Spectral Energy Distributions of 6.7 GHz methanol masers
  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.
