2016K Altcode: ANTARES (A Numerical Tool for Astrophysical RESearch) is a multi-purpose numerical tool to solve different variants of the equations of hydrodynamics as they appear in problems of astrophysics, geophysics, and engineering sciences and which require the construction of detailed numerical simulation models. A presentation of the current feature set of the code with a focus on recent add-ons is given here in addition to a summary on several results from recent applications of ANTARES to solar physics, the physics of planets, and basic convection studies including the damping of pressure modes (solar oscillations) in numerical simulations of convection at the solar surface and the coupling of layers in numerical simulations of sheared and non-sheared double-diffusive convection. Title: On long-duration 3D simulations of stellar convection using ANTARES Authors: Kupka, F.; Fabbian, D.; Krüger, D.; Kostogryz, N.; Gizon, L. Bibcode: 2020IAUGA..30..373K Altcode: We present initial results from three-dimensional (3-D) radiation hydrodynamical simulations for the Sun and targeted Sun-like stars. We plan to extend these simulations up to several stellar days to study p-mode excitation and damping processes. The level of variation of irradiance on the time scales spanned by our 3-D simulations will be studied too. Here we show results from a first analysis of the computational data we produced so far. Title: 3D Hydrodynamical Simulations of Stellar Convection for Helio- and Asteroseismology Authors: Kupka, F. Bibcode: 2020svos.conf..209K Altcode: Hydrodynamical simulations of stellar convection are an essential theoretical tool for gaining insight into the physics of mixing and heat transport by convection, and also into the interaction of convection with pulsation. They are particularly useful for obtaining an accurate description of the structure of the superadiabatic layer, which is important to explain the observed frequencies of $p$-modes in solar-like oscillating stars. The simulations can also be used to probe analytical models of excitation and damping of modes, and thus explain their amplitudes, and eventually the physical completeness of such models. This presentation discussed general challenges of such 3D hydrodynamical simulations developed for helio- and asteroseismology; it summarized some recent results in this field for the Sun, and which are also relevant to other lower main-sequence stars. Title: Thermal Convection in Stars and in Their Atmosphere Authors: Kupka, Friedrich Bibcode: 2020mdps.conf...69K Altcode: 2020arXiv200111540K Thermal convection is one of the main mechanisms of heat transport and mixing in stars in general and also in the photospheric layers which emit the radiation that we observe with astronomical instruments. The present lecture notes first introduce the role of convection in astrophysics and explain the basic physics of convection. This is followed by an overview on the modelling of convection. Challenges and pitfalls in numerical simulation based modelling are discussed subsequently. Finally, a particular application for the previously introduced concepts is described in more detail: the study of convective overshooting into stably stratified layers around convection zones in stars. Title: Shine BRITE: shedding light on stellar variability through advanced models Authors: Fabbian, D.; Kupka, F.; Krüger, D.; Kostogryz, N. M.; Piskunov, N. Bibcode: 2020svos.conf..155F Altcode: 2020arXiv200201560F The correct interpretation of the large amount of complex data from next-generation (in particular, space-based) observational facilities requires a very strong theoretical underpinning. One can predict that, in the near future, the use of atmospheric models obtained with three-dimensional (3-D) radiation magneto-hydrodynamics (RMHD) codes, coupled with advanced radiative transfer treatment including non-local thermodynamic equilibrium (non-LTE) effects and polarisation, will become the norm. In particular, stellar brightness variability in cool stars (i.e., spectral types F-- M) can be caused by several different effects besides pulsation. In this review we have briefly discussed some published results, and mentioned aspects of recent progress. It then attempted to peek into what the future may hold for understanding this important aspect of the lives of stars. Title: Layer formation in double-diffusive convection over resting and moving heated plates Authors: Zaussinger, Florian; Kupka, Friedrich Bibcode: 2019ThCFD..33..383Z Altcode: 2018arXiv181111800Z We present a numerical study of double-diffusive convection characterized by a stratification unstable to thermal convection, while at the same time a mean molecular weight (or solute concentration) difference between top and bottom counteracts this instability. Convective zones can form in this case either by the stratification being locally unstable to the combined action of both temperature and solute gradients or by another process, the oscillatory double-diffusive convective instability, which is triggered by the faster molecular diffusivity of heat in comparison with that one of the solute. We discuss successive layer formation for this problem in the case of an instantaneously heated bottom (plate) which forms a first layer with an interface that becomes temporarily unstable and triggers the formation of further, secondary layers. We consider both the case of a Prandtl number typical for water (oceanographic scenario) and of a low Prandtl number (giant planet scenario). We discuss the impact of a Couette like shear on the flow and in particular on layer formation for different shear rates. Additional layers form due to the oscillatory double-diffusive convective instability, as is observed for some cases. We also test the physical model underlying our numerical experiments by recovering experimental results of layer formation obtained in laboratory setups. Title: Solar p-mode damping rates: Insight from a 3D hydrodynamical simulation Authors: Belkacem, K.; Kupka, F.; Samadi, R.; Grimm-Strele, H. Bibcode: 2019A&A...625A..20B Altcode: 2019arXiv190305479B Space-borne missions such as CoRoT and Kepler have provided a rich harvest of high-quality photometric data for solar-like pulsators. It is now possible to measure damping rates for hundreds of main-sequence and thousands of red-giant stars with an unprecedented precision. However, among the seismic parameters, mode damping rates remain poorly understood and thus barely used for inferring the physical properties of stars. Previous approaches to model mode damping rates were based on mixing-length theory or a Reynolds-stress approach to model turbulent convection. While they can be used to grasp the main physics of the problem, such approaches are of little help to provide quantitative estimates as well as a definitive answer on the relative contribution of each physical mechanism. Indeed, due to the high complexity of the turbulent flow and its interplay with the oscillations, those theories rely on many free parameters which inhibits an in-depth understanding of the problem. Our aim is thus to assess the ability of 3D hydrodynamical simulations to infer the physical mechanisms responsible for damping of solar-like oscillations. To this end, a solar high-spatial resolution and long-duration hydrodynamical 3D simulation computed with the ANTARES code allows probing the coupling between turbulent convection and the normal modes of the simulated box. Indeed, normal modes of the simulation experience realistic driving and damping in the super-adiabatic layers of the simulation. Therefore, investigating the properties of the normal modes in the simulation provides a unique insight into the mode physics. We demonstrate that such an approach provides constraints on the solar damping rates and is able to disentangle the relative contribution related to the perturbation (by the oscillation) of the turbulent pressure, the gas pressure, the radiative flux, and the convective flux contributions. Finally, we conclude that using the normal modes of a 3D numerical simulation is possible and is potentially able to unveil the respective role of the different physical mechanisms responsible for mode damping provided the time-duration of the simulation is long enough. Title: Numerical simulation of DA white dwarf surface convection Authors: Zaussinger, F.; Kupka, F.; Montgomery, M.; Egbers, Ch. Bibcode: 2018JPhCS1031a2013Z Altcode: White dwarfs are compact objects with masses comparable to our Sun, but a radius similar to our Earth. They are the final evolutionary stage for about 95% of all stars in the Galaxy, i.e., for all stars that have a final mass less than the Chandrasekhar mass (about 1.4 times the solar mass), the upper mass limit for which hydrostatic equilibrium can be maintained by the degeneracy pressure of electrons at very high densities. The outermost shell of most white dwarfs contains a convective layer. Even if the latter is very thin (≲ 10 km), it is important for mixing properties, observed radiation, and pulsational stability of the whole object. During a long phase white dwarfs have effective temperatures Teff of about 10000K ∼ 14000K, since the time scale to reach such temperatures by cooling is already ≈ 109 years. Here, we focus on DA (hydrogen-rich) white dwarfs with Teff ≈ 12000K. This is at the transition from shallow to deep convection zones. Due to very high gravitational acceleration (∼ 106 g at the surface) the material is overturned about five times per second over the distance of a few kilometers. Numerical simulations of such objects have to be done for a compressible flow and feature highly turbulent granules at the surface, which are qualitatively comparable to the convection cells observed at the surface of the Sun. For this study we compare three white dwarf surface simulations with realistic microphysical properties and full 3D radiative transport. The simulations differ in effective temperature, namely, Teff = 11800K, 12100K, and 12400K. A statistical analysis of the convective processes as function of Teff is presented. Title: Mixing and overshooting in surface convection zones of DA white dwarfs: first results from ANTARES Authors: Kupka, F.; Zaussinger, F.; Montgomery, M. H. Bibcode: 2018MNRAS.474.4660K Altcode: 2017arXiv171200641K We present results of a large, high-resolution 3D hydrodynamical simulation of the surface layers of a DA white dwarf (WD) with Teff = 11 800 K and log (g) = 8 using the ANTARES code, the widest and deepest such simulation to date. Our simulations are in good agreement with previous calculations in the Schwarzschild-unstable region and in the overshooting region immediately beneath it. Farther below, in the wave-dominated region, we find that the rms horizontal velocities decay with depth more rapidly than the vertical ones. Since mixing requires both vertical and horizontal displacements, this could have consequences for the size of the region that is well mixed by convection, if this trend is found to hold for deeper layers. We discuss how the size of the mixed region affects the calculated settling times and inferred steady-state accretion rates for WDs with metals observed in their atmospheres. Title: Studying p-mode damping and the surface effect with hydrodynamical simulations Authors: Kupka, F.; Belkacem, K.; Samadi, R.; Deheuvels, S. Bibcode: 2017sbcs.conf..222K Altcode: Hydrodynamical simulations can be used as a complementary tool to observations for the study of the damping of p-modes and the so-called surface effect in solar-like oscillators. Here, we present the state-of-the-art in this research. Examples of applications include our Sun and the CoRoT target stars HD 49385 and HD 49933. Title: Modelling of stellar convection Authors: Kupka, Friedrich; Muthsam, Herbert J. Bibcode: 2017LRCA....3....1K Altcode: The review considers the modelling process for stellar convection rather than specific astrophysical results. For achieving reasonable depth and length we deal with hydrodynamics only, omitting MHD. A historically oriented introduction offers first glimpses on the physics of stellar convection. Examination of its basic properties shows that two very different kinds of modelling keep being needed: low dimensional models (mixing length, Reynolds stress, etc.) and "full" 3D simulations. A list of affordable and not affordable tasks for the latter is given. Various low dimensional modelling approaches are put in a hierarchy and basic principles which they should respect are formulated. In 3D simulations of low Mach number convection the inclusion of then unimportant sound waves with their rapid time variation is numerically impossible. We describe a number of approaches where the Navier-Stokes equations are modified for their elimination (anelastic approximation, etc.). We then turn to working with the full Navier-Stokes equations and deal with numerical principles for faithful and efficient numerics. Spatial differentiation as well as time marching aspects are considered. A list of codes allows assessing the state of the art. An important recent development is the treatment of even the low Mach number problem without prior modification of the basic equation (obviating side effects) by specifically designed numerical methods. Finally, we review a number of important trends such as how to further develop low-dimensional models, how to use 3D models for that purpose, what effect recent hardware developments may have on 3D modelling, and others. Title: Semi-convective layer formation Authors: Zaussinger, F.; Kupka, F.; Egbers, Ch.; Neben, M.; Hücker, S.; Bahr, C.; Schmitt, M. Bibcode: 2017JPhCS.837a2012Z Altcode: Semi-convective mixing, as an example of double-diffusive convection, is of general importance in multi-component fluid mixing processes. In astrophysics it occurs when the mean molecular weight gradient caused by a mixture of light material on top of heavier one counteracts the convective instability caused by a temperature gradient. Direct numerical simulations of double-diffusive fluid flows in a realistic stellar or planetary parameter space are currently non-feasible. Hence, a model describing incompressible semi-convection was developed, which allows to investigate semi-convective layer formation. A detailed parameter study with varying Rayleigh number and stability parameter has been performed for the giant planet case. We conclude that semi-convective layering may not play that important role as suggested in earlier works for the planetary case. Title: Multidimensional modelling of classical pulsating stars Authors: Muthsam, H. J.; Kupka, F. Bibcode: 2016CoKon.105..117M Altcode: 2016arXiv160103325M After an overview of general aspects of modelling the pulsation- convection interaction we present reasons why such simulations (in multidimensions) are needed but, at the same time, pose a considerable challenge. We then discuss, for several topics, what insights multidimensional simulations have either already provided or can be expected to yield in the future. We finally discuss properties of our ANTARES code. Many of these features can be expected to be characteristic of other codes which may possibly be applied to these physical questions in the foreseeable future. Title: The ANTARES Code: New Developments Authors: Blies, P. M.; Kupka, F.; Muthsam, H. J. Bibcode: 2015ASPC..498..191B Altcode: We give an update on the ANTARES code. It was presented by Muthsam et al. (2010) and has since experienced various improvements and has also been extended by new features which we will mention in this paper. Two new features will be presented in a bit more detail: the parallel multigrid solver for the 2D non-linear, generalized Helmholtz equation by Happenhofer (2014) and the capability to use curvilinear grids by Grimm-Strele (2014). Title: Multidimensional realistic modelling of Cepheid-like variables - II. Analysis of a Cepheid model Authors: Mundprecht, Eva; Muthsam, Herbert J.; Kupka, Friedrich Bibcode: 2015MNRAS.449.2539M Altcode: 2015arXiv150200449M Non-local, time-dependent convection models have been used to explain the location of double-mode pulsations in Cepheids in the HR diagram as well as the existence and location of the red edge of the instability strip. These properties are highly sensitive to model parameters. We use 2D radiation-hydrodynamical simulations with realistic microphysics and grey radiative transfer to model a short-period Cepheid. The simulations show that the strength of the convection zone varies significantly over the pulsation period and exhibits a phase shift relative to the variations in radius. We evaluate the convective flux and the work integral as predicted by the most common convection models. It turns out that over one pulsation cycle the model parameter αc, has to be varied by up to a factor of beyond 2 to match the convective flux obtained from the simulations. To bring