Protostellar angular momentum evolution during gravoturbulent fragmentation

Abstract

Using hydrodynamic simulations we investigate the rotational properties and angular momentum evolution of prestellar and protostellar cores formed from gravoturbulent fragmentation of interstellar gas clouds. We find the specific angular momentum j of molecular cloud cores in the prestellar phase to be on average 〈j〉 = 7 × 1020 cm2 s-1 in our models. This is comparable to the observed values. A fraction of those cores is gravitationally unstable and goes into collapse to build up protostars and protostellar systems, which then have 〈j〉 = 8 × 10 19cm2s-1. This is one order of magnitude lower than their parental cores and in agreement with observations of main-sequence binaries. The loss of specific angular momentum during collapse is mostly due to gravitational torques exerted by the ambient turbulent flow as well as by mutual protostellar encounters in a dense cluster environment. Magnetic torques are not included in our models, these would lead to even larger angular momentum transport. The ratio of rotational to gravitational energy β in cloud cores that go into gravitational collapse turns out to be similar to the observed values. We find that β is roughly conserved during the main collapse phase. This leads to the correlation j ∝ M2/3, between specific angular momentum ;' and core mass M. Although the temporal evolution of the angular momentum of individual protostars or protostellar systems is complex and highly time-variable, this correlation holds well in a statistical sense for a wide range of turbulent environmental parameters. In addition, high turbulent Mach numbers result in the formation of more numerous protostellar cores with, on average, lower mass. Therefore, models with larger Mach numbers result in cores with lower specific angular momentum. We find, however, no dependence on the spatial scale of the turbulence. Our models predict a close correlation between the angular momentum vectors of neighboring protostars during their initial accretion phase. Possible observational signatures are aligned disks and parallel outflows. The latter are indeed observed in some low-mass isolated Bok globules.