2015A&A...574L...6F

The Pipe nebula is a molecular cloud that lacks star formation feedback and has a relatively simple morphology and velocity structure. This makes it an ideal target for testing cloud evolution through collisions. We aim at drawing a comprehensive picture of this relatively simple cloud to better understand the formation and evolution of molecular clouds on large scales. We use archival data to compare the optical polarization properties, the visual extinction, and the ¹³CO velocities and linewidths of the entire cloud in order to identify trends among the observables. The Pipe nebula can be roughly divided into two filaments with different orientations and gas velocity ranges: E-W at 2-4km/s and N-S at 6-7km/s. The two filaments overlap at the bowl, where the gas shows a velocity gradient spanning from 2 to 7km/s. Compared to the rest of the Pipe nebula, the bowl gas appears to be denser and exhibits larger linewidths. In addition, the polarization data at the bowl shows lower angular dispersion and higher polarization degree. Cores in the bowl tend to cluster in space and to follow the ¹³CO velocity gradient. In the stem, cores tend to cluster in regions with properties similar to those of the bowl. The velocity pattern points to a collision between the filaments in the bowl region. The magnetic field seems to be compressed and strengthened in the shocked region. The proportional increase in density and magnetic field strength by a factor similar to the Alfvenic Mach number suggests a continuous shock at low Alfvenic Mach number under the flux-freezing condition. Shocked regions seem to enhance the formation and clustering of dense cores.