THE DISCOVERY OF A NEW, VERY STRONG, AND WIDESPREAD INTERSTELLAR METHANOL MASER LINE

We report the first detection of very strong maser emission in the 6.6 GHz 5(1) --> 6(0) A+ transition of methanol toward a large number of star-forming regions. Essentially all 6.6 GHz maser sources found are associated with interstellar OH masers. Toward a number of sources, the 6.6 GHz emission has flux densities of several thousand Jy, exceeding that of any known OH maser. Since all known masers in the 12 GHz 2(0) --> 3-1 E transition observationally accesible to us have been found to have 6.6 GHz counterparts, the latter line is undoubtedly a Class II methanol maser transition. In all cases the 6.6 GHz luminosity exceeds the 12 GHz luminosity, often by large factors. Toward Class I methanol maser regions, the 6.6 GHz line is observed in absorption, as has been predicted by pumping models for this variety of sources.