Submillimetre molecular line observations and modelling of molecular clouds.

Abstract

Submillimetre molecular line observations of molecular clouds in our galaxy are presented, and the data analysed using various alternative cloud models. A critical review is given of the methods commonly used to interpret molecular line data, including both theoretical considerations and issues relating to calibration and comparability of results obtained with different telescopes. A detailed comparison is made between results predicted from large velocity gradient (LVG) models, including the generalisation to non-monotonic velocity flows, and those given by "microturbulent" clouds. An LVG model is employed in an investigation of conditions in the molecular outflows frequently found in star formation regions, for which observations in the CO J=3-2 rotational transition at 345 GHz are presented. These are combined with lower frequency data from the literature to derive various properties of the outflows for a sample of 13 sources. The most important result is that local H2 densities exist in the outflows which are higher, typically by an order of magnitude, than previously derived average values obtained using only lower frequency data. Observations are presented of the S255 and DR21 clouds in the transitions CO J=2-1, CO J=3-2, CS J=7-6, HCN J=4-3, HCO+ J=4-3 and -3- H13CO+ J=4-3 and are supplemented by continuum data at 350 s. n and (for DR21) at 20 pm. It is shown that, although some features of the data can be understood in terms of an LVG model, there is compelling evidence for fragmentation of the clouds on length scales much smaller than the cloud sizes. The data are used to constrain the local H2 densities and relative molecular abundances in the clumpy cloud cores, and compared with lower frequency results from the literature. The implications of these results for the star formation environment are discussed, and an assessment made of possible strategies for their further investigation.