The structure of the cometary globule CG 12:a high-latitude star-forming region

Artikel i vetenskaplig tidskrift, 2007

We have investigated the structure of the high galactic-latitude cometary globule 12 (CG 12) by means of radio molecular-line observations. Detailed, high signal-to-noise ratio maps in C18O (1-0), C18O (2-1) and molecules tracing high-density gas, CS (3-2), DCO+ (2-1), and H13CO+ (1-0), are presented. The C18O line emission is distributed in a $10\arcmin$ long North-South elongated lane with two strong maxima, CG 12-N(orth) and CG 12-S(outh). In CG 12-S the high-density tracers delineate a compact core, DCO+ core, which is offset by 15´´ from the C18O maximum. The observed strong C18O emission traces either the surface of the DCO+ core or a separate, adjacent cloud component. The driving source of the collimated molecular outflow detected in 1993 is located in the DCO+ core. The C18O lines in CG 12-S have low-intensity wings possibly caused by the outflow. The emission in high-density tracers is weak in CG 12-N and especially the H13CO+, DCO+, and N2H+ lines are +0.5 ${\mathrm{km\,s^{-1}}}$ offset in velocity with respect to the C18O lines. Evidence is presented that the molecular gas is highly depleted. The observed strong C18O emission towards CG 12-N originates in the envelope of this depleted cloud component or in a separate entity seen in the same line of sight. The C18O lines in CG 12 were analysed using positive matrix factorization, PMF. The shape and the spatial distribution of the individual PMF factors fitted separately to the C18O (1-0) and (2-1) transitions were consistent with each other. The results indicate a complex velocity and line excitation structure in the cloud. Besides separate cloud velocity components the C18O line shapes and intensities are influenced by excitation temperature variations caused by e.g., the molecular outflow or by molecular depletion. Assuming a distance of 630 pc the size of the CG 12 compact head, 1.1 pc by 1.8 pc, and the C18O mass larger than 100 $M_{\odot}$ are comparable to those of other nearby low/intermediate mass star formation regions.