A Multitransition Methanol Survey toward a Large Sample of High-mass Star-forming Regions

We carried out a spectral line survey of CH3OH toward a large sample of 175 high-mass star-forming regions in the 3 mm, 2 mm, and 1.3 mm bands with the Institut de Radioastronomie Millimetrique (IRAM) 30 m telescope. Out of our 175 targets, 148 sources were detected with one or more CH3OH transition lines. Nineteen CH3OH transition lines, including 13 thermal lines and 6 maser lines, were detected. The 8(0) -> 7(1) A(+) (similar to 95.169 GHz) CH3OH maser line, one of the strongest class I CH3OH maser lines, was detected in 52 sources. Forty-two of them are previously reported masers and the other 10 are new detections. Through analyzing the rotational diagram of the detected CH3OH emission lines (nonmasing lines), we obtained the rotational temperature and the column density for 111 sources. Our results show that E-type CH3OH tends to have lower column density than A-type CH3OH. The column density ratio of E/A was derived in 55 sources with the majority having a ratio less than 1.0 (about 70%), with a peak ratio of similar to 0.6. This is consistent with theoretical predictions, i.e., overabundance of A-type CH3OH at low temperature leading to a low E/A ratio. Furthermore, we found that CH3OH abundance decreases beyond T (dust) similar to 30 K, which is supported by modeling results. All these support the fact that CH3OH is easily formed at low-temperature environments, via successive hydrogenation of CO on cold dust surfaces.