Formation of dense cores in Taurus by fragmentation of velocity-coherent filaments
Observatorio Astronomico Nacional, Madrid, Spain
|Understanding how cores condense inside a molecular cloud remains a major missing piece in the star-formation puzzle. To better characterize this process, we have carried out a series of molecular-line observations in several core-forming regions of the Taurus molecular cloud, using tracers sensitive to the cloud and the core gas. From these observations, we have compared the velocity field of the cloud and the cores, and tried to infer the kinematic changes that the gas undergoes as it makes the transition from cloud to core. >From an initial study of the L1517 region (Hacar & Tafalla 2011), we find that the cloud gas consists of 4 filamentary structures with typical lengths of 0.5 pc and subsonic linewidths. The cores in L1517 seem to have condensed out of this velocity-coherent gas without suffering appreciable changes in its kinematics, in contradiction with the expectations from gravo-turbulent models of core formation. A more extended study of the 10 pc-long Barnard 213 region (Hacar, Tafalla, Kauffmann, & Kovacs 2012, in prep), which contains about 20 cores, confirms the results from the L1517 analysis. This region appears as a single filament in the extinction/continuum maps, but our analysis of the velocity field reveals that it is a collection of several dozen smaller filaments, typically 0.5 pc long and also coherent in velocity. Core formation in B213 seems to be occurring in only a few of these filaments, and in each case, also involving subsonic motions, like found in L1517. Taken together, our observations suggest that core formation in Taurus occurs in two steps. First, 0.5 pc-long velocity-coherent filaments condense out of the cloud gas, probably as a result of the turbulent cascade. From these filaments, dense cores condense quasi-statically, likely due to gravitational instability when the filament has accumulated enough mass.
M. Tafalla, OAN, Spain
J. Kauffmann, JPL, USA
A. Kovacs, UMN, USA