Hydroxyl as a Tracer of H2 in the Envelope of MBM40

TL;DR

This study demonstrates that the OH 1667 MHz line effectively traces molecular gas in low-extinction regions of translucent clouds, revealing substantial mass in their envelopes and peripheries.

Contribution

It introduces a method to estimate molecular cloud mass using OH emission in low-extinction regions, highlighting the importance of envelope mapping.

Findings

OH emission detected in most of the envelope and core regions.

Mass estimates show significant molecular material in cloud peripheries.

OH line is a reliable tracer for low-extinction molecular gas.

Abstract

We observed 51 positions in the OH 1667 MHz main line transitions in the translucent, high latitude cloud MBM40. We detected OH emission in 8 out of 8 positions in the molecular core of the cloud and 24 out of 43 in the surrounding, lower extinction envelope and periphery of the cloud. Using a linear relationship between the integrated OH line intensity and E(B-V), we estimate the mass in the core, the envelope, and the periphery of the cloud to be 4, 8, and 5 solar masses. As much as a third of the total cloud mass may be found in the in the periphery (E(B-V) $<$ 0.12 mag) and about a half in the envelope (0.12 $\le$ E(B-V) $\le$ 0.17 mag). If these results are applicable to other translucent clouds the OH 1667 MHz line is an excellent tracer of gas in very low extinction regions and high-sensitivity mapping of the envelopes of molecular clouds may reveal the presence of significant quantities of molecular mass.