Stellar Atmospheres and Pulsating Stars

Atmospheres are the windows of stars to the outside world. Stellar atmospheres are the basis for interpreting many astronomical observations from X-rays to the Infrared. About 80% of the mass of the visible Universe is contained in stars and virtually all information about this Universe is carried to us by electromagnetic radiation. Stellar atmospheres are the origin of most photons registered by our telescopes. Hence, modeling of stellar atmospheres is of fundamental importance for a wide range of astrophysical topics and often a limiting factor due to over- simplification introduced by "standard" assumptions and insufficient quality of spectrum analysis techniques. The demand for improving model atmospheres has therefor enormously increased over the last few decades. Detailed spectral syntheses and abundance studies (chemical evolution of the universe), the study of horizontal and vertical inhomogeneities in stellar atmospheres (Doppler imaging and diffusion processes), determination of stellar magnetic fields based on spectral line profiles and polarization measurements (stellar activity), and stellar winds (recycling stellar matter to the ISM) are only few examples. Oscillations, in addition, carry information about the internal state of a star outward to the reflecting stellar boundary, where the oscillations can be observed (asteroseismology as an `X-ray` technique for stars allowing to investigate their interiors). The importance of investiga-ting pulsation properties, like the range of frequencies and amplitudes and their stability, coincidence with instability regions in the HR diagram, etc., is evident. The description of the following physical processes are intended for implementation or improvement in our model atmosphere code: convection - diffusion - rotation Our new atmosphere models will be compared with spectroscopic and photometric observations of stars at or close to the Main and Pre-Main Sequence phase in the range of 1.4 to 2.5 solar mass. They are the most promising candidates for testing the influence of physical processes relevant for the current proposal, because - these stars require the least complex physics for modelling and hence may allow fast progress of our project. - these stars are expected to have the entire range of high to low level of convection in the envelope and photosphere and which therefor allows direct observation. - the effects of diffusion are immediately observable. Stellar wind effects dominate for the more massive stars, for the less massive stars diffusion is compensated by convection. - this range includes the classical instability strip, allowing the use of asteroseismic techniques for independently testing stellar models. No other region in the HR-diagram can be compared in diversity of observationally dominating physical effects in stars with otherwise similar temperature and luminosity! It also is interesting to note that three different types of pulsation can be observed for different groups of stars with otherwise similar fundamental parameters: low and high order non-radial pressure modes (delta Sct, lambda Boo, roAp) which are sensitive to the outer stellar regions, and gravity modes (gamma Dor) which are sensitive to the core. In addition, according to generally accepted theory, the coolest gamma Dor stars share the HR diagram with pulsators stochastically driven by convection, hence providing potentially an interesting link also to solar type stars.