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.

With the advent of modern space and ground-based telescopes our understanding of how stars and planets form has improved significantly over the past decades. Yet there is a class of young protostars, named FUors after the first discovered prototype FU Orionis in the constellation Orion, whose nature is elusive. FUors are distinguished by spontaneous and unpredictable flares of luminosity lasting tens and even hundreds of years, during which the amount of emitted energy can increase by factors of hundreds. This project has brought our understanding of these enigmatic objects to a qualitatively new level. We have demonstrated that luminosity flares caused by in-falling gaseous clumps formed in circumstellar gas-dust disks are inherent to most young stars with various masses ranging from that of the Sun to tens of solar masses. This suggests that the formation of stars follows laws and processes that are common across the entire stellar mass spectrum. We employed numerical modeling complemented with observations on the Spitzer and Herschel space telescopes to study FUors and found that luminosity flares leave long-lasting signatures in the circumstellar environment in the form of spectacular knotty jets and rings of carbon monoxide. Another class of enigmatic stellar objects that is distinguished by luminosities 10 times as low as that of the Sun appears to be intrinsically linked to FUors and can be considered as the manifestation of one fundamental process, namely variable protostellar accretion with episodic flares. We conclude that FUor flares can be fundamental to chemical synthesis and the formation of primary protoplanetary blocks in the gas-dust disks surrounding stars. Some data in primary meteorites may indicate the existence of such flares in the early stages of the formation of our own Protosun.