Spectral Energy Distributions of 6.7 GHz methanol masers

TL;DR

This study models the spectral energy distributions of 6.7 GHz methanol masers to understand their association with early massive star formation stages, confirming they often precede hypercompact HII region development.

Contribution

It provides the first comprehensive SED analysis of methanol masers across multiple wavelengths, linking them to early massive star formation stages.

Findings

70% of masers lack radio continuum emission

SEDs indicate high accretion rates above 10^{-3} M_sun/yr

Most masers are associated with early star formation phases

Abstract

The 6.7 GHz maser transition of methanol has been found exclusively towards massive star forming regions. A majority of the masers have been found to lack the presence of any associated radio continuum. This could be due to the maser emission originating prior to the formation of an HII region around the central star, or from the central object being too cool to produce a HII region. One way to distinguish between the two scenarios is to determine and model the spectral energy distributions (SEDs) of the masers. We observed a sample of 20 6.7 GHz methanol masers selected from the blind Arecibo survey, from centimeter to submillimeter wavelengths. We combined our observations with existing data from various Galactic plane surveys to determine SEDs from centimeter to near-infrared wavelengths. We find that 70% of the masers do not have any associated radio continuum, with the rest of the sources being associated with hypercompact and ultracompact HII regions. Modeling the SEDs shows them to be consistent with rapidly accreting massive stars, with accretion rates well above 10^{-3} M_sun/yr. The upper limits on the radio continuum are also consistent with any ionized region being confined close to the stellar surface. This confirms the paradigm of 6.7 GHz methanol masers being signposts of early phases of massive star formation, mostly prior to the formation of a hypercompact HII region.