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Pumping of Class II methanol masers
II. The transition
A.M. Sobolev 1,
D.M. Cragg 2 and
P.D. Godfrey 2
Received 24 June 1996 / Accepted 14 January 1997
We present large velocity gradient (LVG) model calculations which explain the observed intensities () of the methanol line at 6 GHz, which is the brightest of the strong Class II methanol masers. Our model of radiative transfer in the maser source was described in the first paper of this series devoted to the excitation of the transition at 12 GHz (Sobolev & Deguchi, 1994a, Paper I). We consider several collisional models for A -species methanol. Line overlap is found to have little effect on the intensities of the brightest methanol maser lines.
The present calculations confirm that pumping operating through the levels of the second and first torsionally excited states can explain the existence of masers, and the observed brightnesses of the and lines in W3(OH). The pumping mechanism requires ambient dust of temperature K with the maser regions having methanol column densities and hydrogen number densities . The strongest masers present in the vicinity of H II regions should be beamed. We find that the required methanol abundance is such that conditions in Class II methanol masers are likely to be influenced by the passage of shock waves. Recently discovered variability in the strongest methanol maser lines could be explained by movements of the medium.
The brightness of the methanol line in our model is strongly determined by the free-free radio continuum emission from the underlying ultracompact H II region. This emission strongly influences the excitation of the saturated transition, as well as providing a source of background radiation for amplification. It is shown that to produce the observed intensities of the strongest Class II methanol maser lines in W3(OH) the H II region emission should be highly diluted (). This implies that a substantial portion of the maser radiation forms in regions which are situated at a considerable distance from the H II region. Therefore, the conditions necessary for the appearance of masers with an underlying continuum source are likely to be produced by the shock wave preceding the ionization front which forms the ultracompact H II region. It is shown that Class II maser spots most probably correspond to radial velocity correlation paths in the turbulent medium. This example shows how the combined observation of 6 and 12 GHz methanol masers can be used to delimit physical conditions in star-forming regions.
Key words: masers radiative transfer H ii regions ISM: molecules radio lines: ISM
Send offprint requests to: D.M. Cragg
© European Southern Observatory (ESO) 1997
Online publication: May 26, 1998