Regions of massive star formation have been extensively studied in the lines of simple molecules like CO, CS, or NH3. In contrast, only a few have been carefully studied in thermal lines of more complex molecules, such as methanol (CH3 OH) or methyl cyanide (CH3 CN). Complex molecules are less abundant than simpler ones and their thermal lines are typically weak, making observations of them difficult. However, observations of these molecules are very important for interstellar chemistry; in addition, the richness of their spectra and tendency to form groups of lines, which can be observed simultaneously with the same telescope and receiver, allows one to obtain information about physical parameters of the sources which sometimes cannot be obtained from observations of simple molecules.
Methanol is one of the most abundant complex molecules in Galactic star-forming regions. The methanol molecule is a slightly asymmetric top with hindered internal rotation, and possesses a large number of allowed transitions at radio frequencies. Methanol in space has been intensively studied since its discovery by Ball et al. (1970), but the attention has mostly been concentrated on methanol masers, and much less effort has been spent on studying thermal methanol. Recently, Slysh et al. (1995, 1996) made a large survey of Galactic star-forming regions in the series of lines near 157 GHz and in the line near 133 GHz. They detected 4 masers and 73 thermal sources at 157 GHz and 6 masers and 33 thermal sources at 133 GHz. The 157 and 133 GHz lines alone proved to be insufficient to determine some parameters of interest. We therefore observed 12 sources from the lists of Slysh et al. in the series near 96 GHz. These lines were detected in space by Gottlieb et al. (1979). We chose this series because the levels , , , and are common for the , lines at 96 GHz and the , lines at 157 GHz. This allowed us to estimate source parameters combining the 157 and 96 GHz data.
In this paper, we describe the method and present our results; comparison of our results with the data of other investigators and more detailed analysis of source structures using the available data on other molecular species will be given in subsequent papers.
© European Southern Observatory (ESO) 1997
Online publication: June 30, 1998