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 (5<SUB>1)</SUB> yields (6<SUB>0)</SUB> 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 (2<SUB>0)</SUB> yields (3<SUB>-1)E</SUB> 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.