Quiescent and coherent cores from gravoturbulent fragmentation

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 sigma(turb) less than or equal to c(s). A total of 46% are transonic,'' with c(s) < sigma(turb) <= 2c(s). More than half of our sample cores are identified as coherent,'' i.e., with sigma(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 aminimum. The apparent coherencemay 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, sigma(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 M-vir for the mass of a core based on sigma(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 M-vir/M, supporting the notion that molecular cloud turbulence is driven at large scales.