A Search for 85.5- and 86.6-Ghz Methanol Maser Emission

We have used the Australia Telescope National Facility Mopra 22-m millimetre telescope to search for emission from the 85.5-GHz 6(-2)-7(-1) E and 86.6-GHz 7(2)-6(3) A(-) transitions of methanol. The search was targeted towards 22 star formation regions which exhibit maser emission in the 107.0-GHz 3(1)- 4(0) A(+) methanol transition, as well as in the 6.6-GHz 5(1)-6(0) A(+) transition characteristic of class II methanol maser sources. Each of these regions was searched at 85.5 GHz, resulting in five detections, of which one appears to be a newly discovered maser. For the 86.6-GHz transition, observations were made of 18 regions, which yielded two detections, but no new maser sources. This search demonstrates that emission from the 6(-2)-7(-1) E and 7(2-)6(3) A(-) transitions is rare. Detection of maser emission from either of these transitions therefore indicates the presence of special conditions, different from those in the majority of methanol maser sources. We have observed temporal variability in the 86.6-GHz emission towards 345.010+1.792, which - along with the very narrow line width - confirms that the emission is a maser in this source.We have combined our current observations with published data for the 6.6-, 12.1-, 85.5-, 86.6-, 107.0-, 108.8- and 156.6- GHz transitions for comparison with the maser model of Sobolev & Deguchi. Both detections and non-detections are useful for setting limits on the physical conditions in star-forming regions which contain methanol maser emission. This has allowed us to estimate the likely ranges of dust temperature, gas density and methanol column density, both for typical methanol maser sources and for those sources which also show 107.0-GHz emission. The gas temperature can also be estimated for those sources exhibiting masers at 85.5 and/or 86.6 GHz.