convective fluxes integrated over the He II convection zone and the overshoot zone below into agreement, this parameter has to be varied by a factor of up to ∼7.5 (Kuhfuß). We then present results on the energetics of the convection and overshoot zone by radially symmetric and fluctuating quantities. To successfully model this scenario by a static, 1D or even by a simple time-dependent model appears extremely challenging. We conclude that significant improvements are needed to make predictions based on 1D models more robust and to improve the reliability of conclusions on the convection-pulsation coupling drawn from them. Multidimensional simulations can provide guidelines for developing descriptions of convection then applied in traditional 1D modelling. Title: Achievable efficiency of numerical methods for simulations of solar surface convection Authors: Grimm-Strele, H.; Kupka, F.; Muthsam, H. J. Bibcode: 2015CoPhC.188....7G Altcode: 2014arXiv1406.1891G We investigate the achievable efficiency of both the time and the space discretisation methods used in Antares for mixed parabolic-hyperbolic problems. We show that the fifth order variant of WENO combined with a second order Runge-Kutta scheme is not only more accurate than standard first and second order schemes, but also more efficient taking the computation time into account. Then, we calculate the error decay rates of WENO with several explicit Runge-Kutta schemes for advective and diffusive problems with smooth and non-smooth initial conditions. With this data, we estimate the computational costs of three-dimensional simulations of stellar surface convection and show that SSP RK(3,2) is the most efficient scheme considered in this comparison. Title: Realistic simulations of stellar surface convection with ANTARES: I. Boundary conditions and model relaxation Authors: Grimm-Strele, H.; Kupka, F.; Löw-Baselli, B.; Mundprecht, E.; Zaussinger, F.; Schiansky, P. Bibcode: 2015NewA...34..278G Altcode: 2013arXiv1305.0743G We have implemented open boundary conditions into the ANTARES code to increase the realism of our simulations of stellar surface convection. Even though we greatly benefit from the high accuracy of our fifth order numerical scheme (WENO5) the broader stencils needed for the numerical scheme complicate the implementation of boundary conditions. We show that the effective temperature of a numerical simulation cannot be changed by corrections at the lower boundary since the thermal stratification does only change on the Kelvin-Helmholtz time scale. Except for very shallow models this time scale cannot be covered by multidimensional simulations due to the enormous computational requirements. We demonstrate to what extent numerical simulations of stellar surface convection are sensitive to the initial conditions and the boundary conditions. An ill-conceived choice of parameters for the boundary conditions can have a severe impact. Numerical simulations of stellar surface convection will only be (physically) meaningful and realistic if the initial model, the extent and position of the simulation box and the parameters from the boundary conditions are chosen adequately. Title: The PLATO 2.0 mission Authors: Rauer, H.; Catala, C.; Aerts, C.; Appourchaux, T.; Benz, W.; Brandeker, A.; Christensen-Dalsgaard, J.; Deleuil, M.; Gizon, L.; Goupil, M. -J.; Güdel, M.; Janot-Pacheco, E.; Mas-Hesse, M.; Pagano, I.; Piotto, G.; Pollacco, D.; Santos, Ċ.; Smith, A.; Suárez, J. -C.; Szabó, R.; Udry, S.; Adibekyan, V.; Alibert, Y.; Almenara, J. -M.; Amaro-Seoane, P.; Eiff, M. Ammler-von; Asplund, M.; Antonello, E.; Barnes, S.; Baudin, F.; Belkacem, K.; Bergemann, M.; Bihain, G.; Birch, A. C.; Bonfils, X.; Boisse, I.; Bonomo, A. S.; Borsa, F.; Brandão, I. M.; Brocato, E.; Brun, S.; Burleigh, M.; Burston, R.; Cabrera, J.; Cassisi, S.; Chaplin, W.; Charpinet, S.; Chiappini, C.; Church, R. P.; Csizmadia, Sz.; Cunha, M.; Damasso, M.; Davies, M. B.; Deeg, H. J.; Díaz, R. F.; Dreizler, S.; Dreyer, C.; Eggenberger, P.; Ehrenreich, D.; Eigmüller, P.; Erikson, A.; Farmer, R.; Feltzing, S.; de Oliveira Fialho, F.; Figueira, P.; Forveille, T.; Fridlund, M.; García, R. A.; Giommi, P.; Giuffrida, G.; Godolt, M.; Gomes da Silva, J.; Granzer, T.; Grenfell, J. L.; Grotsch-Noels, A.; Günther, E.; Haswell, C. A.; Hatzes, A. P.; Hébrard, G.; Hekker, S.; Helled, R.; Heng, K.; Jenkins, J. M.; Johansen, A.; Khodachenko, M. L.; Kislyakova, K. G.; Kley, W.; Kolb, U.; Krivova, N.; Kupka, F.; Lammer, H.; Lanza, A. F.; Lebreton, Y.; Magrin, D.; Marcos-Arenal, P.; Marrese, P. M.; Marques, J. P.; Martins, J.; Mathis, S.; Mathur, S.; Messina, S.; Miglio, A.; Montalban, J.; Montalto, M.; Monteiro, M. J. P. F. G.; Moradi, H.; Moravveji, E.; Mordasini, C.; Morel, T.; Mortier, A.; Nascimbeni, V.; Nelson, R. P.; Nielsen, M. B.; Noack, L.; Norton, A. J.; Ofir, A.; Oshagh, M.; Ouazzani, R. -M.; Pápics, P.; Parro, V. C.; Petit, P.; Plez, B.; Poretti, E.; Quirrenbach, A.; Ragazzoni, R.; Raimondo, G.; Rainer, M.; Reese, D. R.; Redmer, R.; Reffert, S.; Rojas-Ayala, B.; Roxburgh, I. W.; Salmon, S.; Santerne, A.; Schneider, J.; Schou, J.; Schuh, S.; Schunker, H.; Silva-Valio, A.; Silvotti, R.; Skillen, I.; Snellen, I.; Sohl, F.; Sousa, S. G.; Sozzetti, A.; Stello, D.; Strassmeier, K. G.; Švanda, M.; Szabó, Gy. M.; Tkachenko, A.; Valencia, D.; Van Grootel, V.; Vauclair, S. D.; Ventura, P.; Wagner, F. W.; Walton, N. A.; Weingrill, J.; Werner, S. C.; Wheatley, P. J.; Zwintz, K. Bibcode: 2014ExA....38..249R Altcode: 2014ExA...tmp...41R; 2013arXiv1310.0696R PLATO 2.0 has recently been selected for ESA's M3 launch opportunity (2022/24). Providing accurate key planet parameters (radius, mass, density and age) in statistical numbers, it addresses fundamental questions such as: How do planetary systems form and evolve? Are there other systems with planets like ours, including potentially habitable planets? The PLATO 2.0 instrument consists of 34 small aperture telescopes (32 with 25 s readout cadence and 2 with 2.5 s candence) providing a wide field-of-view (2232 deg 2) and a large photometric magnitude range (4-16 mag). It focusses on bright (4-11 mag) stars in wide fields to detect and characterize planets down to Earth-size by photometric transits, whose masses can then be determined by ground-based radial-velocity follow-up measurements. Asteroseismology will be performed for these bright stars to obtain highly accurate stellar parameters, including masses and ages. The combination of bright targets and asteroseismology results in high accuracy for the bulk planet parameters: 2 %, 4-10 % and 10 % for planet radii, masses and ages, respectively. The planned baseline observing strategy includes two long pointings (2-3 years) to detect and bulk characterize planets reaching into the habitable zone (HZ) of solar-like stars and an additional step-and-stare phase to cover in total about 50 % of the sky. PLATO 2.0 will observe up to 1,000,000 stars and detect and characterize hundreds of small planets, and thousands of planets in the Neptune to gas giant regime out to the HZ. It will therefore provide the first large-scale catalogue of bulk characterized planets with accurate radii, masses, mean densities and ages. This catalogue will include terrestrial planets at intermediate orbital distances, where surface temperatures are moderate. Coverage of this parameter range with statistical numbers of bulk characterized planets is unique to PLATO 2.0. The PLATO 2.0 catalogue allows us to e.g.: - complete our knowledge of planet diversity for low-mass objects, - correlate the planet mean density-orbital distance distribution with predictions from planet formation theories,- constrain the influence of planet migration and scattering on the architecture of multiple systems, and - specify how planet and system parameters change with host star characteristics, such as type, metallicity and age. The catalogue will allow us to study planets and planetary systems at different evolutionary phases. It will further provide a census for small, low-mass planets. This will serve to identify objects which retained their primordial hydrogen atmosphere and in general the typical characteristics of planets in such low-mass, low-density range. Planets detected by PLATO 2.0 will orbit bright stars and many of them will be targets for future atmosphere spectroscopy exploring their atmosphere. Furthermore, the mission has the potential to detect exomoons, planetary rings, binary and Trojan planets. The planetary science possible with PLATO 2.0 is complemented by its impact on stellar and galactic science via asteroseismology as well as light curves of all kinds of variable stars, together with observations of stellar clusters of different ages. This will allow us to improve stellar models and study stellar activity. A large number of well-known ages from red giant stars will probe the structure and evolution of our Galaxy. Asteroseismic ages of bright stars for different phases of stellar evolution allow calibrating stellar age-rotation relationships. Together with the results of ESA's Gaia mission, the results of PLATO 2.0 will provide a huge legacy to planetary, stellar and galactic science. Title: Improved Time Integration for WENO Methods in Astrophysical Applications Authors: Kupka, F.; Grimm–Strele, H.; Happenhofer, N.; Higueras, I.; Koch, O.; Muthsam, H. J. Bibcode: 2014ASPC..488..243K Altcode: Weighted essentially non-oscillatory methods are a powerful approach to discretize advection and pressure gradient terms in the hydrodynamical equations, since they yield higher effective resolution than traditional methods. But in some astrophysical problems, low Mach number flows have to be tackled or the flow may change from a low to a high Mach number flow, spatially or in time, and diffusion or radiative transfer can impose severe limitations on time steps allowed in explicit time integration methods. We provide a summary of new developments on semi-implicit time integration methods useful for astrophysical problems such as numerical simulations of stellar surfaces and envelopes, and the basic physical question of how to improve models of double-diffusive convection. We discuss several applications of these new methods. Title: The effects of rotation on a double-diffusive layer in a rotating spherical shell Authors: Blies, Patrick; Kupka, Friedrich; Zaussinger, Florian; Hollerbach, Rainer Bibcode: 2014arXiv1404.6086B Altcode: So far, numerical studies of double-diffusive layering in turbulent convective flows have neglected the effects of rotation. We undertake a first step into that direction by investigating how Coriolis forces affect a double-diffusive layer inside a rotating spherical shell. For this purpose we have run simulations in a parameter regime where these layers are expected to form and successively increased the rate of rotation with the result that fast rotation is found to have a similar stabilising effect on the overall convective flux as an increase of the stability ratio $R_{\rho}$ has in a non-rotating setup. We have also studied to what extent the regimes of rotational constraints suggested by King, Stellmach, and Buffett (2013) for rotation in the case of Rayleigh-Bénard convection are influenced by double-diffusive convection: their classification could also be applicable to the case of double-diffusive convection in a spherical shell if it is extended to be also a function of the stability ratio $R_{\rho}$. Furthermore, we examined the ratio of saline and thermal Nusselt numbers and compared our results with models of Spruit (2013), Rosenblum et al. (2011) and Wood, Garaud, and Stellmach (2013). We find our data to be fitted best by Spruit's model. Our result that fast rotation further decreases the convective transport, which is already lowered by double-diffusive convection, could play a major role for e.g. the modeling of the interior of some rapidly rotating giant planets, as gaseous giant planets have recently been proposed to be influenced by double-diffusive convection. Title: Curvilinear grids for WENO methods in astrophysical simulations Authors: Grimm-Strele, H.; Kupka, F.; Muthsam, H. J. Bibcode: 2014CoPhC.185..764G Altcode: 2013arXiv1308.3066G We investigate the applicability of curvilinear grids in the context of astrophysical simulations and WENO schemes. With the non-smooth mapping functions from Calhoun et al. (2008), we can tackle many astrophysical problems which were out of scope with the standard grids in numerical astrophysics. We describe the difficulties occurring when implementing curvilinear coordinates into our WENO code, and how we overcome them. We illustrate the theoretical results with numerical data. The WENO finite difference scheme works only for high Mach number flows and smooth mapping functions, whereas the finite volume scheme gives accurate results even for low Mach number flows and on non-smooth grids. Title: Pulsation - convection interaction Authors: Kupka, F.; Mundprecht, E.; Muthsam, H. J. Bibcode: 2014IAUS..301..177K Altcode: A lot of effort has been devoted to the hydrodynamical modelling of Cepheids in one dimension. While the recovery of the most basic properties such as the pulsational instability itself has been achieved already a long time ago, properties such as the observed double-mode pulsation of some objects and the red-edge of the classical instability strip and their dependence on metallicity have remained a delicate issue. The uncertainty introduced by adjustable parameters and further physical approximations introduced in one-dimensional model equations motivate an investigation based on numerical simulations which use the full hydrodynamical equations. In this talk, results from such two-dimensional numerical simulations of a short period Cepheid are presented. The importance of a carefully designed numerical setup, in particular of sufficient resolution and domain extent, is discussed. The problematic issue of how to reliably choose fixed parameters for the one-dimensional model is illustrated. Results from an analysis of the interaction of pulsation with convection are shown concerning the large-scale structure of the He ii ionization zone. We also address the influence of convection on the atmospheric structure. Considering the potential of hydrodynamical simulations and the wealth of ever improving observational data an outlook on possible future work in this field of research is given. Title: Multidimensional realistic modelling of Cepheid-like variables - I. Extensions of the ANTARES code Authors: Mundprecht, Eva; Muthsam, Herbert J.; Kupka, Friedrich Bibcode: 2013MNRAS.435.3191M Altcode: 2012arXiv1209.2952M; 2013MNRAS.tmp.2236M We have extended the ANTARES code to simulate the coupling of pulsation with convection in Cepheid-like variables in an increasingly realistic way, in particular in multidimensions, 2D at this stage. Present-day models of radially pulsating stars assume radial symmetry and have the pulsation-convection interaction included via model equations containing ad hoc closures and moreover parameters whose values are barely known. We intend to construct ever more realistic multidimensional models of Cepheids. In this paper, the first of a series, we describe the basic numerical approach and how it is motivated by physical properties of these objects which are sometimes more, sometimes less obvious. For the construction of appropriate models a polar grid comoving with the mean radial velocity has been introduced to optimize radial resolution throughout the different pulsation phases. The grid is radially stretched to account for the change of spatial scales due to vertical stratification and a new grid refinement scheme is introduced to resolve the upper, hydrogen ionization zone where the gradient of temperature is steepest. We demonstrate that the simulations are not conservative when the original weighted essentially non-oscillatory method implemented in ANTARES is used and derive a new scheme which allows a conservative time evolution. The numerical approximation of diffusion follows the same principles. Moreover, the radiative transfer solver has been modified to improve the efficiency of calculations on parallel computers. We show that with these improvements, the ANTARES code can be used for realistic simulations of the convection-pulsation interaction in Cepheids. We discuss the properties of several numerical models of this kind which include the upper 42 per cent of a Cepheid along its radial coordinate and assume different opening angles. The models are suitable for an in-depth study of convection and pulsation in these objects. Title: Semi-convection Authors: Zaussinger, F.; Kupka, F.; Muthsam, H. J. Bibcode: 2013LNP...865..219Z Altcode: Double-diffusive processes play an important role in various astrophysical and geophysical systems. Especially the case which considers a thermally unstable configuration stabilized by mean molecular weight and which