Chemical environments of 6.7 GHz methanol maser sources
  6.7 GHz methanol masers are the brightest of class II methanol masers that
are regarded as excellent signposts in the formation of young massive stars. We
present here a molecular line study of 68 6.7 GHz methanol maser hosts chosen
from the MMB catalogue, that have MALT90 data available. We performed (1)
pixel-by-pixel study of 9 methanol maser sources that have high signal-to-noise
ratio and (2) statistical study taking into account the entire 68 sources. We
estimated the molecular column densities and abundances of N$_2$H$^+$(1-0),
HCO$^+$(1-0), HCN(1-0) and HNC(1-0) lines. The derived abundances are found to
be in congruence with the typical values found towards high mass star forming
regions. We derived the column density and abundance ratios between these
molecular species as an attempt to unveil the evolutionary stage of methanol
maser sources. We found the column density and abundance ratio of HCN to HNC to
increase and that of N$_2$H$^+$ to HCO$^+$ to decline with source evolution, as
suggested by the chemical models. The HCN/HNC, N$_2$H$^+$/HCO$^+$, HNC/HCO$^+$
and N$_2$H$^+$/HNC ratios of the methanol maser sources are consistent with
them being at a later evolutionary state than quiescent phase and possibly
protostellar phase, but at an earlier stage than HII regions and PDRs.
