Fragmentation of a Molecular Cloud Core versus Fragmentation of the Massive Protoplanetary Disk in the Main Accretion Phase

Abstract

The fragmentation of molecular cloud cores a factor of 1.1 denser than the critical Bonnor-Ebert sphere is examined though three-dimensional numerical simulations. A nested grid is employed to resolve fine structure down to 1 AU while following the entire structure of the molecular cloud core of radius 0.14 pc. A barotropic equation of state is assumed to take account of the change in temperature during collapse, allowing simulation of the formation of the first core. A total of 225 models are shown to survey the effects of initial rotation speed, rotation law, and amplitude of bar mode perturbation. The simulations show that the cloud fragments whenever the cloud rotates sufficiently slowly to allow collapse but fast enough to form a disk before first-core formation. The latter condition is equivalent to 0 t ff e0:05, where 0 and t ff denote the initial central angular velocity and the freefall time measured from the central density, respectively. Fragmentation is classified into six types: disk-bar, ring-bar, satellite, bar, ring, and dumbbell types according to the morphology of collapse and fragmentation. When the outward decrease in initial angular velocity is more steep, the cloud deforms from spherical at an early stage. The cloud deforms into a ring only when the bar mode (m ¼ 2) perturbation is very minor. The ring fragments into two or three fragments via ring-bar type fragmentation and into at least three fragments via ring type fragmentation. When the bar mode is significant, the cloud fragments into two fragments via either bar or dumbbell type fragmentation. These fragments eventually merge because of their low angular momenta, after which several new fragments form around the merged fragment via satellite type fragmentation. This satellite type fragmentation may be responsible for the observed wide range of binary separation.

Disk formation during collapse of magnetized protostellar cores

“…Starting with a level of rotation consistent with observations, hydrodynamical numerical simulations discovered that centrifugally supported disks form with characteristic diameters of the order of a few hundred AU (Matsumoto & Hanawa 2003;Hennebelle et al 2004;Goodwin et al 2007;Commerçon et al 2008). These structures are the unavoidable consequence of angular momentum conservation during the collapse phase.…”

Spin-orbit alignment in the very low mass binary regime

“…In the former scenario, fragments may share the core's net angular momentum vector (Matsumoto & Hanawa 2003). We note that while equal inclinations is a necessary condition for aligned angular momenta, it is not a sufficient one, unless the inclinations are zero.…”

Accretion phase of star formation in clouds with different metallicities

“…To construct the BE profile, we set the central density as n = 10 4 cm −3 in all models and adopt different isothermal temperatures that are estimated by a one-zone calculation (see below). To promote contraction, we enhance the cloud density by a factor of 1.8 (Matsumoto & Hanawa 2003). For each initial cloud, the mass and radius are different, while the initial cloud stability (the ratio of thermal to gravitational energy), which affects the fragmentation condition, is the same (for details, see §5.2).…”