Formation of dense structures induced by filament collisions.
  Correlation of density, kinematics and magnetic field in the Pipe nebula
  Context. 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 to test cloud evolution through collisions. Aims. 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. Methods. 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. Results.
The Pipe nebula can be roughly divided in two filaments with different
orientations and gas velocity ranges: E-W at 2-4 km s-1 and N-S at 6-7 km s-1.
The two filaments overlap at the bowl, where the gas shows a velocity gradient
spanning from 2 to 7 km s-1. 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 tend to
follow the 13CO velocity gradient. In the stem, cores tend to cluster in
regions with properties similar to those of the bowl. Conclusions. 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 of density and magnetic field strength by a factor
similar to the Alfv\'enic Mach number suggests a continuous shock at low
Alfv\'enic Mach number under flux-freezing. Shocked regions seem to enhance the
formation and clustering of dense cores.
