Structure of the class I methanol masers OMC-2 and NGC 2264 |
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| Authors: | V I Slysh S V Kalenskii |
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| Institution: | (1) Astrospace Center, Lebedev Institute of Physics, Russian Academy of Sciences, Profsoyuznaya ul. 84/32, Moscow, 117997, Russia |
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| Abstract: | Results of interferometric observations of the class I methanol masers OMC-2 and NGC 2264 in the 70-61 A + and 80-71 A + lines at 44 and 95 GHz, respectively, are presented. The maser spots are distributed along the arcs bent toward infrared sources, which are young stellar objects. The distributions of the maser spots at 44 and 95 GHz are virtually identical, and the fluxes from the brightest spots are similar. The measured sizes of the maser spots at 44 GHz are, on average, about 50 AU. The brightness temperature of the strongest components at 44 GHz is 1.7 × 107 K and 3.9 × 107 K for OMC-2 and NGC 2264, respectively. A simple model for the excitation of Class I methanol masers is proposed; it yields an estimate of the limiting brightness temperature of the emission. The model is based solely on the properties of the methanol molecule without invoking the physical parameters of the medium. Using it, we showed that the emission opening angles for NGC 2264 and OMC-2 do not exceed 3° and 4.5°, respectively. The depth of the masing region is about 1000 AU. The emission directivity is naturally realized in the model of of maser consisting of a thermalized core and a thin inverted envelope, probably, with an enhanced methanol abundance. The maser emission has the greatest intensity in the direction tangential to the envelope. The size of the masing envelope estimated from the measured depth and spot extens is ～2 × 104 AU, or 0.15 pc. This size is close to the sizes of the dense molecular cores surrounding the young stellar objects IRS 4 in OMC-2 and IRS 1 in NGC 2264. |
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