Evolution of First Cores in Rotating Molecular Cores

Abstract

We investigate the effect of rotation on the star formation process quantitatively using axisymmetric numerical calculations. An adiabatic hydrostatic object (the so-called first core) forms in a contracting cloud core, after the central region becomes optically thick and continues to contract, driven by mass accretion onto it. The structure of a rotating first core is characterized by its total angular momentum Jcore and mass Mcore, both of which increase by accretion with time. We find that the first core evolves with a constant Jcore/Mimg1.gif. Evolutionary paths of first cores can be classified into two types. In a slowly rotating core with Jcore/Mimg1.gif < 0.015G/(img2.gifciso), where ciso and G represent the isothermal sound speed in the molecular cloud core and the gravitational constant, respectively, the core begins "second collapse" after the central density exceeds the H2 dissociation density. This is the same evolution as a standard scenario for a spherically symmetric, nonrotating core. On the other hand, a core with Jcore/Mimg1.gif > 0.015G/(img2.gifciso) stops its contraction before the central density reaches the H2 dissociation density and does not begin the second collapse. These rapidly rotating first cores suffer from nonaxisymmetric instabilities, such as formation of massive spiral arms, deformation into a bar, or fragmentation. Although the rotating first cores have small average luminosities of Lcore = 0.003-0.03 img3.gif Lsun, assuming a constant mass accretion rate img4.gifcore. Their lifetimes last several thousand years or more, which is much longer than those expected for nonrotating clouds (~1000 yr). We expect that at least several percent of prestellar cores contain first cores as very low luminosity objects. Furthermore, we find a core with 0.012G/(img2.gifciso) < 0.015G/(img2.gifciso) may form close binary systems with initial separation of 0.02-0.1 AU after the second collapse phase.