Nonthermal OH main lines and the abundance of OH in interstellar dust clouds

Whether OH main lines in interstellar dust clouds are thermal (thermal means that the excitation temperatures of the two transitions are equal) has been a matter of some controversy. New observations toward 3C 133, 3C 154, and position 2C demonstrate conclusively that OH main-line anomalies exist in those clouds. The anomaly ΔTe is 1-2 K. The effect of the small anomalies is often major, if line ratios are used together with the assumption of thermal main lines to derive OH optical depths and column densities, overestimates by an order of magnitude or more may result. We suggest that typically τ(OH) < 1 in dust clouds, that N(OH)/Aυ 8 × 1013 cm-2 mag-1 (OH/H 4 × 10-8) over the range Aυ ≈ 0.47 mag, and that there is no evidence for very small-scale clumping in the spatial distribution of OH.