is known as semi-convection has been studied for several decades. Whether a semi-convective region in a star should be treated layered or fully mixed has not yet been conclusively answered. However, in recent years numerical simulations have been used to investigate this fluid dynamical instability. With high resolution methods we can now develop a better understanding of this mixing process. For this review our intention is to present semi-convection from different points of view. At first, a summary of studies made during the last decades is used to demonstrate the continuous importance of semi- convection for stellar evolution. The physical process itself as well as linear stability criteria are explained subsequently. Finally, models, experiments, and the study through numerical simulations are discussed. Semi-convective mixing and related questions in stellar evolution and, recently, also in exoplanet science continue to be a vivid research field and have never ceased to surprise. Title: Total-variation-diminishing implicit-explicit Runge-Kutta methods for the simulation of double-diffusive convection in astrophysics Authors: Kupka, Friedrich; Happenhofer, Natalie; Higueras, Inmaculada; Koch, Othmar Bibcode: 2012JCoPh.231.3561K Altcode: 2011arXiv1106.5672K We put forward the use of total-variation-diminishing (or more generally, strong stability preserving) implicit-explicit Runge-Kutta methods for the time integration of the equations of motion associated with the semiconvection problem in the simulation of stellar convection. The fully compressible Navier-Stokes equation, augmented by continuity and total energy equations, and an equation of state describing the relation between the thermodynamic quantities, is semi-discretized in space by essentially non-oscillatory schemes and dissipative finite difference methods. It is subsequently integrated in time by Runge-Kutta methods which are constructed such as to preserve the total variation diminishing (or strong stability) property satisfied by the spatial discretization coupled with the forward Euler method. We analyse the stability, accuracy and dissipativity of the time integrators and demonstrate that the most successful methods yield a substantial gain in computational efficiency as compared to classical explicit Runge-Kutta methods. Title: Divisions Iv-V / Working Group ap & Related Stars Authors: Mathys, Gautier; Cunha, Margarida; Dworetsky, Michael; Kochukhov, Oleg; Kupka, Friedrich; LeBlanc, Francis; Monier, Richard; Paunzen, Ernst; Pintado, Olga; Piskunov, Nikolai; Ziznovsky, Jozef Bibcode: 2012IAUTA..28..203M Altcode: The purpose of the Working Group on Ap and Related Stars (ApWG) is to promote and facilitate research about stars in the spectral type range from B to early F that exhibit surface chemical peculiarities and related phenomena. This is a very active field of research, in which a wide variety of new developments have taken place since 2009, as illustrated by the following selected highlights. Title: Layered convection in double diffusive fluids Authors: Zaussinger, F.; Kupka, F.; Muthsam, H. J.; Happenhofer, N.; Grimm-Strele, H. Bibcode: 2012EGUGA..14.1830Z Altcode: Double diffusive convection plays an important role in astrophysics and oceanography where under certain conditions a thermally unstable temperature gradient is counteracted by a stable solute gradient. This configuration is well known from salt lakes, where the salt concentration stabilizes convective motions and a layered structure emerges. Similar conditions are found in stellar interiors, where helium as the stabilizing component inhibits the development of convection and the occurrence of double-diffusive staircases is assumed. We investigate mixing timescales and stability conditions using theoretical estimates and numerical simulations covering a broad range of parameter sets by varying Prandtl-, Lewis- and Rayleigh numbers. To shed light on the numerically inaccessible astrophysical case we extrapolate to the relevant parameter range. We investigate the initial layer formation process as well as the stability of evolved layers by performing direct numerical simulations in 2D and 3D using the Boussinesq approximation. A fitting formula for the Nusselt numbers and the effective mixing rates is given. Finally, we present a semi-implicit method to solve the compressible counterpart of the governing equations which has the advantage to cover the entire relevant Mach number range. Title: Simulations of stellar convection, pulsation and semiconvection Authors: Muthsam, Herbert J.; Kupka, Friedrich; Mundprecht, Eva; Zaussinger, Florian; Grimm-Strele, Hannes; Happenhofer, Natalie Bibcode: 2011IAUS..271..179M Altcode: 2010arXiv1009.2409M We report on modelling in stellar astrophysics with the ANTARES code. First, we describe properties of turbulence in solar granulation as seen in high-resolution calculations. Then, we turn to the first 2D model of pulsation-convection interaction in a cepheid. We discuss properties of the outer and the HeII ionization zone. Thirdly, we report on our work regarding models of semiconvection in the context of stellar physics. Title: Vamdc as a Resource for Atomic and Molecular Data and the New Release of Vald Authors: Kupka, F.; Dubernet, M. -L.; VAMDC Collaboration Bibcode: 2011BaltA..20..503K Altcode: 2011OAst...20..503K; 2012arXiv1201.0154K The Virtual Atomic and Molecular Data Centre (VAMDC) is an EU-FP7 project devoted to build a common electronic infrastructure for the exchange and distribution of atomic and molecular data. Within VAMDC scientists from many different disciplines in atomic and molecular physics collaborate with users of their data and also with scientists and engineers from the information and communication technology community. In this presentation, an overview of the current status of VAMDC and its capabilities is provided. In the second part of the presentation I will focus on one of the databases which have become a part of the VAMDC platform, the Vienna Atomic Line Data Base (VALD). VALD has developed into a well-known resource of atomic data for spectroscopy, particularly in astrophysics. A new release, VALD-3, will provide numerous improvements over its predecessor. This particularly relates to the data contents where new sets of atomic data for both precision spectroscopy (i.e., with data for observed energy levels) as well as opacity calculations (i.e., with data involving predicted energy levels) have been included. Data for selected diatomic molecules have been added, and a new system for data distribution and data referencing provides more convenience in using the third release of VALD. Title: Asteroseismology of solar-type stars with Kepler I: Data analysis Authors: Karoff, C.; Chaplin, W. J.; Appourchaux, T.; Elsworth, Y.; Garcia, R. A.; Houdek, G.; Metcalfe, T. S.; Molenda-Żakowicz, J.; Monteiro, M. J. P. F. G.; Thompson, M. J.; Christensen-Dalsgaard, J.; Gilliland, R. L.; Kjeldsen, H.; Basu, S.; Bedding, T. R.; Campante, T. L.; Eggenberger, P.; Fletcher, S. T.; Gaulme, P.; Handberg, R.; Hekker, S.; Martic, M.; Mathur, S.; Mosser, B.; Regulo, C.; Roxburgh, I. W.; Salabert, D.; Stello, D.; Verner, G. A.; Belkacem, K.; Biazzo, K.; Cunha, M. S.; Gruberbauer, M.; Guzik, J. A.; Kupka, F.; Leroy, B.; Ludwig, H. -G.; Mathis, S.; Noels, A.; Noyes, R. W.; Roca Cortes, T.; Roth, M.; Sato, K. H.; Schmitt, J.; Suran, M. D.; Trampedach, R.; Uytterhoeven, K.; Ventura, R. Bibcode: 2010AN....331..972K Altcode: 2010arXiv1005.0507K We report on the first asteroseismic analysis of solar-type stars observed by Kepler. Observations of three G-type stars, made at one-minute cadence during the first 33.5 days of science operations, reveal high signal-to-noise solar-like oscillation spectra in all three stars: About 20 modes of oscillation can clearly be distinguished in each star. We discuss the appearance of the oscillation spectra, including the presence of a possible signature of faculae, and the presence of mixed modes in one of the three stars. Title: VAMDC: The Virtual Atomic and Molecular Data Centre: a Service Orientated Data Infrastructure for e- Research Authors: Mason, N. J.; Dubernet, M. L.; Benson, K. M.; Bureau, J.; Heiter, U.; Kupka, F.; Le Sidaner, P.; Piskunov, N.; Rixon, G. T.; Schlemmer, S.; Shih, A.; Tennyson, J.; Walton, N.; Witherick, D. W. Bibcode: 2010epsc.conf..861M Altcode: No abstract at ADS Title: ANTARES - A Numerical Tool for Astrophysical RESearch with applications to solar granulation Authors: Muthsam, H. J.; Kupka, F.; Löw-Baselli, B.; Obertscheider, C.; Langer, M.; Lenz, P. Bibcode: 2010NewA...15..460M Altcode: 2009arXiv0905.0177M We discuss the general design of the ANTARES code which is intended for simulations in stellar hydrodynamics with radiative transfer and realistic microphysics in 1D, 2D and 3D. We then compare the quality of various numerical methods. We have applied ANTARES in order to obtain high resolution simulations of solar granulation which we describe and analyze. In order to obtain high resolution, we apply grid refinement to a region predominantly occupied by an exploding granule. Strong, rapidly rotating vortex tubes of small diameter (∼100km) generated by the downdrafts and ascending into the photosphere near the granule boundaries evolve, often entering the photosphere from below in an arclike fashion. They essentially contribute to the turbulent velocity field near the granule boundaries. Title: INTER-DIVISION IV-V WORKING GROUP on Ap and Related Stars Authors: Cunha, Margarida S.; Weiss, Werner; Dworetsky, Mike; Kochukhov, Oleg; Kupka, Friedrich; Leblanc, Francis; Monier, Richard; Paunzen, Ernst; Piskunov, Nikolai; Shibahashi, Hiromoto; Smalley, Barry; Ziznovsky, Jozef Bibcode: 2010IAUTB..27..205C Altcode: The business meeting started at 11h00, in the presence of 18 members, with a brief summary of the activities and achievements of the Working group during the triennium 2006-2009. Title: The Asteroseismic Potential of Kepler: First Results for Solar-Type Stars Authors: Chaplin, W. J.; Appourchaux, T.; Elsworth, Y.; García, R. A.; Houdek, G.; Karoff, C.; Metcalfe, T. S.; Molenda-Żakowicz, J.; Monteiro, M. J. P. F. G.; Thompson, M. J.; Brown, T. M.; Christensen-Dalsgaard, J.; Gilliland, R. L.; Kjeldsen, H.; Borucki, W. J.; Koch, D.; Jenkins, J. M.; Ballot, J.; Basu, S.; Bazot, M.; Bedding, T. R.; Benomar, O.; Bonanno, A.; Brandão, I. M.; Bruntt, H.; Campante, T. L.; Creevey, O. L.; Di Mauro, M. P.; Doǧan, G.; Dreizler, S.; Eggenberger, P.; Esch, L.; Fletcher, S. T.; Frandsen, S.; Gai, N.; Gaulme, P.; Handberg, R.; Hekker, S.; Howe, R.; Huber, D.; Korzennik, S. G.; Lebrun, J. C.; Leccia, S.; Martic, M.; Mathur, S.; Mosser, B.; New, R.; Quirion, P. -O.; Régulo, C.; Roxburgh, I. W.; Salabert, D.; Schou, J.; Sousa, S. G.; Stello, D.; Verner, G. A.; Arentoft, T.; Barban, C.; Belkacem, K.; Benatti, S.; Biazzo, K.; Boumier, P.; Bradley, P. A.; Broomhall, A. -M.; Buzasi, D. L.; Claudi, R. U.; Cunha, M. S.; D'Antona, F.; Deheuvels, S.; Derekas, A.; García Hernández, A.; Giampapa, M. S.; Goupil, M. J.; Gruberbauer, M.; Guzik, J. A.; Hale, S. J.; Ireland, M. J.; Kiss, L. L.; Kitiashvili, I. N.; Kolenberg, K.; Korhonen, H.; Kosovichev, A. G.; Kupka, F.; Lebreton, Y.; Leroy, B.; Ludwig, H. -G.; Mathis, S.; Michel, E.; Miglio, A.; Montalbán, J.; Moya, A.; Noels, A.; Noyes, R. W.; Pallé, P. L.; Piau, L.; Preston, H. L.; Roca Cortés, T.; Roth, M.; Sato, K. H.; Schmitt, J.; Serenelli, A. M.; Silva Aguirre, V.; Stevens, I. R.; Suárez, J. C.; Suran, M. D.; Trampedach, R.; Turck-Chièze, S.; Uytterhoeven, K.; Ventura, R.; Wilson, P. A. Bibcode: 2010ApJ...713L.169C Altcode: 2010arXiv1001.0506C We present preliminary asteroseismic results from Kepler on three G-type stars. The observations, made at one-minute cadence during the first 33.5 days of science operations, reveal high signal-to-noise solar-like oscillation spectra in all three stars: about 20 modes of oscillation may be clearly distinguished in each star. We discuss the appearance of the oscillation spectra, use the frequencies and frequency separations to provide first results on the radii, masses, and ages of the stars, and comment in the light of these results on prospects for inference on other solar-type stars that Kepler will observe. Title: Effects of resolution and helium abundance in A star surface convection simulations Authors: Kupka, F.; Ballot, J.; Muthsam, H. J. Bibcode: 2009CoAst.160...30K Altcode: 2009arXiv0905.0070K We present results from 2D radiation-hydrodynamical simulations of fully compressible convection for the surface layers of A-type stars with the ANTARES code. Spectroscopic indicators for photospheric convective velocity fields show a maximum of velocities near Teff ∼ 8000 K with the largest values measured for the subgroup of Am stars. Thus far, no prognostic model, neither theoret ical nor numerical, is able to exactly reproduce the line profiles of sharp line A and Am stars in that temperature range. As the helium abundance of A stars is not known from observations, we have considered two extreme cases for our simulations: a solar helium abundance as an upper limit and zero helium abundance as a lower limit. The simulation for the helium free case is found to differ from the case with solar helium abundance by larger velocities, larger flow structures, and by a sign reversal of the flux of kinetic energy inside the hydrogen ionisation zone. Both simulations show extended shock fronts emerg- ing from the optical surface, as well as mixing far below the region of partial ionisation of hydrogen, and vertical oscillations emerging after initial perturba- tions have been damped. We discuss problems related to the rapid radiative cooling at the surface of A-type stars such as resolution and efficient relax- ation. This includes identifying the radiative cooling rate Qrad , which poses the most severe time step limitation for the dynamical equation for the evolution of energy density, as a stiff term. It is thus a numerical rather than a physical constraint for the numerical simulation of these objects. The present work is considered as a step towards a systematic study of convection in A- to F-type stars, encouraged by the new data becoming available for these objects from both asteroseismological missions and from high resolution spectroscopy. Title: Atmospheric velocity fields in tepid main sequence stars Authors: Landstreet, J. D.; Kupka, F.; Ford, H. A.; Officer, T.; Sigut, T. A. A.; Silaj, J.; Strasser, S.; Townshend, A. Bibcode: 2009A&A...503..973L Altcode: 2009arXiv0906.3824L Context: The line profiles of the stars with {ve sin i} below a few km s-1 can reveal direct signatures of local velocity fields such as convection in stellar atmospheres. This effect is well established in cool main sequence stars, and has been detected and studied in three A stars.
Aims: This paper reports observations of main sequence B, A and F stars (1) to identify additional stars with sufficiently low values of {ve sin i} to search for spectral line profile signatures of local velocity fields and (2) to explore how the signatures of the local velocity fields in the atmosphere depend on stellar parameters such as effective temperature and peculiarity type.
Methods: We have carried out a spectroscopic survey of B and A stars of low {ve sin i} at high resolution. Comparison of model spectra with those observed allows us to detect signatures of the local velocity fields such as asymmetric excess line wing absorption, best-fit {ve sin i} parameter values that are found to be larger for strong lines than for weak lines, and discrepancies between observed and modelled line profile shapes.
Results: Symptoms of local atmospheric velocity fields are always detected through a non-zero microturbulence parameter for main sequence stars having {Te} below about 10 000 K, but not for hotter stars. Direct line profile tracers of the atmospheric velocity field are found in six very sharp-lined stars in addition to the three reported earlier. Direct signatures of velocity fields are found to occur in A stars with and without the Am chemical peculiarities, although the amplitude of the effects seems larger in Am stars. Velocity fields are also directly detected in spectral line profiles of A and early F supergiants, but without significant line asymmetries.
Conclusions: We confirm that several atmospheric velocity field signatures, particularly excess line wing absorption which is stronger in the blue line wing than in the red, are detectable in the spectral lines of main sequence A stars of sufficiently low {ve sin i}. We triple the sample of A stars known to show these effects, which are found both in Am and normal A stars. We argue that the observed line distortions are probably due to convective motions reaching the atmosphere. These data still have not been satisfactorily explained by models of atmospheric convection, including numerical simulations.

