Pumping of Class 2 methanol masers. 1: The 20 - 3-1E transition

Details Pumping of Class 2 methanol masers. 1: The 20 - 3-1E transition Pumping of Class 2 methanol masers. 1: The 20 - 3-1E transition

Nov 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...291..569s&link_type=abstract

Astronomy and Astrophysics (ISSN 0004-6361), vol. 291, no. 2, p. 569-576

Astronomy and Astrophysics

Astrophysics

50

Astronomical Models, H Ii Regions, Maser Pumping, Masers, Methyl Alcohol, Molecular Excitation, Radiative Transfer, Radio Emission, Rotational States, Cosmic Dust, Stimulated Emission, Variations

Scientific paper

We present a large velocity gradient (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 greater than 150 K. 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 greater than 1010K) should be beamed. The value of (CH3OH)/H2 in the strong MMII sources should exceed 7 x 10-7. Results suggest that the chemistry of the strong MMII was influenced by the shock wave passage. The strong MMII sources should have hydrogen number densities greater than 3 x 106/cu cm and gas temperatures less than 50 K. 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 greater than or equal to 109 K can be produced in the sources with the optical depth equal in all directions. Like the strong MMII they should be dense nH greater than (106/cu cm - 3), methanol-abundant ((CH3OH)/H2) greater than 10-7 and relatively cold (T less than 75 K).

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