The relationship between Class I and Class II methanol masers at high angular resolution

TL;DR

This study uses high-resolution observations to explore the spatial and flux relationships between Class I and Class II methanol masers, revealing their proximity to sources and association with outflows, enhancing understanding of star formation regions.

Contribution

First high-resolution 95-GHz observations of Class I methanol masers in sources with Class II masers, comparing multiple transitions and analyzing their spatial and flux relationships.

Findings

95-GHz masers are closer to driving sources.

Flux density ratio of 3:1 between 95- and 44-GHz masers.

Class I masers often associated with outflow candidates.

Abstract

We have used the Australia Telescope Compact Array (ATCA) to make the first high resolution observations of a large sample of class~I methanol masers in the 95-GHz ($8_0$--$7_1$A$^+$) transition. The target sources consist of a statistically complete sample of 6.7-GHz class~II methanol masers with an associated 95-GHz class~I methanol maser, enabling a detailed study of the relationship between the two methanol maser classes at arcsecond angular resolution. These sources have been previously observed at high resolution in the 36- and 44-GHz transitions, allowing comparison between all three class~I maser transitions. In total, 172 95-GHz maser components were detected across the 32 target sources. We find that at high resolution, when considering matched maser components, a 3:1 flux density ratio is observed between the 95- and 44-GHz components, consistent with a number of previous lower angular resolution studies. The 95-GHz maser components appear to be preferentially located closer to the driving sources and this may indicate that this transition is more strongly inverted nearby to background continuum sources. We do not observe an elevated association rate between 95-GHz maser emission and more evolved sources, as indicated by the presence of 12.2-GHz class~II masers. We find that in the majority of cases where both class~I and class~II methanol emission is observed, some component of the class~I emission is associated with a likely outflow candidate.