Probing accretion of ambient cloud material into the Taurus B211/B213
  filament
  Herschel observations have emphasized the role of molecular filaments in star
formation. However, the origin and evolution of these filaments are not yet
well understood, partly because of the lack of kinematic information. To
examine whether the B211/B213 filament is accreting background gas due to its
gravitational potential, we produced a toy accretion model and compared its
predictions to the 12CO(1--0) and 13CO(1--0) velocity patterns. We also
examined the spatial distributions of Halpha, 857 GHz continuum, and HI
emission to search for evidence of large-scale external effects. We estimated
the depth of the cloud around the B211/B213 filament to be 0.3--0.7 pc under
the assumption that the density of the gas is the same as the 13CO critical
density. Compared to a linear extent of >10 pc in the plane of the sky, this
suggests that the 3D morphology of the cloud is sheet-like. 12CO and 13CO PV
diagrams perpendicular to the filament axis show that the emission from the gas
surrounding B211/B213 is redshifted to the northeast of the filament and
blueshifted to the southwest, respectively, and that the velocities of both
components approach the filament velocity as the line of sight approaches the
filament crest. The PV diagrams predicted by our accretion model are in good
agreement with the observed 12CO and 13CO PV diagrams, supporting the scenario
of mass accretion into the filament proposed by Palmeirim et al. Moreover,
inspection of the distribution of the Halpha and 857 GHz emission in the
Taurus-California-Perseus region suggests that the B211/B213 filament may have
formed as a result of an expanding supershell generated by the Per OB2
association. Based on these results, we propose a scenario in which the
B211/B213 filament was initially formed by large-scale compression of HI gas
and then is now growing in mass due to the gravitational accretion of ambient
cloud molecular gas.
