A Study of 6.7 GHz Methanol Absorption in the Galaxy

One of the most prominent masers found in regions of high-mass star formation is the 6.7GHz transition of methanol. This line has been detected in emission towards hundreds of sources, however detection of 6.7GHz methanol absorption is far less common. We discuss detection of 6.7GHz methanol absorption towards G45.12+0.13, a high-mass star-forming region observed with the Arecibo Telescope in Puerto Rico. We also present results of a complete literature review of 6.7GHz methanol absorption. A total of 30 regions have been reported to show 6.7GHz methanol absorption; in most cases, the absorption is likely against compact radio continuum sources. In only 2 cases the absorption must be against the Cosmic Microwave Background, which indicates that the transition can be anti-inverted. Our study shows that 6.7GHz methanol absorption mostly traces active high-mass star forming regions, based on detection of HII regions and relatively large spectral line-widths, which are indicative of turbulent environments. Even though some blind high-sensitivity surveys have been done, the sensitivity of most studies was optimized for the detection of masers and not weak absorption features, thus a population of weak 6.7GHz methanol absorbers below the ~100mJy level may be present.