The gravitational collapse of Plummer protostellar clouds

Abstract

The growing observational evidence that main-sequence and pre-main-sequence protostars are found in binary and multiple systems suggests that they are formed by a fragmentation of collapsing molecular cloud cores. In this paper we present the results of a set of numerical simulations aimed to study the gravitational collapse and fragmentation of a centrally condensed cloud with a nearly flat central density region and a surrounding gas envelope. In order to describe this cloud structure, we use the Plummer radial density profile which satisfies the observed fact that protostellar clouds have a flat central density profile in their innermost region. We consider the cloud to be made of molecular hydrogen and describe the thermodynamics with a single barotropic equation of state, which includes a critical density ρcrit as a unique free parameter that determines a thermodynamical change on the collapsing gas: from an isothermal to an adiabatic regime. In this paper we consider four different values for the initial radius Rc of the cloud, ranging from 2675 to 19 913 AU. In the models, for each ratio Rc/R0 of the cloud to core radius, we use two critical density values: ρcrit = 5.0 × 10 −14 and 5.0 × 10 −12 gr cm −3 . When the adiabatic change regime starts earlier, we find interesting gas structures as a result of the collapse, although these structures are different according to the initial mass content of the envelope and the initial angular velocity of the cloud. When the thermodynamical change occurs later, i.e., for ρcrit = 5.0 × 10 −12 gr cm −3 , we observe that the previously found structure is almost erased to give place instead to a single clump of gas without any adorning spiral arms. In general, we find that as the extension of the envelope mass increases, the possibility of a model to produce a multiple system decreases. This is a result of the initial configuration of our models, namely that with bigger envelopes their cores have a lower ratio of rotational to gravitational energy β0, a lower ratio of thermal plus rotational to gravitational energy α0 + β0, and a lower angular velocity Ω0 ,w hich induces a stronger collapse which in turn contributes to the destruction of the structure that is formed during the initial phases of the collapse. Thus in a sufficient quantity rotational energy is crucial for the fragmentation to occur and survive.