Quiescent and Coherent Cores from Gravoturbulent Fragmentation

Abstract

We investigate the velocity structure of protostellar cores that result from nonmagnetic numerical models of the gravoturbulent fragmentation of molecular cloud material. A large fraction of the cores analyzed are ‘‘quiescent’’; i.e., they have nonthermal line widths smaller or equal to the thermal line width. Specifically, about 23% of the cores have subsonic turbulent line-of-sight velocity dispersions turb cs. A total of 46% are ‘‘transonic,’’ with cs < turb 2cs. More than half of our sample cores are identified as ‘‘coherent,’’ i.e., with turb roughly independent of column density. Of these, about 40% are quiescent, 40% are transonic, and 20% are supersonic. The fact that dynamically evolving cores in highly supersonic turbulent flows can be quiescent may be understood because cores lie at the stagnation points of convergent turbulent flows, where compression is at a maximum and relative velocity differences are at a minimum. The apparent coherence may be due, at least in part, to an observational effect related to the length and concentration of the material contributing to the line. In our simulated cores, turb often has its local maximum at small but finite offsets from the column density maximum, suggesting that the core is the dense region behind a shock. Such a configuration is often found in observations of nearby molecular cloud cores and argues in favor of the gravoturbulent scenario of stellar birth as it is not expected in star formation models based on magnetic mediation. A comparison between the virial estimate Mvir for the mass of a core based on turb and its actual value M shows that cores with collapsed objects tend to be near equipartition between their gravitational and kinetic energies, while cores without collapsed objects tend to be gravitationally unbound, suggesting that gravitational collapse occurs immediately after gravity becomes dominant. Finally, cores in simulations driven at large scales are more frequently quiescent and coherent and have more realistic ratios of Mvir=M, supporting the notion that molecular cloud turbulence is driven at large scales.