37 GHz methanol masers : Horsemen of the Apocalypse for the class II
  methanol maser phase?
  We report the results of a search for class II methanol masers at 37.7, 38.3
and 38.5 GHz towards a sample of 70 high-mass star formation regions. We
primarily searched towards regions known to show emission either from the 107
GHz class II methanol maser transition, or from the 6.035 GHz excited OH
transition. We detected maser emission from 13 sources in the 37.7 GHz
transition, eight of these being new detections. We detected maser emission
from three sources in the 38 GHz transitions, one of which is a new detection.
We find that 37.7 GHz methanol masers are only associated with the most
luminous 6.7 and 12.2 GHz methanol maser sources, which in turn are
hypothesised to be the oldest class II methanol sources. We suggest that the
37.7 GHz methanol masers are associated with a brief evolutionary phase (of
1000-4000 years) prior to the cessation of class II methanol maser activity in
the associated high-mass star formation region.
