CH as a Molecular Gas Tracer and C-Shock Tracer Across a Molecular Cloud Boundary in Taurus

TL;DR

This study uses new observations of CH and OH radicals in the Taurus molecular cloud boundary to demonstrate that CH is a reliable tracer of molecular gas and C-shocks, revealing gas dynamics and shock presence.

Contribution

It introduces CH as a superior tracer for total column density in intermediate extinction regions and links CH overabundance to C-shocks in molecular clouds.

Findings

CH abundance is consistent with previous translucent cloud observations.

CH X-factor remains nearly constant across a range of extinctions.

CH shows overabundance below 1 mag extinction, indicating C-shocks.

Abstract

We present new observations of all three ground-state transitions of the methylidyne (CH) radical and all four ground-state transitions of the hydroxyl (OH) radical toward a sharp boundary region of the Taurus molecular cloud. These data were analyzed in conjunction with existing CO and dust images. The derived CH abundance is consistent with previous observations of translucent clouds ($0.8\le A_{v} \le 2.1$ mag). The $X({\rm CH})$-factor is nearly a constant at $(1.0\pm0.06)\times 10^{22}$ $\rm {cm^{-2}~K^{-1}~km^{-1}~s}$ in this extinction range, with less dispersion than that of the more widely used molecular tracers CO and OH. CH turns out be a better tracer of total column density in such an intermediate extinction range than CO or OH. Compared with previous observations, CH is overabundant below 1 mag extinction. Such an overabundance of CH is consistent with the presence of a C-shock. CH has two kinematic components, one of which shifts from 5.3 to 6 km s$^{-1}$, while the other stays at 6.8 km s$^{-1}$ when moving from outside toward inside of the cloud. These velocity behaviors exactly match with previous OH observation. The shifting of the two kinematic components indicates colliding streams or gas flow at the boundary region, which could be the cause of the C-shock.