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. Schöier
2002 Astronomy and Astrophysics  
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
more » ... ecule 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.
doi:10.1051/0004-6361:20021226 fatcat:cez4zkkzfzcxreaw6edlssqqqe