Multiwavelength study of low-luminosity 6.7-GHz methanol masers
  We present results of 13CO(1-0), C18O(1-0), and HCO+(1-0) map observations
and N2H+2 (1-0) single point observations directed towards a sample of nine
low-luminosity 6.7-GHz masers. N2H+ line emission has been detected from six
out of nine sources, C18O line emission has been detected from eight out of
nine sources, and HCO+ and 13CO emission has been detected in all sources. In
particular, a "blue profile" of the HCO+ spectrum, a signature of inflow, is
found towards one source. From integrated intensity emission maps, we
identified 17 cores in the sample. Among them, nine cores are closely
associated with low-luminosity methanol masers. For these cores, we derive the
column densities, core sizes, masses and molecular abundances. Comparison of
our results with similar molecular line surveys towards the southern sky
methanol masers indicates that linewidths of our sample, including only the
low-luminosity masers, are smaller than the sample that includes both low- and
high-luminosity masers. For the maser associated cores, their gas masses have
the same order of magnitude as their virial masses, indicating that these cores
are gravitationally bound systems. In addition, we have found from our
observations that the low-luminosity methanol masers tend to coexist with H2O
masers and outflows rather than with OH masers.
