Protostellar collapse induced by compression - II. Rotation and fragmentation

Abstract

We investigate numerically and semi-analytically the collapse of low-mass, rotating prestellar cores. Initially, the cores are in approximate equilibrium with low rotation (the initial ratio of thermal to gravitational energy is α 0 0.5, and the initial ratio of rotational to gravitational energy is β 0 = 0.02-0.05). They are then subjected to a steady increase in external pressure. Fragmentation is promoted -in the sense that more protostars are formed -both by more rapid compression, and by higher rotation (larger β 0 ).In general, the large-scale collapse is non-homologous, and follows the pattern described by Hennebelle et al. (Paper I) for non-rotating clouds, namely a compression wave is driven into the cloud, thereby increasing the density and the inflow velocity. The effects of rotation become important at the centre, where the material with low angular momentum forms a central primary protostar (CPP), whilst the material with higher angular momentum forms an accretion disc around the CPP. More rapid compression drives a stronger compression wave and delivers material more rapidly into the outer parts of the disc. Consequently, (i) there is more mass in the outer parts of the disc; (ii) the outer parts of the disc are denser (because the density of the material running into the accretion shock at the edge of the disc is higher); and (iii) there is less time for the gravitational torques associated with symmetry breaking to redistribute angular momentum and thereby facilitate accretion on to the CPP. The combination of a massive, dense outer disc and a relatively low-mass CPP renders the disc unstable against fragmentation, and leads to the formation of one or more secondary protostars. At their inception, these secondary protostars are typically four or five times less massive than the CPP.For very rapid compression there is no CPP and the disc becomes more like a ring, which then fragments into two or three protostars of comparable mass.For more rapid rotation (larger β 0 ), the outer disc is even more massive in comparison to the CPP, even more extended, and therefore even more prone to fragment.

The gravitational collapse of Plummer protostellar clouds

“…They found that fragmentation is promoted when both the rotation of the cloud and the compression rate are faster. In this sense, our models with the more massive envelopes and smaller initial rotation velocities would be similar to those of Hennebelle et al (2004) with a low compression rate.…”

“…But as we observed in models A3 and B3, there is not enough rotation inertia for the spiral arms to grow up, and most of the particles go directly to the central clump. Hennebelle et al (2004) have considered the collapse of a low-mass, rotating prestellar core which is subject to a steady increase of external pressure. They found that fragmentation is promoted when both the rotation of the cloud and the compression rate are faster.…”

Magnetically Self-Regulated Formation of Early Protoplanetary Disks

“…Apart for δ, which is a coefficient on the order of a few, the first term simply correponds to the singular isothermal sphere (Shu 1977) while the second one is a correction that must be included when rotation is significant, particularly in the inner part of the envelope close to the disc edge, as discussed in Hennebelle et al (2004) (see their appendix and Fig. 2).…”

Simulating star formation in molecular cores

“…In the highly dynamic scenario the collapse of a prestellar core is far more likely to lead to fragmentation and the formation of multiple systems (e.g., Whitworth et al 1995;Turner et al 1995;Whitworth et al 1996;Klein et al 2001Klein et al , 2003Bate et al 2002aBate et al ,b, 2003Bonnell et al 2003;Delgado-Donate 2003, 2004Goodwin et al 2004a,b;Hennebelle et al 2003Hennebelle et al , 2004. This is because in the highly dynamic scenario prestellar cores are formed non-quasistatically and therefore (a) they are launched directly into the non-linear regime of gravitational collapse, and (b) they are likely to have retained some internal turbulence.…”