OH and CO as tracers of molecular gas in MBM 53

We observed the OH 18 cm lines in the northern portion of the high-latitude diffuse molecular cloud, MBM 53. The 1667MHz line was detected in 11 of 44 lines of sight at rms levels of 7-10 mK. This region was previously observed in the CO(1-0) line at high sensitivity and spectral resolution. Donate & Magnani (2017) detected CO in 19 of the 44 lines of sight with an A(V) threshold of 0.25mag, while the OH detection threshold in A(V) is 0.35 mag. We see more of the extent of the cloud with the CO(1-0) line when both transitions are observed with similar integration times. However, using standard assumptions for converting the OH and CO observations to the molecular mass in the region lead to three times greater value in the OH-derived mass. This somewhat puzzling result likely indicates that the OH/CO abundance varies significantly in the outer, low-extinction regions of MBM 53 and implies that a larger CO-H-2 conversion factor may apply there.