How do methanol masers manage to appear in the youngest star vicinities and isolated molecular clumps?

TL;DR

This paper reviews methanol maser characteristics, their association with star-forming regions, and models their excitation mechanisms, revealing insights into the physical conditions and triggering processes in young stellar environments.

Contribution

It provides a comprehensive analysis of methanol maser properties, their environmental associations, and models the physical conditions and excitation mechanisms involved.

Findings

Methanol masers are concentrated in spiral arms and associated with star-forming regions.

Class II masers can be pumped by dust radiation and free-free emission from hypercompact HII regions.

Periodic flux variations may be caused by changes in dust temperature related to stellar accretion.

Abstract

General characteristics of methanol (CH3OH) maser emission are summarized. It is shown that methanol maser sources are concentrated in the spiral arms. Most of the methanol maser sources from the Perseus arm are associated with embedded stellar clusters and a considerable portion is situated close to compact HII regions. Almost 1/3 of the Perseus Arm sources lie at the edges of optically identified HII regions which means that massive star formation in the Perseus Arm is to a great extent triggered by local phenomena. A multiline analysis of the methanol masers allows us to determine the physical parameters in the regions of maser formation. Maser modelling shows that class II methanol masers can be pumped by the radiation of the warm dust as well as by free-free emission of a hypercompact region hcHII with a turnover frequency exceeding 100 GHz. Methanol masers of both classes can reside in the vicinity of hcHIIs. Modelling shows that periodic changes of maser fluxes can be reproduced by variations of the dust temperature by a few percent which may be caused by variations in the brightness of the central young stellar object reflecting the character of the accretion process. Sensitive observations have shown that the masers with low flux densities can still have considerable amplification factors. The analysis of class I maser surveys allows us to identify four distinct regimes that differ by the series of their brightest lines.