Episodic accretion in star formation

This proposal aims at addressing the following major astrophysical questions: How does matter get accreted onto stars? How do accreting young stars interact with their environment? What is the impact of episodic accretion on circumstellar matter? The character of mass accretion in low-mass protostars has come into focus due to recent observational studies showing that classic theoretical models have a difficulty in explaining the variety of mass accretion rates observed in star-forming regions. A spectacular example for the failure of classic models comes from a class of young stars called FU Orionis objects (FUors) and their related EXors (from the prototype, EX Lup). Both classes were discovered observationally: FUors display luminosity outbursts of about 4 magnitudes that last several decades to hundreds of years, whereas EXors show somewhat smaller outbursts on a much shorter timescale, from a few months to a few years, and may occur repeatedly. Such outbursts are thought to be due to drastic increases in mass accretion rate in such young stars.
These observational facts have brought about a new paradigm of episodic accretion, whereby matter is accreted onto protostars in a highly variable manner, unlike the classic models predicting smooth accretion. The emerging question is, thus, how important are outbursting sources and, more generally, episodic accretion in the star formation process?
Many theoretical and numerical models have been developed to explain the origin of accretion bursts in young stars. Nevertheless, the actual mechanism of such accretion outbursts still remains unclear. We will focus on two promising mechanisms: a combination of gravitational instability in the outer protostellar disk and the triggering of the magnetorotational instability in the inner disk and also accretion of dense gaseous clumps formed in a gravitationally fragmenting disk. Given the large number of competing theoretical models, observations may be the key for understanding the outburst phenomenon.
This joint Swiss-Austrian proposal aims to tackle the problem of episodic accretion from both the theoretical and observational sides. Numerical models will be employed to study the origin of the outburst mechanism and the effect of such outbursts onto the chemistry in the circumstellar envelopes and disks of young stars, making valuable predictions for future observational programs. Space and ground-based data will be analyzed making use of the numerical models to actually search for the effect of episodic outbursts. In a full circular loop, the data sets will help to further constrain and modify the model of episodic accretion.
The SNF part will fund a 3-year PhD student in Switzerland, while the FWF part will fund parts of the salary for the Austrian PI and an Austrian PhD student who will collaborate in the project and provide his expertise in the thermo-chemical code ProDiMo.