Line radiative transfer computations

V. Ossenkopf

My present occupation deals with radiative transfer in rotational lines in molecular clouds. I lay special attention on lines which are neither optically thin nor thick and on regions where systematic velocities are comparable to turbulent velocities.

In the last months I have concentrated onto two topics:

1D numerical simulation of the line radiative transfer in regions with arbitrary density and velocity structure
Extension of the common approximations of complete redistribution and large velocity gradients beyond their limits

1D Simulations

The main product of work on the first problem is the program LTR. It is one of the most flexible codes for the treatment of line radiative transfer in molecular rotational lines presently available and certainly the most accurate one. In the present state, the code can treat linear molecules in spherically symmetric configurations with arbitrary velocity structures and optical depths between 0 and about 3000.

The general ideas are quite close to those of the code described by Dickel and Auer (1994, ApJ 437,222), but due to a completely adaptive discretization the resulting accuracy is much higher. Moreover, a statistical treatment of local turbulence is implemented.

The program manual describing the detailed properties of the code is available as dvi file. The most recent program version is 1.40 released on June 17th 1996. If you have used an earlier version of LTR, have a look at the list of recent changes. At the moment, most experience on the code was collected by the observers from the Max Planck Group "Dust in Star-Forming regions". If you would like to test the code, you can find binaries for most platforms at our ftp server, but, please, mention the origin of the program when publishing results obtained with the help of it.

Further investigations and improvements will deal with:

Introduction of a GUI
Nonlinear molecules
Intermittent turbulence
Photodissociation regions

Basic theory of line transfer

I have found that many of the usually applied approximations have only a very weak theoretical foundation. This especially concerns the assumption of complete redistribution and the application of the large velocity gradient (LVG) approximation in regions with steep density gradients.

By applying exact methods, I could set up criteria for the applicability of these approximations and I could derive more accurate numerical tools for cases beyond those limits.

First results concerning the question of complete redistribution were published in a Poster at the AG-Tagung der Astronomischen Gesellschaft. Here, I have compared complete redistribution, the application of a redistribution function, and the exact integration of the velocity space, providing exact criteria for the applicability of each way. An extended Sobolev approximation was introduced in a paper recently submitted to A&A.

Further investigations include the application to the special situation in highly collimated molecular outflows.

V. Ossenkopf
Dec. 2, 1996