New class I methanol masers

TL;DR

This paper reviews the properties and distribution of collisionally pumped class I methanol masers, highlighting new rare maser types, their possible origins, and implications for star formation evolution.

Contribution

It provides a comprehensive observational overview of class I methanol masers, introduces a new rare maser transition, and discusses diverse shock origins and evolutionary implications.

Findings

36 and 84 GHz masers are most widespread

23.4 GHz maser is a newly detected rare type

Shocks causing masers may arise from various phenomena

Abstract

We review properties of all known collisionally pumped (class I) methanol maser series based on observations with the Australia Telescope Compact Array (ATCA) and the Mopra radio telescope. Masers at 36, 84, 44 and 95 GHz are most widespread, while 9.9, 25, 23.4 and 104 GHz masers are much rarer, tracing the most energetic shocks. A survey of many southern masers at 36 and 44 GHz suggests that these two transitions are highly complementary. The 23.4 GHz maser is a new type of rare class I methanol maser, detected only in two high-mass star-forming regions, G357.97-0.16 and G343.12-0.06, and showing a behaviour similar to 9.9, 25 and 104 GHz masers. Interferometric positions suggest that shocks responsible for class I masers could arise from a range of phenomena, not merely an outflow scenario. For example, some masers might be caused by interaction of an expanding HII region with its surrounding molecular cloud. This has implications for evolutionary sequences incorporating class I methanol masers if they appear more than once during the evolution of the star-forming region. We also make predictions for candidate maser transitions at the ALMA frequency range.