On the Excitation of "Quasi-thermal” Lines of Methanol

Details On the Excitation of "Quasi-thermal” Lines of Methanol On the Excitation of "Quasi-thermal” Lines of Methanol

Jan 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011aas...21725121m&link_type=abstract

American Astronomical Society, AAS Meeting #217, #251.21; Bulletin of the American Astronomical Society, Vol. 43, 2011

Astronomy and Astrophysics

Astronomy

Scientific paper

We revisit the problem of the excitation of "quasi-thermal” lines of methanol in dense molecular clouds. Our analysis is based on the observations of 13 clouds in the four rotational lines of E-methanol (J0 - J-1 J =1-4) at 157 GHz using the 12-m ARO telescope on Kitt Peak (AZ) and on comparison of the results with the steady-state solutions for the level populations in a Monte-Carlo simulation of radiative transfer. A higher spectral resolution allowed us to deconvolve the blended 10-1-1, 20-2-1 and 30-3-1 lines better than in previous works. Treating the 157 GHz lines as spontaneous and optically thin, we estimate the relative populations of the J =1 to 4 levels in the K= 0 stack. Most of the sources demonstrate strong deviations from the thermal population distribution with a single excitation temperature. A number of them can be divided into two groups showing the signatures of either "Class I” or "Class II” pumping. This dichotomy is probably determined by the distance of the bulk of the observed gas from the closest IR source controlling the strength of the "Class II” pumping. However, a considerable number of sources do not fit into either class, some of them showing, in particular, a significant overheating of the 40-30 transition not predicted by either pumping model. This discrepancy can be due to oversimplification of our theoretical model. This project was supported by NSF/REU grant AST-0851892 and the Nantucket Maria Mitchell Association.

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