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Astron. Astrophys. 322, 943-961 (1997)

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Non-LTE radiative transfer in clumpy molecular clouds

M. Juvela

Helsinki University Observatory, Tähtitorninmäki, P.O.Box 14, FIN-00014 University of Helsinki, Finland

Received 8 August 1996 / Accepted 2 December 1996


We present some results of the simulation of radiative transfer in clumpy molecular clouds. For this study we have developed a program that can be used to calculate the observed molecular line profiles with one-, two- and three-dimensional model clouds. The three-dimensional models enable us to study the effect of small-scale density fluctuations on the observed molecular emission lines. The density distributions can be generated by using either fractal models or structure tree statistics.

We have made some modifications to the traditional Monte Carlo simulation scheme. These changes can in some cases significantly speed up the simulation and e.g. enable efficient calculations of clouds with high optical depths. We shall describe the principles of the implemented algorithms and present some CS spectra calculated using different clumpy cloud models.

The model calculations have confirmed that clumpy cloud structure combined with macroturbulent motions can decisively reduce self-absorption seen in molecular lines. Such effects indicate that the estimation of e.g. the column density is very uncertain without detailed knowledge of the density structure of the cloud.

Key words: radiative transfer – line: profiles – ISM: clouds – ISM: molecules – radio lines: ISM

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998