This work has confirmed that torsional pumping can account for the simultaneous occurrence of 6 and 12 GHz methanol masers, with the 6 GHz maser generally being brighter. The general conditions required for strong masers governed by this pumping mechanism have been defined: methanol rich () gas with moderate densities (), in the vicinity of warm dust (), with a beamed geometry ().
More specifically, an attempt has been made to use the observed maser ratio to delimit conditions in three different cases of Class II methanol maser formation with a strong continuum source in the background:
(1) The observations in W3(OH) and other strong 6 GHz sources (median ) define a narrow column density regime where the 6 GHz maser turns on first, and highly diluted H II region emission for a prominent part of the maser source. Possibly the species abundance is enhanced following evaporation of cold methanol from grains.
(2) In a 'typical' methanol maser source (median ) the masers are likely to be greatly influenced by the effects of saturation.
(3) In sources where such as NGC6334F the maser geometry involves less beaming and hydrogen density .
These conclusions are subject to uncertainties of modelling, specifically in the collisional excitation rates and also in the nature of the sources and the treatment of radiative transfer. It is clear however that the existence of two strong and spatially coincident maser transitions in methanol provides a sensitive probe of the physical conditions in star formation regions.
© European Southern Observatory (ESO) 1997
Online publication: May 26, 1998