Based in part on observations obtained at the

Canada-France-Hawaii Telescope (CFHT) which is operated by the

National Research Council of Canada, the Institut National des

Sciences de l'Univers of the Centre National de la Recherche

Scientifique of France, and the University of

Hawaii.

Based in part on observations made at

Observatoire de Haute Provence (CNRS), France. Title: Using p-mode excitation rates for probing convection in solar-like stars Authors: Kupka, F.; Belkacem, K.; Goupil, J. -M.; Samadi, R. Bibcode: 2009CoAst.159...24K Altcode: We discuss how the possibility to measure mode excitation rates through means of helio- and asteroseismology has improved our capabilities to test convection models and numerical simulations of surface convection and avoids ambiguities that have limited previous approaches. Title: 3D stellar atmospheres for stellar structure models and asteroseismology. Authors: Kupka, F. Bibcode: 2009MmSAI..80..701K Altcode: 2009arXiv0911.3353K Convection is the most important physical process that determines the structure of the envelopes of cool stars. It influences the surface radiation flux and the shape of observed spectral line profiles and is responsible for both generating and damping solar-like oscillations, among others. 3D numerical simulations of stellar surface convection have developed into a powerful tool to model and analyse the physical mechanisms operating at the surface of cool stars. This review discusses the main principles of 3D stellar atmospheres used for such applications. The requirements from stellar structure and evolution theory to use them as boundary conditions are analysed as well as the capabilities of using helio- and asteroseismology to reduce modelling uncertainties and probing the consistency and accuracy of 3D stellar atmospheres as part of this process. Simulations for the solar surface made by different teams are compared and some issues concerning the uncertainties of this modelling approach are discussed. Title: An Introduction to Turbulence Authors: Hillebrandt, W.; Kupka, F. Bibcode: 2009LNP...756....1H Altcode: No abstract at ADS Title: Turbulent Convection and Numerical Simulations in Solar and Stellar Astrophysics Authors: Kupka, F. Bibcode: 2009LNP...756...49K Altcode: No abstract at ADS Title: Interdisciplinary Aspects of Turbulence Authors: Hillebrandt, Wolfgang; Kupka, Friedrich Bibcode: 2009LNP...756.....H Altcode: No abstract at ADS Title: Inter-Division IV-V / Working Group Ap and Related Stars Authors: Cunha, Margarida S.; Weiss, Werner W.; Dworetsky, Michael M.; Kochukhov, Oleg; Kupka, Friedrich; Leblanc, Francis; Monier, Richard; Paunzen, Ernst; Piskunov, Nikolai E.; Shibahashi, Hiromoto; Smalley, Barry; Ziznovsky, Jozef Bibcode: 2009IAUTA..27..245C Altcode: The diversity of physical phenomena embraced by the study of Chemically Peculiar (CP) stars results in an associated research community with interests that are equally diverse. This fact became once more evident during the CP#Ap Workshop that took place in Vienna (Austria) in September 2007, and which gathered over 80 members of this research community. Besides the excellent scientific outcome of the meeting, during the workshop the community had the opportunity to discuss its organization and plans for the future. Following on those plans, the Working Group has submitted a proposal for a Joint Discussion during the IAU XXVII General Assembly, in Rio de Janeiro, which has meanwhile been accepted. Moreover, through an ApN newsletter forum, the Working Group has compiled requests from the community concerning atomic and related data. These requests have been put together and will be shared with Commission 14. Title: Vald Authors: Obbrugger, M.; Heiter, U.; Kupka, F.; Lüftinger, T.; Nesvacil, N.; Piskunov, N.; Ryabchikova, T. A.; Stempels, H. C.; Stütz, Ch.; Weiss, W. W. Bibcode: 2008asvo.proc..215O Altcode: VALD is a collection of atomic transition parameters and supporting extraction software. VALD services are available via Email (VALD-EMS) and the Web interface. Different kinds of requests are useful for several needs like abundance analysis, radial velocity measurements, or line identification. Since 1994, the early days of VALD, the database has been constantly improved and the release of VALD-3 - incorporating, e.g., molecular data and new line lists - is in preparation. With meanwhile more than 5 000 000 requests VALD developed to a much appreciated tool. Title: Analysing the Contributions in Moment Equations of Reynolds Stress Models of Convection with Numerical Simulations Authors: Kupka, F.; Muthsam, H. J. Bibcode: 2008IAUS..252..463K Altcode: We discuss how 3D numerical simulations can be used to analyse the different contributions within dynamical equations of non-local Reynolds stress models of convection. Title: Shear Driven Turbulence and Coherent Structures in Solar Surface Simulations Authors: Kupka, F. Bibcode: 2008IAUS..252..451K Altcode: Numerical simulations of convection near the solar surface are now advanced enough to reproduce both a large set of observational data and provide tests for convection models. We discuss the role of coherent structures in models of solar p-mode excitation, for which the analysis of numerical simulations has provided key inputs in the modelling. The robustness of these simulations is shown by a comparison illustrating the influence of boundary conditions on ensemble averaged quantities. In a concluding example advanced high resolution simulations are shown to resolve the onset of shear driven turbulence generated by up- and downflow structures. Title: Modeling the excitation of acoustic modes in α Centauri A Authors: Samadi, R.; Belkacem, K.; Goupil, M. J.; Dupret, M. -A.; Kupka, F. Bibcode: 2008A&A...489..291S Altcode: 2008arXiv0806.0725S From different seismic observations we infer the energy supplied per unit of time by turbulent convection to the acoustic modes of α Centauri A (HD 128620), a star that is similar but not identical to the Sun. The inferred rates of energy supplied to the modes (i.e. mode excitation rates) are found to be significantly higher than in the Sun. They are compared with those computed with an excitation model that includes two sources of driving, the Reynolds stress contribution and the advection of entropy fluctuations. The model also uses a closure model, the Closure Model with Plumes (CMP hereafter), that takes the asymmetry between the up- and down-flows (i.e. the granules and plumes, respectively) into account. Different prescriptions for the eddy-time correlation function are also compared to observational data. Calculations based on a Gaussian eddy-time correlation underestimate excitation rates compared with the values derived from observations for α Centauri A. On the other hand, calculations based on a Lorentzian eddy-time correlation lie within the observational error bars. This confirms results in the solar case. Compared to the helioseismic data, those obtained for α Centauri A constitute an additional support for our model of excitation. We show that mode masses must be computed taking turbulent pressure into account. Finally, we emphasize the need for more accurate seismic measurements in order to distinguish between the CMP closure model and the quasi-normal approximation in the case of α Centauri A, as well as to confirm or not the need to include the excitation by the entropy fluctuations. Title: VALD — an atomic and molecular database for astrophysics Authors: Heiter, U.; Barklem, P.; Fossati, L.; Kildiyarova, R.; Kochukhov, O.; Kupka, F.; Obbrugger, M.; Piskunov, N.; Plez, B.; Ryabchikova, T.; Stempels, H. C.; Stütz, Ch; Weiss, W. W. Bibcode: 2008JPhCS.130a2011H Altcode: The VALD database of atomic and molecular data aims to ensure a robust and consistent analysis of astrophysical spectra. We offer a convenient e-mail and web-based user interface to a vast collection of spectral line parameters for all chemical elements and in the future also for molecules. An international team is working on the following tasks: collecting line parameters from relevant theoretical and experimental publications, computing line parameters, evaluating the data quality by comparison of similar data from different sources and by comparison with astrophysical observations, and incorporating the data into VALD. A unique feature of VALD is its capability to provide the most comprehensive spectral line lists for specific astrophysical plasma conditions defined by the user. Title: High-resolution models of solar granulation: the two-dimensional case Authors: Muthsam, H. J.; Löw-Baselli, B.; Obertscheider, Chr.; Langer, M.; Lenz, P.; Kupka, F. Bibcode: 2007MNRAS.380.1335M Altcode: 2007arXiv0706.3349M; 2007MNRAS.tmp..777M Using advanced numerical schemes and grid refinement, we present 2D high-resolution models of solar granulation with particular emphasis on downflowing plumes. In the high-resolution portion of our simulation, a box measuring 1.97 × 2.58Mm2 (vertical × horizontal), the grid size is 1.82 × 2.84km2. Calculations at the resolution usually applied in this type of simulations amount to only a few horizontal gridpoints for a downflowing plume. Due to the increased number of gridpoints in our high-resolution domain, the simulations show the development of vigorous secondary instabilities of both the plume's head and stem. The plume's head produces counterrotating vortex patches, a topology due to the 2D nature of the simulations. Below a depth of about 1Mm, the plume's head and stem instabilities produce, in these 2D models, patches of low density, temperature, pressure and high vorticity which may last for all of our simulation time, ~10min, and probably considerably longer. Centrifugal forces acting in these patches counteract the strong inward pressure. Probably most importantly, the plume's instabilities give rise to acoustic pulses created predominantly down to ~1.5Mm. The pulses proceed laterally as well as upwards and are ubiquitous. Ultimately, most of them emerge into the photosphere. A considerable part of the photospheric `turbulence' in these models is due to those pulses rather than to some sort of eddies. The upflows in granules are smooth where they reach the photosphere from below even in the present calculations; however, the pulses may enter in the photosphere also in granular upflows. Title: Eclipsing binaries as a test for synthetic photometry Authors: Heiter, U.; Smalley, B.; Stütz, Ch.; Kupka, F.; Kochukhov, O. Bibcode: 2007IAUS..240..328H Altcode: Narrow band photometry is a viable tool to characterize large numbers of stars. The connection between observed colors and astrophysical parameters has to rely on synthetic photometry calculated from stellar atmosphere models. Here, we present synthetic Hbeta indices calculated from 1D model atmospheres, which implement various treatments of convection. The calculated indices are transformed to the standard system using observed medium-resolution spectra from recently published stellar libraries. We test how well the synthetic photometry reproduces observed indices by using a number of eclipsing binary systems. For these stars, atmospheric parameters can be determined independently from the models with highest possible accuracy. As a preliminary conclusion, the computed indices deviate from the observed ones by an amount expected from the observational errors and the accuracy of the atmospheric parameters. Title: Convection in Astrophysics (IAU S239) Authors: Kupka, Friedrich; Roxburgh, Ian; Chan, Kwing Lam Bibcode: 2007IAUS..239.....K Altcode: Preface; Session A. Modelling convection and radiative transfer; Session B. Observational probes of convection; Session C. Convection in planets and brown dwarfs; Session D. Stellar evolution, nucleosynthesis and convective mixing; Session E. Oscillations, mass loss and convection; Session F. Convection and rotation; Session G. MHD convection and dynamos. Title: Two-scale mass-flux closure models for turbulence: p-mode amplitudes in solar-like stars Authors: Belkacem, K.; Samadi, R.; Goupil, M. -J.; Kupka, F.; Dupret, M. -A. Bibcode: 2007CoAst.150..153B Altcode: A new closure model has been developed, which takes into account both the skewness of the velocity distribution induced by the presence of two flows in the convection zone, and the effects of turbulence onto each flow (Belkacem et al. 2006a). Applied to the formalism of p-mode excitation, it has been possible to validate this theoretical model by a comparison with the observational excitation rates in the solar case using GOLF data (see Belkacem et al. 2006b). The next step is to consider α Cen A for which observations of the mode-damping rates are available. Title: Reynolds stress models of convection in convective cores Authors: Roxburgh, Ian W.; Kupka, Friedrich Bibcode: 2007IAUS..239...77R Altcode: No abstract at ADS Title: Probing Reynolds stress models of convection with numerical simulations: I. Overall properties: fluxes, mean profiles Authors: Kupka, F.; Muthsam, H. J. Bibcode: 2007IAUS..239...80K Altcode: No abstract at ADS Title: Mixing length model of convection in stellar cores Authors: Roxburgh, Ian W.; Kupka, Friedrich Bibcode: 2007IAUS..239...98R Altcode: No abstract at ADS Title: Some open questions concerning the modelling of non-locality in Reynolds stress type models of stellar convection. Authors: Kupka, F. Bibcode: 2007IAUS..239...92K Altcode: No abstract at ADS Title: Coherent structures in granulation convection and their importance for higher order closure models Authors: Kupka, F.; Robinson, F. J. Bibcode: 2007IAUS..239...74K Altcode: No abstract at ADS Title: A closure model for turbulent convection. Application to the excitation of p modes Authors: Belkacem, K.; Samadi, R.; Goupil, M. -J.; Kupka, F.; Baudin, F. Bibcode: 2007IAUS..239..376B Altcode: No abstract at ADS Title: Eclipsing binaries as a test for synthetic photometry Authors: Heiter, U.; Smalley, B.; Stütz, Ch.; Kupka, F.; Kochukhov, O. Bibcode: 2007IAUS..239..169H Altcode: No abstract at ADS Title: Modelling of solar granulation Authors: Muthsam, H. J.; Löw-Baselli, B.; Obertscheider, Chr.; Langer, M.; Lenz, P.; Kupka, F. Bibcode: 2007IAUS..239...89M Altcode: No abstract at ADS Title: Probing Reynolds stress models of convection with numerical simulations: II. Non-locality and third order moments. Authors: Kupka, F.; Muthsam, H. J. Bibcode: 2007IAUS..239...83K Altcode: No abstract at ADS Title: Solar-like oscillation amplitudes and line-widths as a probe for turbulent convection in stars Authors: Samadi, R.; Belkacem, K.; Goupil, M. -J.; Kupka, F.; Dupret, M. -A. Bibcode: 2007IAUS..239..349S Altcode: 2006astro.ph.11760S Excitation of solar-like oscillations is attributed to turbulent convection and takes place at the upper-most part of the outer convective zones. Amplitudes of these oscillations depend on the efficiency of the excitation processes as well as on the properties of turbulent convection. We present past and recent improvements on the modeling of those processes. We show how the mode amplitudes and mode line-widths can bring information about the turbulence in the specific cases of the Sun and Alpha Cen A. Title: Probing Reynolds stress models of convection with numerical simulations: III. Compressibility modelling and dissipation Authors: Kupka, F.; Muthsam, H. J. Bibcode: 2007IAUS..239...86K Altcode: No abstract at ADS Title: Effect of the microturbulence parameter on the Color-Magnitude Diagram Authors: Montalbán, J.; Nendwich, J.; Heiter, U.; Kupka, F.; Paunzen, E.; Smalley, B. Bibcode: 2007IAUS..239..166M Altcode: No abstract at ADS Title: Round table discussion of session A: modelling convection and radiative transfer Authors: Kupka, F. Bibcode: 2007IAUS..239...64K Altcode: No abstract at ADS Title: On the effects of coherent structures on higher order moments in models of solar and stellar surface convection Authors: Kupka, F.; Robinson, F. J. Bibcode: 2007MNRAS.374..305K Altcode: Non-local models of stellar convection usually rely on the assumption that the transfer of convective heat flux, turbulent kinetic energy and related quantities can be described as a diffusion process or that the fourth-order moments of velocity and temperature fluctuations follow a Gaussian distribution (quasi-normal approximation). The latter is also assumed in models of solar p-mode excitation.

We have used realistic numerical simulations of solar granulation and of granulation in a K dwarf to test the quasi-normal approximation and several alternatives. For the superadiabatic layer of the Sun and for the quasi-adiabatic zone underneath, we find that the hypothesis of quasi-normality is a rather poor approximation. In the superadiabatic layer, it overestimates some of the fourth-order moments of vertical velocity and temperature by up to a factor of 2 while it underestimates them in the quasi-adiabatic layers by up to a factor of 3.5. The model proposed by Gryanik & Hartmann and Gryanik et al. reduces the discrepancies within the quasi-adiabatic zone to typically less than 30 per cent and is partially comparable and partially in better agreement with the simulation data than two earlier models by Grossman & Narayan. Simulation data for the K dwarf confirm these results. However, none of the proposed models works well in the superadiabatic layer nor in the photospheric layers above. For the Sun, we provide evidence that the fourth-order moments of horizontal velocity fields can be estimated to within about 30 per cent with the quasi-normal approximation despite the complexity of the horizontal flow. Comparing our results to those from solar simulations with idealized microphysics and with related studies of geophysical convection zones confirms our conclusions about the quasi-normal approximation and the new models.

The improvements come from including the effects of coherent structures (such as granules or plumes), while the limitations are tied to the transition regions or boundaries such as the rapid radiative cooling that occurs at the top of the convection zone. Incorporating the model of Gryanik & Hartmann and Gryanik et al. into non-local convection models may well produce a significant improvement in the modelling of convection or of solar-like p-mode excitation in the quasi-adiabatic part of convection zones. For application to entire convection zones, modifications are necessary which can account for the change in background properties of the convective medium near boundaries or transition regions. Title: A closure model with plumes. I. The solar convection Authors: Belkacem, K.; Samadi, R.; Goupil, M. J.; Kupka, F. Bibcode: 2006A&A...460..173B Altcode: 2006astro.ph..7568B Context: .Oscillations of stellar p modes, excited by turbulent convection, are investigated. In the uppermost part of the solar convection zone, radiative cooling is responsible for the formation of turbulent plumes, hence the medium is modelled with downdrafts and updrafts.
Aims: .We take into account the asymmetry of the up- and downflows created by turbulent plumes through an adapted closure model. In a companion paper, we apply it to the formalism of excitation of solar p modes developed by Samadi & Goupil (2001).
Methods: .Using results from 3D numerical simulations of the uppermost part of the solar convection zone, we show that the two-scale mass-flux model (TFM) is valid only for quasi-laminar or highly skewed flows (Gryanik & Hartmann 2002) and does not reproduce turbulent properties of the medium such as velocity-correlation products. We build a generalized two-scale mass-flux Model (GTFM) model that takes both the skew introduced by the presence of two flows and the effects of turbulence in each flow into account. In order to apply the GTFM to the solar case, we introduce the plume dynamics as modelled by Rieutord & Zahn (1995) and construct a closure model with plumes (CMP).
Results: .The CMP enables expressing the third- and fourth-order correlation products in terms of second-order ones. When compared with 3D simulation results, the CMP improves the agreement for the fourth-order moments by a factor of two approximately compared with the use of the quasi-normal approximation or a skewness computed with the classical TFM.
Conclusions: .The asymmetry of turbulent convection in the solar case has an important impact on the vertical-velocity fourth-order moment, which has to be accounted for by models. The CMP is a significant improvement and is expected to improve the modelling of solar p-mode excitation. Title: A closure model with plumes. II. Application to the stochastic excitation of solar p modes Authors: Belkacem, K.; Samadi, R.; Goupil, M. J.; Kupka, F.; Baudin, F. Bibcode: 2006A&A...460..183B Altcode: 2006astro.ph..7570B Context: . Amplitudes of stellar p modes result from a balance between excitation and damping processes taking place in the uppermost part of convective zones in solar-type stars and can therefore be used as a seismic diagnostic for the physical properties of these external layers.
Aims: . Our goal is to improve the theoretical modelling of stochastic excitation of p modes by turbulent convection.
Methods: . With the help of the closure model with plume (CMP) developed in a companion paper, we refine the theoretical description of the excitation by the turbulent Reynolds stress term. The CMP is generalized for two-point correlation products so as to apply it to the formalism developed by Samadi & Goupil (2001, A&A, 370, 136). The excitation source terms are then computed with this improvement, and a comparison with solar data from the GOLF instrument is performed.
Results: .The present model provides a significant improvement when comparing absolute values of theoretical amplitudes with observational data. It gives rise to a frequency dependence of the power supplied to solar p modes, which agrees with GOLF observations. It is shown that the asymmetry of the turbulent convection zone (up and downflows) plays a major role in the excitation processes. Despite an increase in the Reynolds stress term contribution due to our improved description, an additional source of excitation, identified as the entropy source term, is still necessary for reproducing the observational data.
Conclusions: .Theoretical excitation rates in the frequency range ν in [2.5 mHz, 4 mHz] now are in agreement with the observational data from the GOLF instrument. However, at lower frequencies, it exhibits small discrepancies at the maximum level of a few per cent. Improvements are likely to come from a better physical description of the excitation by entropy fluctuations in the superadiabatic zone. Title: Seismic diagnostics inferred from the amplitudes of stochastically-excited modes Authors: Samadi, R.; Belkacem, K.; Goupil, M. -J.; Kupka, F. Bibcode: 2006IAUJD..17E..14S Altcode: Solar-like oscillations have been detected in numerous main-sequence stars as well as in red giant stars. The excitation of such oscillations is ensured by turbulent convection at the upper most part of the convective zones. From the measurement of the mode amplitude and line-width it is possible to infer the power supplied to the modes by turbulent convection. Apart from the Sun, α Cen A is the only star for which both mode amplitudes and mode line-widths have been derived. I will first present the diagnostics inferred from the measurement of the amplitudes and line-widths in the Sun and in α Cen A. For other stars, despite the lack of direct constraints on the excitation mechanisms, I will show that the solar-like oscillations observed in different stars can nevertheless offer interesting constraints on the properties of turbulent convection. Title: Excitation of solar p modes. Effect of the asymmetry of the convection zone Authors: Belkacem, K.; Samadi, R.; Goupil, M. J.; Kupka, F.; Baudin, F. Bibcode: 2006sf2a.conf..523B Altcode: Excitation of stellar p modes by turbulent convection is investigated. The aim is to take into account the asymmetry of up- and downflows created by turbulent plumes through an adapted closure model. We built a generalized two scale mass flux model (GTFM) that includes both the skew introduced by the presence of two flows and the effect of turbulence within each flow. The plume dynamics modelled according to Rieutord & Zahn (1995) is used to construct a closure model with plumes (CMP). We apply it to the formalism of excitation of stellar p modes developed by Samadi & Goupil (2001). The new excitation model leads to a frequency dependence, of the power supplied to solar p modes, which is in agreement with GOLF observations. Despite an increase of the Reynolds stress contribution due to our improved description, an additional source of excitation -identified as the entropy source term- is still necessary to reproduce the maximum of excitation rate. Our modelling including the entropy contribution reproduces the maximum but

