Physics of Young Stars

The investigation of the earliest phases of stellar evolution is currently one of the most active research fields in stellar astrophysics. The complex physical processes acting in young stars determine their evolution until the end of stellar lifetimes. Young stellar objects differ from their more evolved counterparts of same temperature, luminosity and mass mostly in the inner regions, whereas their atmospheres are quite similar. Young stars still interact with the remainder of the protostellar environment, from which they were born (e.g. through accretion of the circumstellar material). Hence, they are characterized by a high degree of activity, which can be seen in a number of observable phenomena. Due to this complex scenario the question of the initial conditions of stellar evolution is still unsettled. Asteroseismology - similar to Seismology on the Earth - is the only technique to probe the interior structures of stars by analysis of their pulsation modes. The discovery of pulsating young stars has recently triggered the development of non-radial pulsation theory for these objects. As their eigenfrequency spectra differ from those of their evolved analogs, the study of pulsation in young stars provides the unique chance to determine the evolutionary stage of a field star just from an asteroseismic analysis. Stellar evolution from the birthline (i.e. when the stars are first visible in the optical range) to the zero-age main sequence (i.e. the start of hydrogen burning in the core) occurs quite rapidly. Hence, evolutionary changes in the pulsation periods should be observable already after a couple of years. The first measurements of such changes suggest that stellar evolution is up to a factor of ~50 faster than predicted by theory. Photometric time-series observations from ground and from space together with high-resolution, high signal-to- noise spectroscopy permits to study characteristic phenomena of young objects and to compare them with their more evolved analogs. Simultaneously, the observational results provide input for model calculations, which is needed to constrain the structure and evolution of young stars. Therefore, with this project I hope to significantly contribute to the understanding of the physical processes occurring in the earliest phases of stellar evolution.