Formation of dense structures induced by filament collisions. Correlation of density, kinematics and magnetic field in the Pipe nebula

Abstract

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 Alfvénic Mach number suggests a continuous shock at low Alfvénic Mach number under flux-freezing. Shocked regions seem to enhance the formation and clustering of dense cores.

5 pages, 2 figures. Accepted for publication in A&A

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• Distortion of Magnetic Fields in a Starless Core II: 3D Magnetic Field Structure of FeSt 1-457
• Merging Filaments I: A race against collapse
• A correlation between chemistry, polarization and dust properties in the Pipe Nebula starless core FeSt 1-457
• Distortion of Magnetic Fields in a Starless Core VI: Application of Flux Freezing Model and Core Formation of FeSt 1-457
• Simulation of Head-on Collisions Between Filamentary Molecular Clouds Threaded by a Lateral Magnetic Field and Subsequent Evolution
• Temperature Structure of the Pipe Nebula Studied by the Intensity Anomaly of the OH 18 cm
• A multi-scale view of the magnetic field morphology in the hot molecular core G31.41+0.31
• The kinematics of the magnetised protostellar core IRAS15398-3359
• Evolution of compressed clouds formed by filament coalescence. I. Oblique collisions