Dynamic Stellar Atmospheres: Convection and Pulsation

Astronomy has always been not only a observing, but also a calculating science. This latter approach experiences, since decades, a continuous steep upwards trend due to both the dramatic increase in computer hardware and improvement of numerical methods. - We perform calculations largely concerning properties of stars which cannot be directly observed and need to be understood to in order to have a complete picture of a star. One item we deal with are the atmospheres of A stars (stars somewhat hotter than the Sun). We investigate the atmospheres of those stars and show that vigorous dynamics (shock fronts etc.) play a crucial role. Although these stars have much more modest convection zones than the Sun, their effect in the atmosphere is much more dramatic than in the Sun and provide a large area also for future research. - Cepheids constitute an other type of stars we investigate. Googling for `Cepheids` yields 100.000 hits due to the importance of these pulsating stars for our knowledge of stellar structure, evolution and the cosmic distance scale. We are constructing the first multidimensional models for Cepheids which will allow a proper investigation of the pulsation - convection interaction. The models we have discussed above treat the full equations describing the problem and are essentially complete from the standpoint of physics. However, they need high computational resources. That precludes their use for a large number of stars as appropriate for `everyday` astrophysics. We are therefore also working on simplified models of stellar atmospheres and envelopes with convection. In this area, we have made advances regarding the modelling of A-star atmospheres which is notoriously difficult. These models are now applied to a number of stars in order to test them further resp. to get results concerning the state of the atmospheres of those stars.