Hydroxyl as a tracer of H ₂ in the envelope of MBM40

We observed 51 positions in the OH 1667MHz 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 M, respectively. As much as a third of the total cloud mass may be found in the periphery (E(B - V) < 0.12 mag) and about a half in the envelope (0.12 ≤ E(B - V) ≤ 0.17 mag). If these results are applicable to other translucent clouds, then the OH 1667MHz line is an excellent tracer of gas in very low extinction regions and high-sensitivity mapping of the envelopes of translucent molecular clouds may reveal the presence of significant quantities of molecular mass.