A search for OH 6 GHz maser emission towards supernova remnants

TL;DR

This study investigates the presence of 6049 MHz OH masers in supernova remnants and other regions, finding no detections despite models predicting their existence, suggesting different physical conditions than expected.

Contribution

The paper provides the first observational survey for 6049 MHz OH masers in supernova remnants, challenging existing models of OH column densities and maser excitation conditions.

Findings

No 6049 MHz OH masers detected in supernova remnants

Detected new OH masers in star-forming regions at 6030 and 6035 MHz

OH column densities in remnants are lower than model predictions

Abstract

OH masers at 1720 MHz have proven to be excellent indicators of interactions between supernova remnants and molecular clouds. OH excitation calculations suggest that the 6049 MHz OH maser line is excited for higher column densities than for the 1720 MHz line. Previous observations and modelling of 1612, 1665 and 1667 MHz OH absorption and 1720 MHz OH masers indicated that the column densities in some supernova remnants, ~1e17 cm^-2, may be high enough for 6049 MHz OH masers to exist. It is therefore a potentially valuable indicator of remnant-cloud interaction. We present excitation calculations predicting the formation of 6049 MHz OH masers and results of a survey using the Parkes Methanol Multibeam receiver for 6049, 6035 and 6030 MHz OH masers towards 35 supernova remnants, a star-forming region and 4 fields in the Large and Small Magellanic Clouds. Two new sites of 6035 and 6030 MHz OH maser emission associated with star-forming regions have been discovered, but no 6049 MHz masers were detected to a brightness temperature limit of ~0.3-0.6 K, even though modelling of the OH excitation suggests that maser emission should have been detected. Our upper-limits indicate that the OH column density for a typical remnant is less than 1e16.4 cm^-2, which conflicts with observed and modelled column densities. One possible explanation is that 6049 MHz OH masers may be more sensitive to velocity coherence than 1720 MHz OH masers under some conditions.