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21st of February 2020
Qiao et al. present high spatial resolution observations of ground-state OH masers achieved with the Australia Telescope Compact Array (ATCA). These observations targeted 253 pointing centres containing OH maser candidates at all four ground-state OH transitions identified in the Southern Parkes Large-Area Survey in Hydroxyl (SPLASH) across 96 square degrees of the Southern Galactic plane. Ground-state OH masers are relatively common in the vicinity of high-mass star formation (HMSF), the circumstellar envelopes of evolved giant and supergiant stars, and the interaction regions of supernova remnants (SNRs) and their surrounding molecular clouds. OH masers associated with HMSF regions are primarily from the main line transitions, i.e., 1665 and 1667 MHz, whereas evolved star OH masers are predominantly strong in the 1612 MHz transition.

The Figure shows a Venn diagram of maser transition overlap for the full sample of 158 star formation maser sites. 1665 MHz OH masers are the most common, with the majority of star formation sites (128/158) showing 1665 MHz masers. Of these, 31% (40/128) are solitary, i.e., do not show other groundstate OH transitions. Nevertheless, main line transitions show the largest overlap, with 87% (77/89) of 1667 MHz masers associated with 1665 MHz OH emission, which corresponds to 60% (77/128) of 1665 MHz masers having a 1667 MHz maser counterpart. The second-largest overlap is seen between 1665 and 1720 MHz masers, with 75% (18/24) of 1720 MHz sources showing a 1665 MHz counterpart. The most commonly solitary transition is 1612 MHz, with 39% (12/31) showing no association with other frequencies. More details are given in the paper to be published in Monthly Notices of the Royal Astronomical Society.




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