Astronomy and Astrophysics, volume 291, 569-576 (1994/11-2)
Pumping of Class II methanol masers. I. The 20-3–1E transition.
SOBOLEV A.M. and DEGUCHI S.
Abstract (from CDS):
We present LVG model calculations which explain observed 20-3–1E line brightnesses in the strongest Class II methanol masers (MMII). The model explains the variations in spectral appearance of the different maser lines observed from the same source using single-dish facilities through differences in the sensitivities of the line intensities to the MMII physical parameters. In our model masers are pumped by emission of a nearby layer of hot dust with temperature >150K. The MMII are seen in projection on the H II region producing free-free radio continuum emission. It is shown that radiative excitations from rotational levels of the ground state to the levels of the 2nd and the 1st torsionally excited states both play important role in the pump. We found that the strong MMII (Tb>1010K) should be beamed. The value of [CH3OH]/[H2] in the strong MMII sources should exceed 7x10–7. Result. suggest that the chemistry of the strong MMII was influenced by the shock wave passage. The strong MMII sources should have hydrogen number densities >3x106cm–3 and gas temperatures <50K. Thus, results of the present calculations indicate that MMII clumps were exposed to some fast and efficient cooling process. We suppose that local temperature variations can explain the observed absence of spatial coincidence between the MMII and OH maser spots. The MMII with Tb≥109K can be produced in the sources with the optical depth equal in all directions. Like the strong MMII they should be dense (nH>106 cm-3), methanol-abundant ([CH3OH]/[H2]>10–7) and relatively cold (T<75K).
Key words: masers - radiative transfer - H II regions - ISM: molecules radio lines: interstellar