The Heartbeat of Young Stars

The earliest phases in the lives of stars - from their births in molecular clouds to the onset of hydrogen burning - are characterized by complex physical processes that are a challenge to current theory and observing techniques. Young stars and their photospheres are directly connected to the moving hydrodynamic circumstellar material that is still accreted. It is assumed that in these gas and dust disks planetary systems similar to our solar system are formed. The remnants of the stellar birth clouds also cause typical observational features, such as irregular light variations, spectral emission lines and infrared excesses which complicate the determination of the fundamental parameters (i.e., effective temperatures, luminosities, masses etc.) of young stars. The evolutionary stages of field stars cannot be derived only from their effective temperatures, luminosities and masses because stellar objects before and after the main sequence phase possess similar atmospheric properties. The main difference between stars in both evolutionary phases lies in the inner structures. Asteroseismology is the only tool to investigate the interiors of pulsating stars through the analysis of their pulsation modes, similar to the study of earthquakes that allows gaining information on the interior of the Earth. Within the last years, the discovery of several pulsating pre-main sequence (PMS) stars triggered the special development of non-radial pulsation theory for these young stars. As their eigenfrequency spectra differ from those of their evolved (post-) main sequence analogs, asteroseismology provides us with an independent method to constrain the evolutionary phase of a field star. Recent asteroseismic studies indicate that the inner structures of young stars seem to change significantly on their ways from the birthline (i.e. when stars first appear as optically visible objects) to the main sequence (i.e. when hydrogen ignites in the stellar core) depending on their relative position on these paths. According to most recent MOST and COROT observations, these changes may be even more severe than predicted by current stellar evolution theory. During these early phases, stellar evolution proceeds relatively fast and induces changes in the observed pulsation periods. The first measurements of evolutionary period changes suggest that stellar evolution might be a factor ~50 faster than predicted. With this project I intend to obtain photometric and spectroscopic observations from ground and from space to continue the study of PMS stars using asteroseismology, to compare pulsating stars that are still contracting towards the main sequence to their hydrogen burning analogs of same effective temperature, luminosity and mass, and, hence, to contribute to the understanding of the physical processes acting in young stellar objects.