The 6.7- and 25-GHz methanol masers in OMC-1

Abstract

The Australia Telescope Compact Array (ATCA) has been used to search for methanol maser emission at 6.7 GHz towards the Orion Molecular Cloud 1 (OMC-1). Two features peaking at 7.2 and -1.1 km s -1 have been detected. The former has at least two components close in both velocity and position. It is located south-east of the Orion Kleinmann-Low (Orion-KL) nebula in the region of outflow traced by the 25-GHz methanol masers and the 95-GHz methanol emission. It is shown by modelling that, in contrast to the widespread opinion that simultaneous masing of methanol transitions of different classes is impossible, there are conditions for which simultaneous masing of the class II transition at 6.7 GHz and some class I transitions (e.g. the series at 25 GHz) is possible. A relevant example is provided, in which the pumping occurs via the first torsionally excited state and is driven by radiation of the dust intermixed with the gas in the cloud. In this regime, the dust temperature is significantly lower (T 60 K) than in the case of bright 6.7-GHz masers (T > 150 K). The narrow spectral feature at -1.1 km s -1 has a brightness temperature greater than about 1400 K, which suggests that it is probably a maser. It emanates from the Orion South region and is probably associated with the approaching part of outflow seen in CO. The 25-GHz maser associated with OMC-1 was observed quasi-simultaneously with the 6.7-GHz observations. No 25-GHz emission associated with the -1.1 km s -1 6.7-GHz feature towards Orion South was detected.