Introduction

All the previous versions of the STENHOLM ^4.1 program have relied on spherical symmetry in the cloud being modelled. Whilst this may be a good approximation to use from the point of view of writing a computer program it unfortunately does not correspond to the common state of star formation regions. These tend to be more disk shaped than spherical and often have outflows which are only symmetrical around their central axis (if symmetrical at all). In order to deal with this kind of problem the STENHOLM program has been rewritten to create a program that can deal with the Axially Symmetric Transfer of RAdiation (ASTRA). This is a fully 2-D version of the original program. Whilst converting the geometry to 2-D the lambda iteration part of the program was also re-written as well as most of the setup section. The original version also used the Rybicki core saturation technique [28] which has now been removed. The core saturation technique's main purpose was to speed up the program for lines with high optical depth, however, it carries a significant overhead in calculating where to approximate and where not to. For most of the lines being studied in this thesis it appeared to make very little difference to the run times and therefore seems to be an unnecessary approximation.  

The first section in this chapter describes the main changes that have been made from the earlier versions of the STENHOLM program in order to make it work for non-linear molecules. This restriction was caused by the program assuming certain properties about the molecules that it was modelling that only applied to linear molecules. One such example is that for all linear molecules the selection rules ensure that the only allowed transition from level J is to level J-1. This is rather easier to program than for a molecule such as formaldehyde where the selection rules allow multiple transitions originating from just one level.

Due to all these changes and to distinguish the 2-D program from the old 1-D one it has been re-named as ASTRA. Whilst the conceptual structure of the program remains similar to the original there is now very little original code remaining.