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

Abstract

We carried out a spectral line survey of CH _3 OH 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 Millimétrique (IRAM) 30 m telescope. Out of our 175 targets, 148 sources were detected with one or more CH _3 OH transition lines. Nineteen CH _3 OH transition lines, including 13 thermal lines and 6 maser lines, were detected. The 8 _0 → 7 _1 A ^+ (∼95.169 GHz) CH _3 OH maser line, one of the strongest class I CH _3 OH 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 CH _3 OH emission lines (nonmasing lines), we obtained the rotational temperature and the column density for 111 sources. Our results show that E -type CH _3 OH tends to have lower column density than A -type CH _3 OH. 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 ∼0.6. This is consistent with theoretical predictions, i.e., overabundance of A -type CH _3 OH at low temperature leading to a low E / A ratio. Furthermore, we found that CH _3 OH abundance decreases beyond T _dust ∼ 30 K, which is supported by modeling results. All these support the fact that CH _3 OH is easily formed at low-temperature environments, via successive hydrogenation of CO on cold dust surfaces.

Keywords

• Interstellar molecules
• Astrochemistry
• Molecule formation