12.2-GHz methanol masers associated with 1.2-mm dust clumps: Quantifying
  high-mass star formation evolutionary schemes
  We report the results of a search for 12.2-GHz methanol maser emission,
targeted towards 113 known 6.7-GHz methanol masers associated with 1.2-mm dust
continuum emission. Observations were carried out with the Australia Telescope
National Facility (ATNF) Parkes 64-m radio telescope in the period 2008 June 20
- 25. We detect 68 12.2-GHz methanol masers with flux densities in excess of
our 5-sigma detection limit of 0.55 Jy, 30 of which are new discoveries. This
equates to a detection rate of 60 per cent, similar to previous searches of
comparable sensitivity. We have made a statistical investigation of the
properties of the 1.2-mm dust clumps with and without associated 6.7-GHz
methanol maser and find that 6.7-GHz methanol masers are associated with 1.2-mm
dust clumps with high flux densities, masses and radii. We additionally find
that 6.7-GHz methanol masers with higher peak luminosities are associated with
less dense 1.2-mm dust clumps than those 6.7-GHz methanol masers with lower
luminosities. We suggest that this indicates that more luminous 6.7-GHz
methanol masers are generally associated with a later evolutionary phase of
massive star formation than less luminous 6.7-GHz methanol maser sources.
Analysis of the 6.7-GHz associated 1.2-mm dust clumps with and without
associated 12.2-GHz methanol maser emission shows that clumps associated with
both class II methanol maser transitions are less dense than those with no
associated 12.2-GHz methanol maser emission; suggesting that 12.2-GHz methanol
masers are associated with a later evolutionary phase of massive star
formation. We present an evolutionary sequence for masers in high-mass star
formation regions, placing quantitative estimates on the relative lifetimes for
the first time.
