2004MNRAS.351.1327C

In the early days of methanol maser discoveries, the 9₂-10₁A⁺ transition at 23.1 GHz was found to exhibit maser characteristics in the northern star-forming region W3(OH) and probable maser emission in two other sources. Attention subsequently turned to the 6.6-GHz 5₁-6₀A⁺ methanol maser transition, which has proved a valuable tracer of early high-mass star formation. We have undertaken a new search for 23.1-GHz methanol masers in 50 southern star formation regions using the Parkes radio telescope. The target sources all exhibit class II methanol maser emission at 6.6 GHz, with 20 sources also displaying maser features in the 107.0-GHz 3₁-4₀A⁺ methanol line. Strong emission at 23.1 GHz in NGC 6334F was confirmed, but no emission was detected in the remaining sources. Thus the 23.1-GHz methanol masers are rare.

A maser model in which methanol molecules are pumped to the second torsionally excited state by radiation from warm dust can account for class II maser activity in all the transitions in which it is observed. According to this model the 23.1-GHz maser is favoured by conditions representing low gas temperature, high external dust temperature, low gas density and high column density of methanol; the scarcity of this maser indicates that such combinations of conditions are uncommon. We have undertaken new model calculations to examine the range of parameters compatible with the upper limits on 23.1-GHz emission from our survey. Further constraints apply in sources with upper limits to maser emission at 107.0 GHz and the combination of data for the two transitions delineates a narrow range of gas density and methanol abundance if the dust temperature is 175 K or greater. While the results are subject to the uncertainties of the chosen model, they may be applicable to the majority of methanol maser sites in the vicinity of newborn high-mass stars, in which methanol masers other than the 6.6- and 12.1-GHz transitions are not detected.