over-estimates, at low frequencies, the power and calls for further theoretical improvements. Title: Prospects for population synthesis in the H band: NeMo grids of stellar atmospheres compared to observations Authors: Frémaux, J.; Kupka, F.; Boisson, C.; Joly, M.; Tsymbal, V. Bibcode: 2006A&A...449..109F Altcode: 2005astro.ph.11125F Context: .For applications in population synthesis, libraries of theoretical stellar spectra are often considered an alternative to template libraries of observed spectra, because they allow a complete sampling of stellar parameters. Most of the attention in published theoretical spectral libraries has been devoted to the visual wavelength range.Aims.The goal of the present work is to explore the near-infrared range where few observed fully calibrated spectra and no theoretical libraries are available.Methods.We make a detailed comparison of theoretical spectra in the range 1.57-1.67 μm for spectral types from A to early M and for giant and dwarf stars, with observed stellar spectra at resolutions around 3000, which would be sufficient to disentangle the different groups of late-type stars. We selected the NeMo grids of stellar atmospheres to perform this comparison.Results.We first demonstrate that observed spectral flux distributions can be matched very well with theoretical ones for almost the entire parameter range covered by the NeMo grids at moderate resolution in the visual range. In the infrared range, although the overall shape of the observed flux distributions still matches reasonably well, the individual spectral features are reproduced by the theoretical spectra only for stars earlier than mid F type. For later spectral types the differences increase, and theoretical spectra of K type stars have systematically weaker line features than those found in observations. These discrepancies are traced back to stem primarily from incomplete data on neutral atomic lines, although some of them are also related to molecules.Conclusions.Libraries of theoretical spectra for A to early M type stars can be successfully used in the visual regions for population synthesis, but their application in the infrared is restricted to early and intermediate type stars. Improving atomic data in the near infrared is a key element in making the construction of reliable libraries of stellar spectra feasible in the infrared. Title: Limb-darkening coefficients for the purpose of pulsation mode identification for A-F stars. . Authors: Barban, C.; Goupil, M. J.; van't Veer-Menneret, C.; Garrido, R.; Heiter, U.; Kupka, F. Bibcode: 2006MmSAI..77..101B Altcode: Limb-darkening coefficients are computed from a set of model atmospheres with: a solar chemical composition, 6000 K< Teff < 8500 K (Delta T_eff=250 K), 2.5 < logg < 4.5 (Delta log g=0.1) and a microturbulent velocity of 2 km/s. Convection is included assuming either the turbulent convection approach of \citet{cm} or the classical mixing length prescription with alpha =0.5 and 1.25. Four limb-darkening laws have been used: quadratic, cubic, square root and the one of \citet{cl}. We compare the ATLAS 9 intensities and the ones computed from these laws. We find that Claret's law is the best law for almost all the models, independently of the convection prescription used. Title: Influence of local treatments of convection upon solar p mode excitation rates Authors: Samadi, R.; Kupka, F.; Goupil, M. J.; Lebreton, Y.; van't Veer-Menneret, C. Bibcode: 2006A&A...445..233S Altcode: 2005astro.ph..7243S We compute the rates P at which acoustic energy is injected into the solar radial p modes for several solar models. The solar models are computed with two different local treatments of convection: the classical mixing-length theory (MLT) and the formulation by Canuto et al. (1996, ApJ, 473, 550, CGM). Among the models investigated here, our best models reproduce both (i) the solar radius and the solar luminosity at solar age and (ii) the observed Balmer line profiles. For the MLT treatment, the rates P do significantly depend on the properties of the atmosphere, whereas for the CGM treatment, the dependence of P on the properties of the atmosphere is found to be smaller than the error bars attached to the seismic measurements. The excitation rates P for modes associated with the MLT models are significantly underestimated compared with the solar seismic constraints. The CGM models yield values for P closer to the seismic data than do the MLT models. We conclude that the solar p-mode excitation rates provide valuable constraints and, according to the present investigation, clearly favor the CGM treatment with respect to the MLT, although neither of them yields values of P as close to the observations as recently found for 3D numerical simulations. Title: Workshop on "Interdisciplinary Aspects of Turbulence" Authors: Kupka, F.; Hillebrandt, W. Bibcode: 2005iat..work.....K Altcode: The Workshop on Interdisciplinary Aspects of Turbulence was organized within the framework of the Max-Planck-Institute for Astrophysics in Garching, Germany, and held at Ringberg Castle. It brought together researchers from the fields of Astrophysics, Atmospheric and Ocean Physics, Engineering and Laboratory Turbulence Research, as well as Non-linear Dynamics and Statistical Mechanics who share a common interest in turbulence and its properties of mixing, self-organisation of large scale structures, and related issues and the interdisciplinary aspects underlying these topics. The proceedings volume contains a collection of extended abstracts as well as short review papers based on 30 from a total of 40 contributions presented at the workshop as well as links and references to further material discussed at the workshop. Title: Turbulent convection in astrophysics and geophysics - a comparison Authors: Kupka, F. Bibcode: 2005iat..work..141K Altcode: Energy transport by turbulent convection is studied in both astrophysics and geophysics. We briefly outline the physics of convection as well as the differences between astrophysical and geophysical turbulent convective flows. The case of convection in stars including our sun is described in more detail. Reynolds stress models have been suggested for quantitative predictions for both stellar and geophysical convection. One of the main problems of such models is how to account for the influence of the observed large scale coherent structures. We discuss a possible solution which has first been suggested for the convective planetary boundary layer and show how it performs when applied to convection in our sun. Title: Spectrum synthesis of sharp-line B, A and F stars Authors: Silaj, J.; Townshend, A.; Kupka, F.; Landstreet, J.; Sigut, A. Bibcode: 2005EAS....17..345S Altcode: The spectra of a number of reportedly sharp-line B, A and F stars have been observed in a 60-Å window with a resolving power of 120 000. As a first step in analysis of these data, the spectra have been synthesized to see how well simple models fit. The information obtained from both successful modelling and from discrepant fits is discussed. Title: Direct Simulations of Radiative and Convective Zones Authors: Kupka, F. Bibcode: 2005EAS....17..177K Altcode: Numerical simulations have become one of the main tools in the research on stellar convection zones and they can be expected to become very important for the study of the convection-diffusion interaction as well. In this review, I will first provide some historical background and then select a few sample problems where numerical simulations of stellar convection have provided useful results or are expected to do so in the near future: solar granulation, interacting convection zones, and semi-convection. Title: Some Physics We Can Learn from Spectroscopy of A-Type Stars Authors: Kupka, F. Bibcode: 2005PhST..119...20K Altcode: Stars of spectral type A are distinguished from others by the interesting fact that several physical processes are of equal importance in their outer envelope. The competing processes include diffusion driven by either gravitational settling or radiation pressure, thermal convection driven by partial ionisation of hydrogen and/or helium, their interaction with a global stellar magnetic field in some cases, and large rotation rates or pulsational instabilities in others. Whether one of them dominates over the others is also determined by their initial and present environment. Most of the information on the physics of these processes is obtained from spectroscopy of the surface layers of these stars. In spectroscopic studies of A-stars we can either attempt to obtain a better understanding of the physical processes, if our atomic data are good enough, or alternatively, if the stellar physics provides strong enough constraints, we can learn about atomicphysics: wavelengths, oscillator strengths, pressure broadening, and other quantities which characterise electronic transitions in neutral and ionised atoms within the stellar plasma.

Professor Charles R Cowley has made many important contributions to the spectroscopy of A-type stars throughout his research work and he is also very active in this field today. I have enjoyed being able to work with him on this subject on several occasions. In this paper I present an overview as to why the spectroscopy of A-type stars is of interest to both astrophysicists and laboratory spectroscopists, and I will use various opportunities to illustrate the selected topics through contributions made by Professor Cowley to this field. Title: Observational signatures of atmospheric velocity fields in Main Sequence stars Authors: Kupka, F.; Landstreet, J. D.; Sigut, A.; Bildfell, C.; Ford, A.; Officer, T.; Silaj, J.; Townshend, A. Bibcode: 2004IAUS..224..573K Altcode: In stars with sufficiently small projected rotational velocities (less than a few km s-1), it is often possible to detect signatures of the atmospheric velocity field in line profiles. These signatures may be as subtle as small asymmetries in the profile ("line bisector curvature") or as obvious as profile shapes that strongly depart from those predicted by simple microturbulence models. We have recently carried out a high resolution survey of sharp-line stars to search for these symptoms of local velocity fields. We report the first results of a comparison of models with the observed profiles. Title: Convection in stars Authors: Kupka, F. Bibcode: 2004IAUS..224..119K Altcode: Convection is one of the most intricate processes studied in stellar astrophysics and has challenged both theorists and observers since the beginnings of astrophysics. But during the last two decades observational data of unprecedented resolution and accuracy have been collected in solar and stellar research which permit a new look at the field. An enormous increase of computer speed now permits solving more complete model equations with more accurate numerical approximations. Modelling and theoretical understanding of convection, however, are lagging behind observational progress and are still wanting. Title: Stellar model atmospheres with emphasis on velocity dynamics Authors: Stütz, Ch.; Kupka, F. Bibcode: 2004IAUS..224..570S Altcode: Recently a formalism for solving the open set of non-local hydrodynamic moment equations has been applied to the envelopes of A-stars by Kupka & Montgomery (2002). Tests on numerical simulations and selected properties derived from stellar spectra such as average surface velocities and velocity skewness have shown the applicability of this RSM approach. In addition, a variety of improvements over existing modeling standards have been developed within and around the AMS group at the Institute for Astronomy in Vienna during the last couple of years. For example, individual elemental abundance patterns (Piskunov & Kupka 2001) or stratification as an observed parameter (Shuliak et al. 2004) can be taken into account in our model atmospheres. Once the underlying programs, which currently operate as stand alone applications, are combined with a model atmosphere code, we will obtain a powerful and efficient tool that will allow us the investigation of a number of open problems in the physics of A-stars, such as more realistic models of the temperature gradients of cool CP2 stars. Title: Panel discussion section I Authors: Kupka, F. Bibcode: 2004IAUS..224..465K Altcode: No abstract at ADS Title: Effect of Local Treatments of Convection upon the Solar P-Mode Excitation Rates Authors: Samadi, R.; Goupil, M. J.; Lebreton, Y.; van't Veer, C.; Kupka, F. Bibcode: 2004ESASP.559..611S Altcode: 2004soho...14..611S; 2004astro.ph..9324S We compute, for several solar models, the rates P at which the solar radial p modes are expected to be excited. The solar models are computed with two different local treatments of convection : the classical mixing-length theory (MLT hereafter) and Canuto, Goldmann and Mazzitelli(1996, CGM hereafter)'s formulation. For one set of solar models (EMLT and ECGM models), the atmosphere is gray and assumes Eddington's approximation. For a second set of models (KMLT and KCGM models), the atmosphere is built using a T(tau) law which has been obtained from a Kurucz's model atmosphere computed with the same local treatment of convection. The mixing-length parameter in the model atmosphere is chosen so as to provide a good agreement between synthetic and observed Balmer line profiles, while the mixing-length parameter in the interior model is calibrated so that the model reproduces the solar radius at solar age. For the MLT treatment, the rates P do depend significantly on the properties of the atmosphere. On the other hand, for the CGM treatment, differences in P between the ECGM and the KCGM models are very small compared to the error bars attached to the seismic measurements. The excitation rates P for modes from the EMLT model are significantly under-estimated compared with the solar seismic constraints. The KMLT model results in intermediate values for P and shows also an important discontinuity in the temperature gradient and the convective velocity. On the other hand, the KCGM model and the ECGM model yield values for P closer to the seismic data than the EMLT and KMLT models. We conclude that the solar p-mode excitation rates provide valuable constraints and according to the present investigation cleary favor the CGM treatment with respect to the MLT. Title: The 5200-Åflux depression of chemically peculiar stars - II. The cool chemically peculiar and λ Bootis stars Authors: Kupka, F.; Paunzen, E.; Iliev, I. Kh.; Maitzen, H. M. Bibcode: 2004MNRAS.352..863K Altcode: 2004MNRAS.tmp..204K After establishing the synthetic Δa photometric system in the first paper of this series, we now present model atmospheres computed with individual abundances for a representative sample of chemically peculiar (CP) stars and either confirm or redetermine their input parameters through comparisons with photometric, spectrophotometric and high-resolution spectroscopic data. The final models obtained from this procedure were used to compute synthetic Δa indices which were compared with observations. The observed behaviour of Δa is reproduced for several types of CP stars: models for Am stars show negligible (or marginally positive) values of a few mmag, while for λ Bootis stars - and for metal deficient A stars in general - we obtain negative values (as low as -12 mmag in one case). For the coolest CP2 stars with effective temperatures below about 8500 K, we obtain mild (~+10 mmag) to moderately large (~+30 mmag) flux depressions in agreement with observations. However, Δa values for slightly hotter members of the CP2 group (for which still Teff < 10000 K) are underestimated from these new models. The effect of the microturbulence parameter on the Δa index is revisited and its different role in various types of CP stars for reproducing the flux depression at 5200 Åis explained. We also provide reasons why models based on enhanced microturbulence and scaled solar abundance could not explain the observed flux depression for all types of CP stars. We discuss potential improvements of the current models including the possibility of still missing line opacities (unidentified and autoionization lines), modifications due to an explicit account of a global stellar magnetic field, and the effect of vertical abundance stratification. Title: Interpolation of Stellar Model Grids and Application to the NEMO Grid Authors: Nendwich, J.; Heiter, U.; Kupka, F.; Nesvacil, N.; Weiss, W. W. Bibcode: 2004CoAst.144...43N Altcode: 2004astro.ph..6381N NEMO Grid of Stellar Atmospheres is a DVD with 91,520 stellar model atmospheres representing a 5D grid of modified ATLAS atmospheres; the purpose of the modifications was to include different treatments of convection and higher vertical resolution. In addition, for every model fluxes are provided and color indices for 14 different photometric systems. Because the model grid contained gaps due to non-converging models, we developed and applied a set of 4D interpolation routines to complete the grid. All the data, which will continuously be updated, can be found on the NEMO homepage http://ams.astro.univie.ac.at/nemo/ and are available via DVD. Title: White dwarf envelopes: further results of a non-local model of convection Authors: Montgomery, M. H.; Kupka, F. Bibcode: 2004MNRAS.350..267M Altcode: 2004astro.ph..1489M We present results of a fully non-local model of convection for white dwarf envelopes. We show that this model is able to reproduce the results of numerical simulations for convective efficiencies ranging from very inefficient to moderately efficient; this agreement is made more impressive given that no closure parameters have been adjusted in going from the previously reported case of A-stars to the present case of white dwarfs. For comparison, in order to match the peak convective flux found in numerical simulations for both the white dwarf envelopes discussed in this paper and the A-star envelopes discussed in our previous work requires changing the mixing length parameter of commonly used local models by a factor of 4. We also examine in detail the overshooting at the base of the convection zone, both in terms of the convective flux and in terms of the velocity field; we find that the flux overshoots by ~1.25 HP and the velocity by ~2.5 HP. Because of the large amount of overshooting found at the base of the convection zone, the new model predicts the mixed region of white dwarf envelopes to contain at least 10 times more mass than local mixing length theory (MLT) models having similar photospheric temperature structures. This result is consistent with the upper limit given by numerical simulations which predict an even larger amount of mass to be mixed by convective overshooting. Finally, we attempt to parametrize some of our results in terms of local MLT-based models, insofar as is possible given the limitations of MLT. Title: Convection in the atmospheres and envelopes of Pre-Main Sequence stars Authors: Montalbán, J.; D'Antona, F.; Kupka, F.; Heiter, U. Bibcode: 2004A&A...416.1081M Altcode: 2003astro.ph.10690M The Teff location of Pre-Main Sequence (PMS) evolutionary tracks depends on the treatment of over-adiabaticity (D'Antona & Mazzitelli \cite{Antona1994}, \cite{Antona1998}). Since the convection penetrates into the stellar atmosphere, also the treatment of convection in the modeling of stellar atmospheres will affect the location of the Hayashi tracks. In this paper we present new non-grey PMS tracks for Teff,>4000 K. We compute several grids of evolutionary tracks varying: i) the treatment of convection: either the Mixing Length Theory (MLT) or Canuto et al. (\cite{Canuto1996e}, CGM) formulation of a Full Spectrum of Turbulence; ii) the atmospheric boundary conditions: we use the new Vienna grids of ATLAS9 atmospheres (Heiter et al. \cite{Heiter2002a}), which were computed using either MLT (with α=Λ/Hp=0.5) or CGM treatments. For comparison, we also compute grids of models with the NextGen (Allard & Hauschildt \cite{Allard1997}, AH97) atmosphere models, and a 1 {M⊙} grey MLT evolutionary track using the α calibration based on 2D-hydrodynamical models (Ludwig et al. \cite{Ludwig1999}). These different grids of models allow us to analyze the effects of convection modeling on the non-grey PMS evolutionary tracks. We disentangle the effect of the wavelength dependent opacity on a self-consistent treatment of convection in the atmosphere from the role of the convection model itself in the atmosphere and in the interior. While for some parts of the HR diagram (e.g., A stars) a low efficiency of atmospheric convection is clearly indicated by the data, for others the evidence is conflicting, showing the weaknesses of all the presently adopted local convection models. Nevertheless, the assumption of a low photospheric efficiency permits us to reproduce a larger amount of data and we have hence restricted our study to this case and draw the following conclusions for it: i) in spite of the solar calibration, if MLT convection is adopted a large uncertainty results in the shape and location of PMS tracks, and the MLT calibration loses sense. ii) As long as the model of convection is not the same in the interior and in the atmosphere, the optical depth at which we take the boundary conditions is an additional parameter of the models. iii) Furthermore, very different sub-atmospheric structures are obtained (for MS and PMS stellar models) depending not only on the treatment of convection, but also on the optical depth at which the boundary conditions are taken. iv) The comparison between NextGen based models and ATLAS9 based models shows that in the Teff domain they have in common (4000-10 000 K) the improved opacities in NextGen atmosphere models have no relevant role on the PMS location, this being determined mainly by the treatment of the over-adiabatic convection. v) In the framework of standard stellar modeling (i.e., non-magnetic, non-rotating, spherical models), the comparison between theoretical models and observational data in very young binary systems indicates that, for both treatments of convection (MLT and CGM) and for any of the atmosphere grids (including those based on the 2D-hydrodynamical atmosphere models), the same assumption for convection cannot be used in PMS and MS: either the models fit the MS - and the Sun in particular - or they fit the PMS. Convection in the PMS phase appears to be less efficient than what is necessary to fit the Sun. Title: New grids of ATLAS9 atmospheres. II. Limb-darkening coefficients for the Strömgren photometric system for A-F stars Authors: Barban, C.; Goupil, M. J.; Van't Veer-Menneret, C.; Garrido, R.; Kupka, F.; Heiter, U. Bibcode: 2003A&A...405.1095B Altcode: Using up-to-date model atmospheres (Heiter et al. \cite{Heiter02}) with the turbulent convection approach developed by Canuto et al. (\cite{Canuto96}, CGM), quadratic, cubic and square root limb darkening coefficients (LDC) are calculated with a least square fit method for the Strömgren photometric system. This is done for a sample of solar metallicity models with effective temperatures between 6000 and 8500 K and with log g between 2.5 and 4.5. A comparison is made between these LDC and the ones computed from model atmospheres using the classical mixing length prescription with a mixing length parameter alpha =1.25 and alpha =0.5. For CGM model atmospheres, the law which reproduces better the model intensity is found to be the square root one for the u band and the cubic law for the v band. The results are more complex for the b and y bands depending on the temperature and gravity of the model. Similar conclusions are reached for MLT alpha =0.5 models. As expected much larger differences are found between CGM and MLT with alpha =1.25. In a second part, the weighted limb-darkening integrals, bl, and their derivatives with respect to temperature and gravity, are then computed using the best limb-darkening law. These integrals are known to be very important in the context of photometric mode identification of non-radial pulsating stars. The effect of convection treatment on these quantities is discussed and as expected differences in the bl coefficients and derivatives computed with CGM and MLT alpha =0.5 are much smaller than differences obtained between computations with CGM and MLT alpha =1.25.

