The study of how stars are born and how they evolve during their youth is one of the major research areas in astrophysics, supported by observations across the entire range of the electromagnetic spectrum. This has led to a broad understanding of the various types of young stellar objects and various phases of early stellar evolution.
However, young stars undergo major changes during their evolution towards the main sequence, changes which mainly arise from a circumstellar accretion disk and its magnetic coupling to the central star but also from jets, outflows and dark spots on the stellar photosphere. It is therefore not a major surprise that virtually all young stars are variable, although amplitudes and timescales differ from one type to another, and depend as well on the evolutionary stage. It is becoming increasingly appreciated that the rather static view of pre-main sequence evolution that has prevailed for many years is misleading, and that young stars cannot be understood without taking their continuous changes into account. Indeed, time-dependent phenomena may hold the key to our understanding of the way young stars grow and their circumstellar environments evolve, eventually leading to the formation of planetary systems.
Irregular variability is known as a fundamental property of young stars. However, after wide-field photographic surveys went out of fashion, only limited work has been done on the photometric variability of young stars in newly formed associations. It is important to recognize young stars as the dynamic bodies they are, because the next major steps in our understanding of young stars are likely to be due to studies of variability.
Therefore an ambitious observational program is needed now that systematically monitors the entire nearest star forming regions, thus providing a time-resolved view of early stellar evolution. The occasional observation of an important accretion event presently occurs by sheer luck. Instead one needs to maintain constant vigilance in order to identify all rare and potentially important events that occur in star forming regions along the Galactic plane. Such an effort is currently on its way with the telescopes VYSOS 6 A+B and VYSOS 16 and will dramatically affect our view of young stars and pre-main sequence evolution, probably even in ways one cannot foresee at the moment.
The earliest and most interesting phases occur while a newborn star is still embedded in its parental cloud. In order to study such extremely young stars, one must turn to infrared wavelengths. The 2MASS all sky survey has been an incredible resource for studies of young stars, and has identified tens of thousands of young stars still partly or fully embedded in their natal clouds. These stars have been identified through their characteristic IR excesses – mainly at the K-band. Additionally, there are many more young stars that do not show strong infrared excesses. Such stars can instead be detected through their infrared variability. Long term variability studies at J,H,K will be a powerful signature of stellar youth in the future and will demonstrate that the static view of star forming regions offered e.g. by the 2MASS catalogue, valuable as it is, may be seriously misleading.
For this purpose IRIS will fill a niche by monitoring young stars with higher sensitivity and much better spatial resolution than 2MASS.
Faculty member active in this area: Rolf Chini