A CO observation of the Galactic methanol masers

¹ The Department of Astronomy, Peking University, No.5 Yiheyuan Road, HaiDian District, 100871 Beijing PR China
e-mail: renzy@bao.ac.cn, yfwu@pku.edu.cn
² Key Laboratory of Radio Astronomy, National Astronomical Observatories (NAOC), Chinese Academy of Science, Chaoyang District, Datun Rd A20, 100012 Beijing, PR China
³ The Kavli Institute for Astronomy and Astrophysics (KIAA), Peking University (PKU), No. 5 Yiheyuan Road, HaiDian District, 100871 Beijing, PR China
⁴ Purple Mount Observatory, Chinese Academy of Science, PO Box 26, Delingha, 817000 Qinghai, PR China
⁵ State Key Laboratory of Radio Astronomy, Chinese Academy of Science, PR China

Received: 28 May 2012
Accepted: 10 April 2014

Abstract

Context. We investigated the molecular gas associated with 6.7 GHz methanol masers throughout the Galaxy using a J = 1−0 transition of the CO isotopologues.

Aims. The methanol maser at 6.7 GHz is an ideal tracer for young high-mass star-forming cores. Based on molecular line emissions in the maser sources throughout the Galaxy, we can estimate their physical parameters and, thereby, investigate the forming conditions of the high-mass stars.

Methods. Using the 13.7-m telescope at the Purple Mountain Observatory (PMO), we have obtained ¹²CO and ¹³CO (1−0) lines for 160 methanol masers sources from the first to the third Galactic quadrants. We made efforts to resolve the distance ambiguity by careful comparison with the radio continuum and HI 21 cm observations. We examined the statistical properties in three aspects: first, the variation throughout the Galaxy; second, the correlation between the different parameters; third, the difference between the maser sources and the infrared dark clouds. In addition, we have also carried out ¹³CO mapping for 33 sources in our sample.

Results. First, the maser sources show increased ¹³CO line widths toward the Galactic center, suggesting that the molecular gas are more turbulent toward the Galactic center. This trend can be noticeably traced by the ¹³C line width. In comparison, the Galactic variation for the H₂ column density and the ¹²CO excitation temperature are less significant. Second, the ¹²CO excitation temperature shows a noticeable correlation with the H₂ column density. A possible explanation consistent with the collapse model is that the higher surface-density gas is more efficient to the stellar heating and/or has a higher formation rate of high-mass stars. Third, comparing the infrared dark clouds, the maser sources on average have significantly lower H₂ column densities, moderately higher temperatures, and similar line widths. Fourth, In the mapped regions around 33 masers, 51 ¹³CO cores have been revealed. Among them, only 17 coincide with the radio continuum emission (F_cm> 6 mJy), while a larger fraction (30 cores) coincide with the infrared emissions. Only one maser source has no significant IR emission. The IR-bright and radio-bright sources exhibit significantly higher ¹²CO excitation temperatures than the IR-faint and radio-faint sources, respectively.

Conclusions. The 6.7 GHz masers show a moderately low ionization rate but have a common-existing stellar heating that generates the IR emissions. The relevant properties can be characterized by the ¹²CO and ¹³CO (1–0) emissions in several aspects as described above.