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Astron. Astrophys. 343, 939-942 (1999)


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A search for methanol masers at 9.978 GHz and 10.058 GHz

M. Röllig 1, W.H. Kegel 1, R. Mauersberger 2 and C. Doerr 1

1 Institut für theoretische Physik, Universität Frankfurt, Robert-Mayer-Strasse 10, D-60054 Frankfurt am Main, Germany
(roellig@astro.uni-frankfurt.de)
2 Steward Observatory, The University of Arizona, Tucson, AZ 85721, USA (mauers@as.arizona.edu)

Received 26 October 1998 / Accepted 8 December 1998

Abstract

We performed NLTE-radiative transfer calculations for interstellar A-type methanol maser lines in order to find out under which physical conditions the level populations show strong inversion. In our model we considered collisional as well as radiative pumping and accounted for line overlap as well as for torsional excitation of the methanol molecules. The radiation field was calculated using a simple on-the-spot approximation. Our results show that torsional excitation can be essential for the maser pumping mechanism while line overlap appears to be of minor importance. In addition to the well known Class I and Class II maser lines, we found two new transitions which possibly may show up as strong masers. These are the [FORMULA]-[FORMULA] and [FORMULA] transitions at [FORMULA] and [FORMULA] respectively. These masers are radiatively pumped by IR radiation, i.e. they should be found near IR sources similar to the classical Class II methanol masers. 88 galactic CH3OH maser sources were searched for emission in these two lines, but no detection was made. Upper limits are given and discussed.

Key words: radio lines: ISM – ISM: H ii regions – ISM: molecules – radiative transfer – masers

Send offprint requests to: M. Röllig (roellig@astro.uni-frankfurt.de)

© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999

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