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

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7. Summary

We have developed a new program for solving the radiative transfer problem in molecular clouds. The density distributions of the clouds are created either with fractal models or with the aid of structure trees. Based on theoretical considerations and the results from the test runs we have concluded that:

  1. quasirandom numbers should be used in the simulation instead of pseudorandom numbers
  2. the execution times can in some cases be dramatically reduced with a simple acceleration method involving the changes of the population levels
  3. it may be necessary to reset the random number generators after each iteration in order to be able to follow the convergence; this can usually be done without endangering the correctness of the solution
  4. in clouds with large density fluctuations or high optical depths our simulation method B should be more effective than the traditional Monte Carlo simulation

We have calculated spectra from different types of clumpy clouds and compared the results with the results from simple spherically symmetric cloud models. These results have confirmed that

  1. the clumpy cloud structure and especially the existence of macroturbulence are needed to explain the general lack of self-absorption in the observed molecular spectra
  2. apart from the mass spectrum of the clumps the radiative transfer properties of the clouds are not strongly dependent on the way the model clouds are generated
  3. the estimates of column densities are very uncertain without detailed knowledge of the (clumpy) density distribution

In a forthcoming article we shall use our three-dimensional cloud models to analyze CS and C34 S measurements of southern massive star forming regions (Juvela 1996). The derived physical parameters will be compared with the values calculated assuming LTE and with LVG models.

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998