Table 1 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/405/1095 Title: Observations of Rapid Radial Velocity Variations of Spectral Lines in Rapidly Oscillating Ap (roAp) Stars Authors: Landstreet, J. D.; Kochukhov, O.; Kupka, F.; Ryabchikova, T.; Weiss, W. W. Bibcode: 2003aahd.conf..347L Altcode: No abstract at ADS Title: VizieR Online Data Catalog: Limb-darkening coefficients from ATLAS9 models (Barban+, 2003) Authors: Barban, C.; Goupil, M. J.; van't Veer-Menneret, C.; Garrido, R.; Kupka, F.; Heiter, U. Bibcode: 2003yCat..34051095B Altcode: Using up-to-date model atmospheres (Heiter et al. 2002A&A...392..619H) with the turbulent convection approach developed by Canuto, Goldman & Mazzitelli (1996ApJ...473..550C, CGM), quadratic, cubic and square root limb darkening coefficients (LDC) are calculated with a least square fit method for the Stroemgren photometric system. This is done for a sample of solar metallicity models with effective temperatures between 6000 and 8500K and with logg between 2.5 and 4.5. A comparison is made between these LDC and the ones computed from model atmospheres using the classical mixing length prescription with a mixing length parameter α=1.25 and α=0.5. For CGM model atmospheres, the law which reproduces better the model intensity is found to be the square root one for the u band and the cubic law for the v band. The results are more complex for the b and y bands depending on the temperature and gravity of the model. Similar conclusions are reached for Mixing Length Theory (MLT) α=0.5 models. As expected much larger differences are found between CGM and MLT with α=1.25. In a second part, the weighted limb-darkening integrals, bell, and their derivatives with respect to temperature and gravity, are then computed using the best limb-darkening law. These integrals are known to be very important in the context of photometric mode identification of non-radial pulsating stars. The effect of convection treatment on these quantities is discussed and as expected differences in the bell coefficients and derivatives computed with CGM and MLT α=0.5 are much smaller than differences obtained between computations with CGM and MLT α=1.25.

The limb darkening coefficients are given here for the u, v, b and y bands and for CGM models, MLT α=0.5 models and MLT α=1.25 models.

(3 data files). Title: The 5200-Å flux depression of chemically peculiar stars - I. Synthetic Δa photometry: the normality line Authors: Kupka, F.; Paunzen, E.; Maitzen, H. M. Bibcode: 2003MNRAS.341..849K Altcode: 2003astro.ph..3225K The Δa photometric system provides an efficient observational method to identify and distinguish magnetic and several other types of chemically peculiar (CP) stars of spectral types B to F from other classes of stars in the same range of effective temperatures. We have developed a synthetic photometric system that can be used to explore the capability of model atmospheres with individual element abundances to predict photometric Δa magnitudes, which measure the extent of the flux depression around 5200 Å found in different types of CP stars. In this first paper, we confirm the observed dependency of the a index as a function of various colour indices sensitive to the effective temperature of stars as well as its average scatter expected from surface gravity variations within the main-sequence band. The behaviour of the so-called `normality line' of Δa systems used in photometric observations of CP stars is well reproduced. The metallicity dependence of the normality line of the Δa system was computed for several grids of model atmospheres where the abundances of elements heavier than He had been scaled +/-0.5 dex with respect to the solar value. We estimate a lowering of Δa magnitudes for CP stars within the Magellanic Clouds by ~-3 mmag relative to those in the solar neighbourhood assuming an average metallicity of [Fe/H]=-0.5 dex. Using these results on the metallicity bias of the Δa system we find the observational systems in use suitable to identify CP stars in other galaxies or distant regions of our own Galaxy and capable to provide data samples on a statistically meaningful basis. In turn, the synthetic system is suitable to test the performance of model atmospheres for CP stars. This work will be presented in follow-up papers of this series. Title: Colors of ATLAS9 Atmospheres and Their Interpolation Authors: Nendwich, J.; Nesvacil, N.; Weiss, W. W.; Heiter, U.; Kupka, F. Bibcode: 2003IAUS..210P..A9N Altcode: No abstract at ADS Title: Problems and Difficulties in Building up Stellar Models with Non-grey Boundary Conditions Authors: Montalbán, J.; D'Antona, F.; Kupka, F. Bibcode: 2003IAUS..210P..C6M Altcode: No abstract at ADS Title: Non-local Convection Models for Stellar Atmospheres and Envelopes Authors: Kupka, F. Bibcode: 2003IAUS..210..143K Altcode: No abstract at ADS Title: Pre-Main Sequence and Main Sequence Models Using the Vienna Grids of ATLAS9 Atmospheres Authors: Montalbán, J.; Kupka, F.; D'Antona, F.; Heiter, U. Bibcode: 2003IAUS..210P..C5M Altcode: No abstract at ADS Title: Convection in A Stars Authors: Kupka, F. Bibcode: 2003ASPC..305..190K Altcode: 2003mfob.conf..190K No abstract at ADS Title: Application of New ATLAS9 Model Atmosphere Grids Authors: Heiter, U.; Kupka, F.; Samadi, R.; Barban, C.; van't-Veer Menneret, C.; Lebreton, Y.; Goupil, M. -J.; Garrido, R.; Weiss, W. W. Bibcode: 2003IAUS..210P.E10H Altcode: No abstract at ADS Title: Using Balmer Line Profiles to Investigate Convection in A and F Stars Authors: Smalley, B.; Kupka, F. Bibcode: 2003IAUS..210P.C10S Altcode: 2002astro.ph..7388S Balmer lines are an important diagnostic of stellar atmospheric structure, since they are formed at a wide range of depths within the atmosphere. The different Balmer lines are formed at slightly different depths making them useful atmospheric diagnostics. The low sensitivity to surface gravity for stars cooler than ~8000 K makes them excellent diagnostics in the treatment of atmospheric convection. For hotter stars Balmer profiles are sensitive to both effective temperature and surface gravity. Provided we know the surface gravity of these stars from some other method (e.g. from eclipsing binary systems), we can use them to determine effective temperature. In previous work, we have found no significant systematic problems with using uvby photometry to determine atmospheric parameters of fundamental (and standard) stars. In fact, uvby was found to be very good for obtaining both Teff and log g. Using Halpha and Hbeta profiles, we have found that both the Canuto & Mazzitteli and standard Kurucz mixing-length theory without approximate overshooting are both in agreement to within the uncertainties of the fundamental stars. Overshooting models were always clearly discrepant. Some evidence was found for significant disagreement between all treatments of convection and fundamental values around 8000~9000K, but these results were for fundamental stars without fundamental surface gravities. We have used stars with fundamental values of both Teff and log g to explore this region in more detail. Title: Tools and Methods for Abundance Analysis Authors: Knoglinger, P.; Nesvacil, N.; Kupka, F.; Mittermayer, P.; Piskunov, N.; Weiss, W. W.; Bruntt, H. Bibcode: 2003IAUS..210P.E66K Altcode: No abstract at ADS Title: Discovery of rapid radial velocity variations in the roAp star 10 Aql and possible pulsations of β CrB Authors: Kochukhov, O.; Landstreet, J. D.; Ryabchikova, T.; Weiss, W. W.; Kupka, F. Bibcode: 2002MNRAS.337L...1K Altcode: We report discovery of radial velocity variations in rare earth spectral lines of the roAp star 10 Aql with amplitudes of between 30 and 130 m s-1 and periods of about 11 min. Radial velocity variations with amplitude 70 m s-1 may also have been detected in one spectral line of Fe I in β CrB. If confirmed, our results may indicate that all Ap stars in a certain temperature range pulsate, which means that roAp stars do not exist as a separate class but are only distinguished by higher pulsational amplitudes. Title: On the anomaly of Balmer line profiles of A-type stars. Fundamental binary systems Authors: Smalley, B.; Gardiner, R. B.; Kupka, F.; Bessell, M. S. Bibcode: 2002A&A...395..601S Altcode: 2002astro.ph..9239S In previous work, Gardiner et al. (\cite{GKS99}) found evidence for a discrepancy between the Teff obtained from Balmer lines with that from photometry and fundamental values for A-type stars. An investigation into this anomaly is presented using Balmer line profiles of stars in binary system with fundamental values of both Teff and log g. A revision of the fundamental parameters for binary systems given by Smalley & Dworetsky (\cite{SD95}) is also presented. The Teff obtained by fitting Hα and Hβ line profiles is compared to the fundamental values and those obtained from uvby photometry. We find that the discrepancy found by Gardiner et al. (\cite{GKS99}) for stars in the range 7000 K <~ Teff <~ 9000 K is no longer evident. Partly based on DENIS data obtained at the European Southern Observatory. Title: New grids of ATLAS9 atmospheres I: Influence of convection treatments on model structure and on observable quantities Authors: Heiter, U.; Kupka, F.; van't Veer-Menneret, C.; Barban, C.; Weiss, W. W.; Goupil, M. -J.; Schmidt, W.; Katz, D.; Garrido, R. Bibcode: 2002A&A...392..619H Altcode: 2002astro.ph..6156H We present several new sets of grids of model stellar atmospheres computed with modified versions of the ATLAS9 code. Each individual set consists of several grids of models with different metallicities ranging from [M/H] = -2.0 to +1.0 dex. The grids range from 4000 to 10 000 K in Teff and from 2.0 to 5.0 dex in log {g}. The individual sets differ from each other and from previous ones essentially in the physics used for the treatment of the convective energy transport, in the higher vertical resolution of the atmospheres and in a finer grid in the (Teff, log {g}) plane. These improvements enable the computation of derivatives of color indices accurate enough for pulsation mode identification. In addition, we show that the chosen vertical resolution is necessary and sufficient for the purpose of stellar interior modelling. To explain the physical differences between the model grids we provide a description of the currently available modifications of ATLAS9 according to their treatment of convection. Our critical analysis of the dependence of the atmospheric structure and observable quantities on convection treatment, vertical resolution and metallicity reveals that spectroscopic and photometric observations are best represented when using an inefficient convection treatment. This conclusion holds whatever convection formulation investigated here is used, i.e. MLT(alpha =0.5), CM and CGM are equivalent. We also find that changing the convection treatment can lead to a change in the effective temperature estimated from Strömgren color indices from 200 to 400 K. Title: Mode identification using the exoplanetary camera Authors: Garrido, R.; Moya, A.; Goupil, M. J.; Barban, C.; van't Veer-Menneret, C.; Kupka, F.; Heiter, U. Bibcode: 2002CoAst.141...48G Altcode: No abstract at ADS Title: A-star envelopes: a test of local and non-local models of convection Authors: Kupka, F.; Montgomery, M. H. Bibcode: 2002MNRAS.330L...6K Altcode: 2001astro.ph.12401K We present results of a fully non-local, compressible model of convection for A-star envelopes. This model quite naturally reproduces a variety of results from observations and numerical simulations which local models based on a mixing length do not. Our principal results, which are for models with T eff between 7200 and 8500K, are the following. First, the photospheric velocities and filling factors are in qualitative agreement with those derived from observations of line profiles of A-type stars. Secondly, the Heii and Hi convection zones are separated in terms of convective flux and thermal interaction, but joined in terms of the convective velocity field, in agreement with numerical simulations. In addition, we attempt to quantify the amount of overshooting in our models at the base of the Heii convection zone. Title: Colors in Eddington: implications for mode identification Authors: Garrido, R.; Claret, A.; Moya, A.; Kupka, F.; Heiter, U.; Barban, C.; Goupil, M. -J.; van't Veer-Menneret, C. Bibcode: 2002ESASP.485..103G Altcode: 2002sshp.conf..103G Theoretical radial and non-radial modes of low n orders in main sequence stars, like δ Scuti and β Cephei, do not show regular pattern in their power spectra. Although Barban et al. (2001) have shown that assuming an amplitude distribution certain patterns could be recovered, a method to identify the mode is needed in order to perform real asteroseismology of these objects. Examples are given in the literature where for several stars showing many periods: FG Vir (24): Breger et al. (1999), XX Pyx (22): Handler et al. (2000), 4 CVn (30): Breger et al. (1999) and BI CMi (20): Breger et al. (2002), a physical modeling was not successful, the reason being the mode identification. Multicolor photometry, as well as line profile spectroscopy, have been the usual methods to identify modes. In this work we will focus on the capabilities of color information to discriminate the different angular degrees l of the spherical harmonic associated to a radial or non-radial mode excited in a given δ Scuti pulsating star. Some numerical simulations are also presented for the exoplanetary camera in the space mission COROT where it is shown that, under some physical assumptions, color can be very important in order to identify the angular degree l. Furthermore colors can give relevant information of the non-adiabatic properties of some pulsating stars which, in the case of the δ Scuti stars, are related with the treatment of the convection. Arguments are also given to support the inclusion of color information in Eddington. Title: The Böhm-Vitense Gap: The Role of Turbulent Convection Authors: D'Antona, F.; Montalbán, J.; Kupka, F.; Heiter, U. Bibcode: 2002ApJ...564L..93D Altcode: ``Böhm-Vitense gaps,'' discontinuities in the color distribution of A-F type stars along the main sequence, have traditionally been attributed to the abrupt onset of strong convection (8000 K>=Teff>=6400 K) in stellar atmospheres and envelopes. Using the full spectrum of turbulence model to describe convective transport (Canuto, Goldman, & Mazzitelli) both in the interior and in the atmosphere yields a very sharp transition between structures that are convective only in the surface layers and structures that show a well-developed convection also in the interior. This produces a sudden change in the Teff of stars, around Teff~=6800 K. Using numerical simulations, we show that this feature produces a stellar depletion that is consistent with the gap at 0.33<~B-V<~0.38. The standard mixing-length theory does not show this behavior. Consequently, this particular gap is a ``Teff'' gap and not a ``color'' gap. In fact, it is also present in the V-I color distribution of the Hyades stars. Title: TEMPLOGG for determining stellar parameters of MONS targets Authors: Kupka, F.; Bruntt, H. Bibcode: 2001JAD.....7Q...8K Altcode: We describe the TEMPLOGG tool which has been developed at the Institute of Astronomy in Vienna for the determination of fundamental stellar parameters from photometric observations. TEMPLOGG provides a very convenient access to a variety of calibrations for the Strömgren-β and the Geneva photometric systems. Using observed photometric indices the program automatically selects the most suitable transformation between observed colours and stellar fundamental parameters. We have used it here for a parameter determination with a preliminary list of primary MONS targets. The results of this study are presented as an example for the application and capabilities of TEMPLOGG. Title: Using TEMPLOGG for determining stellar parameters of MONS targets Authors: Kupka, F.; Bruntt, H. Bibcode: 2001fcm..book...39K Altcode: We describe the TEMPLOGG tool which has been developed at the Institute of Astronomy in Vienna for the determination of fundamental stellar parameters from photometric observations. TEMPLOGG provides a very convenient access to a variety of calibrations for the Stromgren-beta and the Geneva photometric systems. Using observed photometric indices the program automatically selects the most suitable transformation between observed colours and stellar fundamental parameters. We have used it here for a parameter determination with a preliminary list of primary MONS targets. The results of this study are presented as an example for the application and capabilities of TEMPLOGG. Title: Convection in the atmospheres and envelopes of turnoff and giant branch stars of globular clusters Authors: Montalbán, J.; Kupka, F.; D'Antona, F.; Schmidt, W. Bibcode: 2001A&A...370..982M Altcode: We explore the dependence of {T_eff}'s and colors of stellar models on the treatment of over-adiabatic convection, both in the atmosphere and in the interior. We compute main sequence, turnoff, and subgiant models for low metallicity stars (Z=2 10-4) using as boundary conditions two new sets of model atmospheres by the Wien group (Kupka \cite{kup96}; Smalley & Kupka \cite{sma97}; Heiter et al. \cite{hei01}). In these models convection is treated either in the Mixing Length Theory (MLT) or in the Full Spectrum of Turbulence (FST) formulation. We check the dependence of the {T_eff} location of the HR diagram both on the optical depth tau at which the atmospheric boundary conditions are fixed, and on the convective model adopted in the atmosphere and interior. Obviously, full selfconsistency of the result is obtained only if the treatment of convection is the same in the outer layers and in the interior. We show that it is not appropriate to use MLT convection in the atmosphere and FST in the interior; if we wish to test the effect of changing the l/H_p in the MLT, the atmospheric integration must be limited to tau =1. We construct isochrones for ages of 10 Gyr and greater, and transform the theoretical {T_eff} and gravity values to the Johnson B and V magnitudes. The two sets of model atmospheres give small differences (up to ~ 0.03 mag) for the B-V color, a result of the different temperature stratification in the model atmosphere with FST convection treatment compared to that one based on MLT. The FST boundary conditions provide relative locations of turnoff and giant branch which differ from the MLT solutions, and are in better agreement with the HR diagram morphology of the Globular Cluster M 92. Title: Model Atmospheres with Individualized Abundances Authors: Piskunov, N.; Kupka, F. Bibcode: 2001ApJ...547.1040P Altcode: We describe a new method for computing opacity distribution functions (ODFs) for model atmosphere calculations. The method is tailored to model the atmospheres of individual stars on a modern workstation. Our goal is the computation of model atmospheres for stars with abundances significantly different from the solar or scaled solar composition typically used for grid calculations. As a consistency test, we show that the new procedure is able to reproduce the ODFs and existing model atmospheres for solar abundances, and we describe models for stars with peculiar abundances. We demonstrate that while mild chemical peculiarities can be well represented by scaled solar models, the extreme cases result in a very different atmospheric structure with no analogs in scaled solar grids. Such a structure influences the emerging spectrum as is clearly seen both in the observed flux distribution and in the line ratios that are much better represented by the new models. Title: A New Project for Theoretical Colors of Globular Cluster Stars Authors: Montalban, J.; D'Antona, F.; Kupka, F.; Schmidt, W. Bibcode: 2001coev.conf..243M Altcode: The following sections are included:

