Pumping of Class II methanol masers. II. The 5_1_-6_0_A^+^ transition.

Details Pumping of Class II methanol masers. II. The 5_1_-6_0_A^+^ transition. Pumping of Class II methanol masers. II. The 5_1_-6_0_A^+^ transition.

Aug 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997a%26a...324..211s&link_type=abstract

Astronomy and Astrophysics, v.324, p.211-220

Astronomy and Astrophysics

Astrophysics

50

Masers, Radiative Transfer, Hii Regions, Ism: Molecules, Radio Lines: Ism

Scientific paper

We present large velocity gradient (LVG) model calculations which explain the observed intensities (>10^12^K) of the 5_1_-6_0_A^+^ methanol line at 6GHz, 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 2_0_-3_-1_E transition at 12GHz (Sobolev & Deguchi, 1994A&A...291..569S, 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 5_1_-6_0_A^+^ and 2_0_-3_-1_E lines in W3(OH). The pumping mechanism requires ambient dust of temperature >150K with the maser regions having methanol column densities >2.x10^15^cm^-2^ and hydrogen number densities <10^8^cm^3^. The strongest masers present in the vicinity of HII 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 5_1_-6_0_A^+^ methanol line in our model is strongly determined by the free-free radio continuum emission from the underlying ultracompact HII region. This emission strongly influences the excitation of the saturated 5_1_-6_0_A^+^ 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 HII region emission should be highly diluted (W_ HII_<3x10^-4^). This implies that a substantial portion of the maser radiation forms in regions which are situated at a considerable distance from the HII 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 HII 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 12GHz methanol masers can be used to delimit physical conditions in star-forming regions.

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