The discovery of a new, very strong, and widespread interstellar methanol maser line
The first detection is reported of very strong maser emission in the 6.6 GHz (51) yields (60) A exp + 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 (20) yields (3-1)E transition that are observationally accessible 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 source.