* Summary

* References Title: Abundances in Przybylski's star Authors: Cowley, C. R.; Ryabchikova, T.; Kupka, F.; Bord, D. J.; Mathys, G.; Bidelman, W. P. Bibcode: 2000MNRAS.317..299C Altcode: We have derived abundances for 54 elements in the extreme roAp star HD101065. ESO spectra with a resolution of about 80000, and S/N of 200 or more were employed. The adopted model has Teff=6600K, and log(g)=4.2. Because of the increased line opacity and consequent low gas pressure, convection plays no significant role in the temperature structure. Lighter elemental abundances through the iron group scatter about standard abundance distribution (SAD) (solar) values. Iron and nickel are about one order of magnitude deficient while cobalt is enhanced by 1.5dex. Heavier elements, including the lanthanides, generally follow the solar pattern but enhanced by 3 to 4dex. Odd-Z elements are generally less abundant than their even-Z neighbours. With a few exceptions (e.g. Yb), the abundance pattern among the heavy elements is remarkably coherent, and resembles a displaced solar distribution. Title: Abundance analysis of roAp stars. V. HD 166473 Authors: Gelbmann, M.; Ryabchikova, T.; Weiss, W. W.; Piskunov, N.; Kupka, F.; Mathys, G. Bibcode: 2000A&A...356..200G Altcode: This fifth paper in a sequence on abundance analyses of roAp stars features several improvements and complements over the previous investigations: i. The new VALD-2 atomic data base was used which significantly improves the analysis of C, N, O, and rare earth elements (REE) and in particular includes also some doubly ionized REE species. ii. An individual opacity distribution function table was generated for a successful synthesis of photometric indices of this very peculiar star. iii. The influence of a (rather strong) magnetic field on abundance determinations is studied and presented for 30 elements. iv. Our investigation of 33 elements (45 ions) is the hitherto most complete chemical analysis of a chemically peculiar star, using modern tools. Similar to the four roAp stars analysed by us so far (alpha Cir: Kupka et al. 1996, HD 203932: Gelbmann et al. 1997, gamma Equ: Ryabchikova et al. 1997a, and HD 24712: Ryabchikova et al. 1997b) we find nearly solar abundances of Fe and Ni, and a definite overabundance of Cr and especially Co. Rare earth elements have large overabundances, whereas C and O are underabundant relative to the Sun. This pattern seems to be a common property of the chemically peculiar (CP2, Ap) stars. A new and most striking result is the discovery of the anomalous line strengths of the second ions of REE resulting in an abundance increase of up to +1.5 dex, compared to values obtained from lines of the first ions. This anomaly is not found in non-roAp and ``normal'' stars. Based on observations obtained at the European Southern Observatory (La Silla, Chile) Title: Spectroscopic Survey of Rapidly Oscillating Ap Stars Authors: Weiss, W. W.; Ryabchikova, T. A.; Kupka, F.; Lueftinger, T. R.; Savanov, I. S.; Malanushenko, V. P. Bibcode: 2000ASPC..203..487W Altcode: 2000ilss.conf..487W; 2000IAUCo.176..487W A spectroscopic survey of roAp stars has been initiated in Vienna in order to determine their fundamental astrophysical parameters and abundances. We report here on our attempt to confirm and elaborate an atmospheric peculiarity recently discovered (Ryabchikova et al. 1999) which should allow to identify roAp stars with a single spectrum. Title: VALD-2 -- The New Vienna Atomic Line Database Authors: Kupka, F. G.; Ryabchikova, T. A.; Piskunov, N. E.; Stempels, H. C.; Weiss, W. W. Bibcode: 2000BaltA...9..590K Altcode: 2000OAst....9..590K We provide a brief outline of the concepts and facilities of the Vienna Atomic Line DataBase in its new version 2. A summary of contents and recommendations how to use the VALD-2 are given. We conclude by a few applications planned for future releases of VALD. Title: Line Blanketing in Przybylski's Star Authors: Cowley, C. R.; Kupka, F.; Mathys, G. Bibcode: 1999AAS...195.5002C Altcode: 1999BAAS...31.1447C Przybylski's star (HD 101065) may be the most heavily blanketed star known. It therefore provides a test of our techniques for line blanketing. The current abstract draws on a paper in preparation by CRC, T. Ryabchikova, F. Kupka, G. Mathys, and D. J. Bord, based on ESO spectra obtained by GM. Unfortunately, the atomic species that provide the majority of the line blanketing in Przybylski's star does not have enough atomic data for realistic calculations of the blanketing. We therefore discuss three models in which iron-group elements were articifically elevated in abundance in the calculation of opacity used to construct the models. We thank Drs. R. L. Kurucz, and Bengt Edvardsson for calculating respectively Models 1 (dashed [Fe/H]=+3) and 2 (dot-dash, [Fe/H]=+2) at our request. Model 3 (line, [Fe/H]) was calculated by FK, using the Canuto-Mazzitelli formalism. Figure 1 (www.astro.lsa.umich.edu/usrs/cowley/models.gif), shows these 3 models in good agreement with one another, and clearly different from a standard solar-abundance Atlas9 model (dashed) with the same effective temperature. All three models are scaled to Te=6600K. The blanketed models have little or no convection, and show the lowered boundary temperature of classical picket-fence models. The true boundary temperature may be still lower than in these numerical models. Abundances from Pr I and Nd I are systematically higher than those from the corresponding second spectra, as are those from Pr III and Nd III. It was noted long ago by Przybylski and others that the Balmer profiles had cores indicative of temperatures of some 6000K; the wings could be fit with much higher temperatures--perhaps as high as 7500K. Molecular species have been sought but not identified. Calculations show CN and CH lines would be very weak, even if the temperature between log(tau5000)=-3.5 and -5.4 were allowed to drop to 3000K. Title: The Hydrodynamic Moment Equations: An Alternative Treatment For Stellar Convection Authors: Kupka, F. Bibcode: 1999POBeo..65...13K Altcode: No abstract at ADS Title: Turbulent Convection: Comparing the Moment Equations to Numerical Simulations Authors: Kupka, F. Bibcode: 1999ApJ...526L..45K Altcode: 1999astro.ph..9331K The nonlocal hydrodynamic moment equations for compressible convection are compared to numerical simulations. Convective and radiative flux typically deviate less than 20% from the three-dimensional simulations, while mean thermodynamic quantities are accurate to at least 2% for the cases we have investigated. The moment equations are solved in minutes rather than days as required on standard workstations. We conclude that this convection model has the potential to considerably improve the modeling of convection zones in stellar envelopes and cores, in particular those of A and F stars. Title: VALD - The Vienna Atomic Line Database: A Survey Authors: Kupka, F.; Ryabchikova, T. A. Bibcode: 1999POBeo..65..223K Altcode: No abstract at ADS Title: Interacting convection zones Authors: Muthsam, Herbert J.; Göb, Wolfgang; Kupka, Friedrich; Liebich, Wolfgang Bibcode: 1999NewA....4..405M Altcode: 3D Numerical simulations of convection zones separated by a stable layer (according to the Schwarzschild criterion) are presented. The compressible case is considered. We make use of idealized microphysics closely related to polytropes. Decreasing the importance of the separating stable layer by diminishing its vertical extent in a series of models we investigate how the two convection zones merge into one. In our parameter range it is the upper zone which increases in size and ultimately squeezes the lower convection zone more or less out of existence. Properties of various fluxes and other physical quantities are discussed. Title: VALD-2: Progress of the Vienna Atomic Line Data Base Authors: Kupka, F.; Piskunov, N.; Ryabchikova, T. A.; Stempels, H. C.; Weiss, W. W. Bibcode: 1999A&AS..138..119K Altcode: We describe the updated version of the Vienna Atomic Line Data Base (VALD, \cite[Piskunov et al. 1995)]{pis95} which represents a considerable improvement over the first installation from 1994. The original line lists have been complemented with critically evaluated data obtained from experimental measurements and theoretical calculations which are necessary for computing state-of-the-art line opacities in stellar atmospheres, as well as for synthesizing spectra for high precision analyses. In this paper, we present new and improved data sets for neutral species and ions of Si, P, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Ru, Xe, La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, Re, Pt, Au, Hg, and Pb. For some species data are available in VALD for the first time. We explain our choice of quality rankings by reviewing the literature for the new data and by comparison with source lists included into VALD. For some cases, we produced new line data by weighted averaging of data from different sources with individual error estimates in order to increase the reliability of VALD line lists. Software modifications allow remote users of VALD to specify individual extraction parameters as an alternative to the default settings of the VALD team and to have direct control over the quality ranking of line data. A World-Wide-Web interface is described which provides easy access to all new features. To simplify proper crediting of all authors of atomic data, VALD now includes a compilation of all publications used in each type of reply. Finally, we briefly discuss the future roadmap of VALD developments, including the incorporation of molecular transitions and integration with external data bases. http://www.astro.univie.ac.at/~vald http://www.astro.uu.se/~vald Title: Testing convection theories using Balmer line profiles of A, F, and G stars Authors: Gardiner, R. B.; Kupka, F.; Smalley, B. Bibcode: 1999A&A...347..876G Altcode: We consider the effects of convection on the Balmer line profiles ({H_α} and {H_β}) of A, F, and G stars. The standard mixing-length theory (MLT) atlas9 models of Kurucz (1993), with and without overshooting, are compared to atlas9 models based on the turbulent convection theory proposed by Canuto & Mazzitelli (1991, 1992) and implemented by Kupka (1996), and the improved version of this model proposed by Canuto et al. (1996) also implemented by Kupka. The Balmer line profiles are a useful tool in investigating convection because they are very sensitive to the parameters of convection used in the stellar atmosphere codes. The {H_α} and {H_β} lines are formed at different depths in the atmosphere. The {H_α} line is formed just above the convection zone. The {H_β} line, however, is partially formed inside the convection zone. We have calculated the {T_eff} of observed stars by fitting Balmer line profiles to synthetic spectra and compared this to: (i) the {T_eff} of the fundamental stars; (ii) the {T_eff} of stars determined by the Infra-Red Flux Method and (iii) the {T_eff} determined by Geneva photometry for the stars in the Hyades cluster. We find that the results from the {H_α} and {H_β} lines are different, as expected, due to the differing levels of formation. The tests are inconclusive between three of the four models; MLT with no overshooting, CM and CGM models, which all give results in reasonable agreement with fundamental values. The results indicate that for the MLT theory with no overshooting it is necessary to set the mixing length parameter alpha equal to 0.5 for stars with {T_eff <= 6000} K or {T_eff >= 7000} K. However for stars with {6000} K{<= T_eff <= 7000} K the required value for the parameter is {alpha >= 1.25}. Models with overshooting are found to be clearly discrepant, consistent with the results with uvby photometry by Smalley & Kupka (1997). Based on observations made at the Observatorio del Roque de los Muchachos using the Richardson-Brealey Spectrograph on the 1.0m Jacobs Kapteyn Telescope. Title: Eu III identification and Eu abundance in CP stars Authors: Ryabchikova, T.; Piskunov, N.; Savanov, I.; Kupka, F.; Malanushenko, V. Bibcode: 1999A&A...343..229R Altcode: We report the first identification of the Eu iii lambda 6666.347 line in optical spectra of CP stars. This line is clearly present in the spectra of HR 4816, 73 Dra, HR 7575, beta CrB, and alpha (2) CVn, while it is marginally present or absent in spectra of the roAp stars (rapidly oscillating Ap stars, cf. Kurtz 1990) alpha Cir, gamma Equ, HD 203932, GZ Lib (33 Lib), and HD 24712. Careful synthetic spectrum calculations for the Eu ii lambda 6645.11 line taking into account hyperfine, isotopic, and magnetic splittings allow us to obtain more accurate Eu abundances in the atmospheres of 9 CP stars. In most cases the derived abundances are significantly lower than the previous results reported for some of the stars based on coarse analysis of the famous blue Eu ii lines. Assuming an ionization balance in the stellar atmospheres we give an estimate of the astrophysical oscillator strength log (gf)=1.18 +/- 0.14 for the Eu iii lambda 6666.347 line. This value is obtained without taking into account a possible hyperfine-splitting which is unknown for this Eu iii line. We also provide astrophysical gf-values for Eu iii lambda lambda 7221.838, 7225.151, and 8079.071. Title: Observational Evidence for Convection in Main Sequence Star Atmospheres Authors: Weiss, W. W.; Kupka, F. Bibcode: 1999ASPC..173...21W Altcode: 1999sstt.conf...21W No abstract at ADS Title: Testing Convection Theories Using Balmer Line Profiles of A, F and G Stars Authors: Gardiner, R.; Smalley, B.; Kupka, F. Bibcode: 1999ASPC..173..213G Altcode: 1999sstt.conf..213G No abstract at ADS Title: Computing Solar and Stellar Overshooting with Turbulent Convection Models. First Tests of a Fully Non-Local Model Authors: Kupka, F. Bibcode: 1999ASPC..173..157K Altcode: 1999sstt.conf..157K No abstract at ADS Title: The acoustic cut-off frequency of roAp stars Authors: Audard, N.; Kupka, F.; Morel, P.; Provost, J.; Weiss, W. W. Bibcode: 1998A&A...335..954A Altcode: 1997astro.ph.12126A Some of the rapidly oscillating (roAp) stars, have frequencies which are larger than the acoustic cut-off frequency determined from published stellar models which usually assume a grey atmosphere. As the cut-off frequency depends on the T(tau ) relation, we have computed models and adiabatic frequencies for pulsating Ap stars with more realistic atmospheres which include a frequency dependent treatment of radiative transfer, take blanketing effects into account, and which have a better treatment of the radiative pressure. In addition, we are using opacity distribution functions specific to the atmospheric composition. With these improvements over the classical stellar models the theoretical acoustic cut-off frequency for roAp stars are increased by about 200 mu Hz, which brings them close to the observations. We restrict the comparison of our computations with observations to those two `pathological' roAp stars for which more reliable astrophysical parameters are available, HD 24712 and alpha Cir, and comment briefly on a third one, HD 134214. For alpha Cir we find models which have indeed a cut-off frequency beyond the largest observed frequency and which are well within the Teff - L/L_⊙ error box. For HD 24712 only models which are hotter by about 100 K and less luminous by nearly 10% than what is actually the most probable value derived by spectroscopy would have an acoustic cut-off frequency large enough. HD 134214 fits our models best, however, the error box for Teff - L/L_⊙ is the largest of all three stars. One may thus speculate that the old controversy about a mismatch between observed largest frequencies and theoretical cut-off frequencies of roAp star models is resolved. Based on hipparcos data. Title: Abundance analysis of the lambda Bootis stars HD 192640, HD 183324, and HD 84123 Authors: Heiter, U.; Kupka, F.; Paunzen, E.; Weiss, W. W.; Gelbmann, M. Bibcode: 1998A&A...335.1009H Altcode: The classification of lambda Bootis stars based on photometric indices or low resolution spectra is not sufficient for a final decision concerning a membership to this group. A detailed spectroscopic investigation is necessary to rule out stars with similar classification spectra but different abundance patterns. Therefore a program on abundance analyses of lambda Bootis candidates was established, which makes use of a software package that enables the analysis of high signal-to-noise spectra with large resolution. In this paper we present the results of the application of these tools on two lambda Bootis stars (HD 192640 and HD 183324) for which the derived abundances agree very well with the literature. For a third star (HD 84123), which shows a very low projected rotational velocity, the newly determined abundance pattern confirms its membership to the lambda Bootis group. We also investigated the effect of using several different codes for the calculation of the model atmospheres on the synthetic spectra of the program stars, which span a wide range in effective temperature, gravity and metallicity. The substitution of opacity distribution functions, which where pretabulated for metal abundances scaled according to the solar composition, by ones calculated for the individual