A search for 4750‐ and 4765‐MHz OH masers in southern star‐forming regions

We have used the Australia Telescope Compact Array (ATCA) to make a sensitive (5 σ ≃100 mJy) search for maser emission from the 4765‐MHz 2 Π 1/2 F =1→0 transition of OH. 55 star formation regions were searched and maser emission with a peak flux density in excess of 100 mJy was detected toward 14 sites, with 10 of these being new discoveries. In addition we observed the 4750‐MHz 2 Π 1/2 F =1→1 transition towards a sample of star formation regions known to contain 1720‐MHz OH masers, detecting marginal maser emission from G348.550−0.979. If confirmed this would be only the second maser discovered from this transition. The occurrence of 4765‐MHz OH maser emission accompanying 1720‐MHz OH masers in a small number of well‐studied star formation regions has led to a general perception in the literature that the two transitions favour similar physical conditions. Our search has found that the presence of the excited‐state 6035‐MHz OH transition is a much better predictor of 4765‐MHz OH maser emission from the same region than 1720‐MHz OH maser emission is. Combining our results with those of previous high‐resolution observations of other OH transitions we have examined the published theoretical models of OH masers and find that none of them predicts any conditions in which the 1665‐, 6035‐ and 4765‐MHz transitions are inverted simultaneously.