OVRO N2H+ Observations of Class 0 Protostars: Constraints on the Formation of Binary Stars

We present the results of an interferometric study of the N₂H⁺ (1-0) emission from nine nearby, isolated, low-mass protostellar cores, using the Owens Valley Radio Observatory (OVRO) millimeter array. The main goal of this study is the kinematic characterization of the cores in terms of rotation, turbulence, and fragmentation. Eight of the nine objects have compact N₂H⁺ cores with FWHM radii of 1200-3500 AU, spatially coinciding with the thermal dust continuum emission. The one more evolved (Class I) object in the sample (CB 188) shows only faint and extended N₂H⁺ emission. The mean N₂H⁺ line width was found to be 0.37 km s^-1. Estimated virial masses range from 0.3 to 1.2 M_solar. We find that thermal and turbulent energy support are about equally important in these cores, while rotational support is negligible. The measured velocity gradients across the cores range from 6 to 24 km s^-1 pc^-1. Assuming these gradients are produced by bulk rotation, we find that the specific angular momenta of the observed Class 0 protostellar cores are intermediate between those of dense (prestellar) molecular cloud cores and the orbital angular momenta of wide pre-main-sequence (PMS) binary systems. There appears to be no evolution (decrease) of angular momentum from the smallest prestellar cores via protostellar cores to wide PMS binary systems. In the context that most protostellar cores are assumed to fragment and form binary stars, this means that most of the angular momentum contained in the collapse region is transformed into orbital angular momentum of the resulting stellar binary systems.