abundance patterns does not change the synthetic spectra significantly. On the other hand, the derived abundances are sensitive to the treatment of overshooting within the calculation of the convective flux for cool stars. Based on observations obtained at the Observatoire de Haute Provence, the Osservatorio Astronomico di Padua-Asiago and with the Hipparcos satellite Title: CP star atmospheres based on individual ODFs Authors: Kupka, F.; Piskunov, N. E. Bibcode: 1998CoSka..27..228K Altcode: 1998astro.ph..5057K We describe a new method for the computation of opacity distribution functions (ODFs) useful to calculate one-dimensional model atmospheres in local thermal equilibrium (LTE). The new method is fast enough to be applied on current workstations and allows the computation of model atmospheres which deviate significantly from (scaled) solar chemical composition. It has reproduced existing ODFs and model atmospheres for solar abundances. Depending on the type of chemical peculiarity the "individual" model atmosphere may have a structure and surface fluxes similar to atmospheres based on (scaled) solar abundances or deviate in a way that cannot be reproduced by any of the conventional models. Examples are given to illustrate this behavior. The availability of models with "individualized" abundances is crucial for abundance analyses and Doppler imaging of extreme CP stars. Title: The effects of convection on the colours of A and F stars Authors: Smalley, B.; Kupka, F. Bibcode: 1998CoSka..27..233S Altcode: 1998astro.ph..5058S We present a discussion on the effects of convection on the uvby colours of A and F stars. The mixing-length theory used in ATLAS9 is compared to the turbulent convection theory of Canuto & Mazzitelli. Comparison with fundamental stars reveals that colours calculated using the Canuto & Mazzitelli convection theory are generally in better agreement than those obtained using mixing-length theory. Title: EU III identification and EU abundance in cool CP stars Authors: Ryabchikova, T.; Piskunov, N.; Savanov, I.; Kupka, F. Bibcode: 1998CoSka..27..359R Altcode: 1998astro.ph..5206R We report the first identification of the Eu III lambda 6666.317 line in optical spectra of CP stars. This line is clearly present in the spectra of HR 4816, 73 Dra, HR 7575, and beta CrB, while it is marginally present or absent in spectra of the roAp stars alpha Cir, gamma Equ, BI Mic, 33 Lib, and HD 24712. Title: Atmospheric structure and acoustic cut-off frequency of roAp stars Authors: Audard, N.; Kupka, F.; Morel, P.; Provost, J.; Weiss, W. W. Bibcode: 1998CoSka..27..304A Altcode: 1998astro.ph..5091A Some of the rapidly oscillating (CP2) stars, have frequencies which are larger than the theoretical acoustic cut-off frequency. As the cut-off frequency depends on the T(tau) relation in the atmosphere, we have computed models and adiabatic frequencies for pulsating Ap stars with T(tau) laws based on Kurucz model atmospheres and on Hopf's purely radiative relation. The frequency-dependent treatment of radiative transfer as well as an improved calculation of the radiative pressure in Kurucz model atmospheres increase the theoretical acoustic cut-off frequency by about 200 microHz, which is closer to the observations. For alpha Cir we find models with Kurucz atmospheres which have indeed a cut-off frequency beyond the largest observed frequency and which are well within the T(eff) - L error box. For HD 24712 only models which are hotter by about 100 K and less luminous by nearly 10% than what is actually the most probable value would have an acoustic cut-off frequency large enough. One may thus speculate that the old controversy about a mismatch between observed largest frequencies and theoretical cut-off frequencies of roAp star models is resolved. However, the observational errors for the astrophysical fundamental parameters have to be reduced further and the model atmospheres refined. Title: The acoustic cut-off frequency of A to F stars Authors: Audard, N.; Kupka, F.; Morel, P.; Provost, J.; Weiss, W. W. Bibcode: 1998IAUS..185..299A Altcode: For some Ap stars, frequencies are observed which are larger than the theoretical acoustic cut-off frequency, which strongly depends on the T tau relation in the atmosphere. We have computed models and eigenmodes for pulsating Ap stars. The internal stellar structure is obtained with the CESAM code, and improved model atmospheres are constructed with T tau laws derived from Kurucz's model atmospheres. These models are compared to models whose atmospheres are derived from the purely radiative Hopf's T tau law. Our main result is that the frequency-dependent treatment of radiative transfer in Kurucz's model atmospheres induces an increase of the theoretical acoustic cut-off frequency, which brings us closer to the observations. As the cut-off frequency is determined by the very outer layers, we point out the necessity to compute very accurate model atmospheres. Title: The role of convection on the UVBY colours of A, F, and G stars Authors: Smalley, B.; Kupka, F. Bibcode: 1997A&A...328..349S Altcode: We discuss the effects of convection on the theoretical uvby colours of A, F, and G stars. The standard mixing-length theory atlas9 models of Kurucz (1993), with and without approximate overshooting, are compared to models using the turbulent convection theory proposed by Canuto & Mazzitelli (1991, 1992) and implemented by Kupka (1996a). Comparison with fundamental T_eff and log g stars reveals that the Canuto & Mazzitelli models give results that are generally superior to standard mixing-length theory (MLT) without convective overshooting. MLT models with overshooting are found to be clearly discrepant. This is supported by comparisons of non-fundamental stars, with T_eff obtained from the Infrared Flux Method and log g from stellar evolutionary models for open cluster stars. The Canuto & Mazzitelli theory gives values of (b-y)_0 and c_0 that are in best overall agreement with observations. Investigations of the m_0 index reveal that all of the treatments of convection presented here give values that are significantly discrepant for models with T_eff < 6000 K. It is unclear as to whether this is due to problems with the treatment of convection, missing opacity, or some other reason. None of the models give totally satisfactory m_0 indices for hotter stars, but the Canuto & Mazzitelli models are in closest overall agreement above 7000 K. Grids of uvby colours, based on the CM treatment of convection, are presented. These grids represent an improvement over the colours obtained from models using the mixing-length theory. The agreement with fundamental stars enables the colours to be used directly without the need for semi-empirical adjustments that were necessary with the earlier colour grids. Table~5 is only available at the CDS via anonymous ftp 130.79.128.5 or via http://cdsweb.u-strasbg.fr/Abstract.html Title: VizieR Online Data Catalog: Role of Convection in A, F, and G stars (Smalley+ 1997) Authors: Smalley, B.; Kupka, F. Bibcode: 1997yCat..33280349S Altcode: We discuss the effects of convection on the theoretical uvby colours of A, F, and G stars. The standard mixing-length theory atlas9 models of Kurucz (1993, ATLAS9, SAO, Cambridge, USA), with and without approximate overshooting, are compared to models using the turbulent convection theory proposed by Canuto & Mazzitelli (1991ApJ...370..295C, 1992ApJ...389..724C) and implemented by Kupka (1996, ASPConf. Proc. 108, 73). Comparison with fundamental Teff and log g stars reveals that the Canuto & Mazzitelli models give results that are generally superior to standard mixing-length theory (MLT) without convective overshooting. MLT models with overshooting are found to be clearly discrepant. This is supported by comparisons of non-fundamental stars, with Teff obtained from the Infrared Flux Method and log g from stellar evolutionary models for open cluster stars. The Canuto & Mazzitelli theory gives values of (b-y)0 and c0 that are in best overall agreement with observations. Investigations of the m0 index reveal that all of the treatments of convection presented here give values that are significantly discrepant for models with Teff<6000K. It is unclear as to whether this is due to problems with the treatment of convection, missing opacity, or some other reason. None of the models give totally satisfactory m0 indices for hotter stars, but the Canuto & Mazzitelli models are in closest overall agreement above 7000K. Grids of uvby colours, based on the CM treatment of convection, are presented. These grids represent an improvement over the colours obtained from models using the mixing-length theory. The agreement with fundamental stars enables the colours to be used directly without the need for semi-empirical adjustments that were necessary with the earlier colour grids. For a description of the uvby photometric system, see e.g. <GCPD/04> (6 data files). Title: (Erratum) Abundance analysis of roAp stars. Authors: Gelbmann, M.; Kupka, F.; Weiss, W. W.; Mathys, G. Bibcode: 1997A&A...322.1026G Altcode: Erratum to Astron. Astrophys. 319, 630-636 (1997). Title: Abundance analysis of roAp stars. II. HD 203932 Authors: Gelbmann, M.; Kupka, F.; Weiss, W. W.; Mathys, G. Bibcode: 1997A&A...319..630G Altcode: A new tool to simplify abundance analyses which is based on stand-alone programs has been applied to the rapidly oscillating Ap star HD 203932 (BI Mic, CD -30 18600, SAO 212996; Ap(SrEu), V=8.82mag). The spectroscopically determined T_eff_=7450+/-100K and logg=4.3+/-0.15 put this star close to the ZAMS. Other fundamental atmospheric parameters are v_micro_<0.6km/s and the total abundance of all iron peak elements [M/H]=0.0+/-0.1. The fundamental parameters put HD 203932 in a region of the HR-diagram where convection starts becoming efficient and the standard mixing length theory models lead to severe problems in the determination of the atmospheric parameters. The difference between the upper limit for logg obtained from several variants of the mixing length theory and the Canuto-Mazzitelli model indicates that the choice of a particular convection model can influence the determination of basic stellar parameters. For the first time abundances were determined for HD 203932 showing a pattern for the 35 investigated elements which is similar to α Cir (Kupka et al. 1996A&A...308..886K, Paper I). Fe and Ni have about solar abundance, Cr and especially Co are clearly overabundant as well as rare earth elements. The most underabundant element is Sc, followed by C, N, and O, which is a common property of CP2 stars. The lack of a correlation in our data between individual line abundances and their effective Lande factors implies a mean magnetic field modulus not exceeding few kG. Compared to the last homogeneous spectroscopic investigation of a large sample of chemically peculiar stars (21 cool Ap stars, Adelman 1973ApJ...183...95A), our analysis is based on data with higher spectral resolution and signal-to-noise ratio. Even more important, we are using a much larger atomic line data base with more precise atomic parameters than available more than twenty years ago. Title: The Vienna Atomic Line Database : Present State and Future Development Authors: Ryabchikova, T. A.; Piskunov, N. E.; Kupka, F.; Weiss, W. W. Bibcode: 1997BaltA...6..244R Altcode: 1997OAst....6..244R We describe the main structure of the Vienna Atomic Line Database, the tools provided for the users and the statistics of its use in the last two years. Our plans for future developments of the database are discussed. Title: Abundance analysis of roAp stars. I. α Circini. Authors: Kupka, F.; Ryabchikova, T. A.; Weiss, W. W.; Kuschnig, R.; Rogl, J.; Mathys, G. Bibcode: 1996A&A...308..886K Altcode: Based on high resolution, low noise spectroscopy in the spectral region from 4200A to 6700A we derived T_eff_=7900+/-200K, logg=4.2+/-0.15, vsin(i)=12.5(-0.5,+1.5)km/s, and elemental abundances for the rapidly oscillating (ro)Ap star αCir(HD128898, HR5463). We used the spectrum synthesis and the equivalent width technique and confirmed the presence of a magnetic field in αCir. The underabundance of C, N, and O, and the overabundances of rare-earth and some other heavy elements are comparable to other cool Ap-stars. The most important peculiarity is that of Co, which is almost as overabundant as Cr, the most overabundant iron peak element in cool Ap-stars. Title: New models for the convective flux in stellar atmospheres Authors: Kupka, F. Bibcode: 1996IAUS..176..557K Altcode: No abstract at ADS Title: Beyond Mixing Length Theory Authors: Kupka, F. Bibcode: 1996ASPC..108...73K Altcode: 1996mass.conf...73K No abstract at ADS Title: Introduction Authors: Griffin, R. E. M.; Kupka, F. Bibcode: 1996ASPC..108..299G Altcode: 1996mass.conf..299G No abstract at ADS Title: M.A.S.S., Model Atmospheres and Spectrum Synthesis Authors: Adelman, Saul J.; Kupka, Friedrich; Weiss, Warner W. Bibcode: 1996ASPC..108.....A Altcode: 1996mass.conf.....A No abstract at ADS Title: M.A.S.S. Model atmospheres and stellar spectra. 5th Vienna workshop Authors: Adelman, Saul J.; Kupka, Friedrich; Weiss, Werner W. Bibcode: 1996mssm.conf.....A Altcode: No abstract at ADS Title: VALD: The Vienna Atomic Line Data Base. Authors: Piskunov, N. E.; Kupka, F.; Ryabchikova, T. A.; Weiss, W. W.; Jeffery, C. S. Bibcode: 1995A&AS..112..525P Altcode: The "Vienna Atomic Line Data Base" (VALD) consists of a set of critically evaluated lists of astrophysically important atomic transition parameters and supporting extraction software. VALD contains about 600000 entries and is one of the largest collections of accurate and homogeneous data for atomic transitions presently available. It also includes specific tools for extracting data for spectrum synthesis and model atmosphere calculations. The different accuracies of data available in the literature made it necessary to introduce a ranking system and to provide a flexible method for extracting the best possible set of atomic line parameters for a given transition from all the available sources. The data base is presently restricted to spectral lines which are relevant for stars in which the LTE approximation is sufficient and molecular lines do not have to be taken into account. The provision was made that these requirements should not restrict the general design of VALD and the possibility of future expansion. In this paper we describe the structure of VALD, the available data sets and specific retrieval tools. The electronic-mail interface (VALD-EMS) created to allow remote access to VALD is also described. Both users and producers of atomic data are invited to explore the database, and to collaborate in improving and extending its contents. Title: Fine Analysis of Pulsating CP Stars Authors: Kupka, F.; Gelbmann, M.; Heiter, U.; Kuschnig, R.; Weiss, W. W.; Ryabchikova, T. A. Bibcode: 1995ASPC...83..317K Altcode: 1995IAUCo.155..317K; 1995aasp.conf..317K No abstract at ADS Title: A numerical study of compressible convection. Authors: Muthsam, H. J.; Goeb, W.; Kupka, F.; Liebich, W.; Zoechling, J. Bibcode: 1995A&A...293..127M Altcode: We perform numerical investigations of (mainly 3D) compressible convection by using simple microphysics (closely related to piecewise polytropes). The convective layer is embedded between stable layers at the top and the bottom. Overshooting above is found to be quite effective within the present parameter range, overshooting below less so. However, consistent with prior 2D calculation the induced motions outside of the directly convectively influenced region are more developed below than above the convection zone. Typical features of the lower parts of the convection zone leading to a downwardly pointing convective flux are discussed as is the existence of a specific cell system near the lower boundary of the convection zone proper. Also, the influence of parameters is discussed. 2D convection tends to lead to more intensive convection than 3D (the physical parameters being equal). Title: The Vienna Atomic Line Data-Base Authors: Piskunov, N. E.; Kupka, F.; Ryabchikova, T. A.; Weiss, W. W.; Jeffery, C. S. Bibcode: 1995ASPC...81..610P Altcode: 1995lahr.conf..610P No abstract at ADS Title: The Evolutionary Status of Lambda Boo Stars Authors: Paunzen, E.; Gelbmann, M.; Heiter, U.; Kupka, F.; Kuschnig, R.; Weiss, W. W. Bibcode: 1995ASPC...83..315P Altcode: 1995IAUCo.155..315P; 1995aasp.conf..315P No abstract at ADS Title: Abundance analysis of cool oscillating CP stars Authors: Kupka, F.; Ryabchikova, T.; Bolgova, G.; Kuschnig, R.; Weiss, W. W.; Mathys, G.; Le Contel, J. M. Bibcode: 1994cpms.conf..130K Altcode: No abstract at ADS Title: Interaction of convection zones: the nonmagnetic case Authors: Muthsam, H. J.; Göb, W.; Kupka, F.; Liebich, W. Bibcode: 1994smf..conf..152M Altcode: No abstract at ADS Title: The peculiar binary system HR 8891 (ET And) Authors: Kuschnig, R.; Weiss, W. W.; Piskounov, N.; Ryabchikova, T.; Kreidl, T. J.; Alvarez, M.; Bedolla, S. G.; Bus, S. J.; Guo, Z.; Hao, J.; Huang, L.; Kupka, F.; Le Contel, D.; Le Contel, J. M.; Osip, D. J.; Panov, K.; Polosukchina, N.; Sareyan, J. P.; Schneider, H.; Valtier, J. C.; Zboril, M.; Ziznovsky, J.; Zverko, J. Bibcode: 1994IAUS..162...43K Altcode: No abstract at ADS