New Numerical Code to Treat Radiative Transfer in Methanol Masers

Details New Numerical Code to Treat Radiative Transfer in Methanol Masers New Numerical Code to Treat Radiative Transfer in Methanol Masers

Jan 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009aas...21348512b&link_type=abstract

American Astronomical Society, AAS Meeting #213, #485.12; Bulletin of the American Astronomical Society, Vol. 41, p.457

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The most common method to account for the effects of resonant radiation transfer in numerical models of methanol masers is the well-known Large Velocity Gradient (LVG) approximation. The assumptions underlying this approximation are plausible when the maximum difference in the line-of-sight velocities across the cloud is much greater than the mean thermal velocity of the molecules. However, the masing methanol clouds seem to be relatively quiescent. Typical observed line widths (≤1; km/s), even accounting for some maser line narrowing (probably by less than a factor of 3), are only moderately greater than the thermal broadening ( 0.5 km/s at the anticipated kinetic temperature of 50K), which puts the validity of the LVG approximation into question. We developed a code treating the radiation transfer in the rotational lines of methanol in a static medium of large optical depth. The code is based on the "Transfer Function Approximation” for the resonant photon escape probability from a plane-parallel medium and allows for numerical modeling of pumping processes involving collisional and radiative transitions. Both external radiation sources and internal sources and sinks can be included in the numerical model. We will demonstrate the first results of simulations using the new code and compare these results with those obtained with an LVG code. This project was supported by the NSF/REU grant AST-0354056 and the Nantucket Maria Mitchell Association.

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