Discovery of weak 6.7-GHz CH3OH masers in a sample of high-mass Hi-GAL
  sources
  Maser lines from different molecular species, including water, hydroxyl, and
methanol, are common observational phenomena associated with massive star
forming regions. In particular, the methanol maser appears as an ideal tool to
study the early phases of massive star formation. However, it is difficult to
establish the exact start of the methanol maser phase, and it would then be
interesting to detect and study low-flux density methanol masers (i.e., < 0.1
Jy or even << 0.1 Jy), in order to determine if they can effectively be used to
mark a specific evolutionary phase in high-mass star formation. Past surveys
have been unable to systematically detect many low-flux density methanol
masers, and thus we do not yet know how many such masers exist in the Galaxy
and what is their physical nature. Out of a sample of 107 observed Hi-GAL
sources we detected a total of 32 methanol masers, with 22 sources being new
and weak (median peak flux density 0.07 Jy) detections, in the Galactic
longitude range [32.0, 59.8]deg. We also detected 12 6.035-GHz OH maser, with 9
objects being new detections. Our survey covers a similar range of source
distances as the "Arecibo Methanol Maser Galactic Plane Survey", but the
methanol masers detected by us are clearly shifted towards lower integrated
flux densities. The newly detected methanol masers are mostly of low-luminosity
and, except for some sources, their weakness is not due to distance effects or
positional offsets. No specific correlation is found with the physical
parameters of the Hi-GAL clumps, except for sources with both CH3OH and OH
masers which tend to have higher mass and luminosity. The intensity of the
methanol masers correlates well with the velocity range of the maser emission,
which suggests that the low brightness of these masers is related to the number
of maser spots in the emitting region and their evolution with time.
