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Numerical methods for non-LTE line radiative transfer: Performance and convergence characteristics

G. -J. van Zadelhoff·C. P. Dullemond·F. F. S. van der Tak·J. A. Yates·S. D. Doty·V. Ossenkopf·M. R. Hogerheijde·M. Juvela·H. Wiesemeyer·F. L. Schoeier
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Abstract

Comparison is made between a number of independent computer programs for radiative transfer in molecular rotational lines. The test models are spherically symmetric circumstellar envelopes with a given density and temperature profile. The first two test models have a simple power law density distribution, constant temperature and a fictive 2-level molecule, while the other two test models consist of an inside-out collapsing envelope observed in rotational transitions of HCO+. For the 2-level molecule test problems all codes agree well to within 0.2%, comparable to the accuracy of the individual codes, for low optical depth and up to 2% for high optical depths (tau=4800). The problem of the collapsing cloud in HCO+ has a larger spread in results, ranging up to 12% for the J=4 population. The spread is largest at the radius where the transition from collisional to radiative excitation occurs. The resulting line profiles for the HCO+ J=4-3 transition agree to within 10%, i.e., within the calibration accuracy of most current telescopes. The comparison project and the results described in this paper provide a benchmark for future code development, and give an indication of the typical accuracy of present day calculations of molecular line transfer.

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G. -J. van Zadelhoff, C. P. Dullemond, F. F. S. van der Tak, J. A. Yates, S. D. Doty, V. Ossenkopf, M. R. Hogerheijde, M. Juvela, H. Wiesemeyer, F. L. Schoeier. 2002-08-29. Numerical methods for non-LTE line radiative transfer: Performance and convergence characteristics. https://doi.org/10.1051/0004-6361%3A20021226

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