2023ApJS..266...29Z

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 Millimétrique (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 8₀ → 7₁ A⁺ (∼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 ∼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 ∼ 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.