The role of disc self-gravity in the formation of protostars and protostellar discs

Abstract

We use time‐dependent, one‐dimensional disc models to investigate the evolution of protostellar discs that form through the collapse of molecular cloud cores and in which the primary transport mechanism is self‐gravity. We assume that these discs settle into a state of thermal equilibrium with Q= 2 and that the strength of the angular momentum transport is set by the cooling rate of the disc. The results suggest that these discs will attain a quasi‐steady state that persists for a number of free‐fall times and in which most of the mass within 100 au is located inside 10–20 au. This pile‐up of mass in the inner disc could result in temperatures that are high enough for the growth of magnetohydrodynamic turbulence which could rapidly drain the inner disc and lead to FU Orionis‐like outbursts. In all our simulations, the inner regions of the discs (r < 40 au) were stable against fragmentation, while fragmentation was possible in the outer regions (r > 40 au) of discs that formed from cores that had enough initial angular momentum to deposit sufficient mass in these outer regions. The large amount of mass in these outer regions, however, suggests that fragmentation will lead to the formation of sub‐stellar and stellar mass companions, rather than planetary mass objects. Although mass accretion rates were largely consistent with observations, the large disc masses suggest that an additional transport mechanism (such as magnetorotational instability occurring in the upper layers of the disc) must operate in order to drain the remaining disc material within observed disc lifetimes.

The Burst Mode of Accretion and Disk Fragmentation in the Early Embedded Stages of Star Formation

“…The growing disk mass outweighs the stabilizing influence of stellar irradiation, at least for stars with M * 1.0 M . Our conclusion is in line with that of Rice et al (2010) who argue that the primary requirement for disk fragmentation is a large enough β to produce disks with radii large enough for fragmentation. Indeed, we may form a disk of greater size not only by increasing β but also by taking a larger (and hence more massive) cloud core.…”

“…3. The amount of rotation in the natal cloud core should be sufficiently large in order to form extended and massive protostellar disks (Vorobyov & Basu 2006;Kratter et al 2008;Vorobyov 2009b;Rice et al 2010;Machida et al 2010).…”

The formation of planets by disc fragmentation

“…repetitive episodes of disc fragmentation [65,[68][69][70][71][72][73][74][75]. Mass infall can have a large impact on the stability of the outer disc and provides a physical mechanism for driving the system to an unstable state.…”

Sub-Keplerian accretion onto circumstellar disks

“…For the current work, the α parameter is set to a constant value of 0.01 (Hartmann et al 1998). Choosing a different value, or allowing α to vary in time or with radius, would affect the evolution of the disk (e.g., Lin & Pringle 1990;Kratter et al 2008;Vorobyov & Basu 2009;Rice & Armitage 2009;Rice et al 2010). In order to keep the current work focused on the new treatment of the sub-Keplerian accretion, the effects of different viscosity prescriptions will be explored separately in a future publication.…”