Christie, H; (2012) Structure and depletion in star forming clouds. Doctoral thesis (PhD), UCL (University College London).

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Abstract

Observations of star forming molecular clouds reveal clumpiness on all scales, both in the spectra of molecules and thermal continuum emission from the solid component of the interstellar medium, the dust. Recent, high resolution maps have allowed us to probe down to extremely small scales at which we see clumps of radii just several hundredths of a parsec. A good knowledge of the structure of these regions, and of the chemical processes occurring within, is crucial if we want to properly understand the early stages of star formation and the resulting stellar population. However, observations of cold, dense environments are challenging. Molecules emit at long wavelengths which are notoriously difficult to observe. A comparison with models is also complicated by the fact that in these conditions molecules will freeze-out onto dust grain surfaces forming icy mantles. We know little about the rate at which this process occurs in interstellar conditions, or the chemical reactions that happen on the grain surfaces. In this thesis we present two alternative methods by which to investigate the underlying clumpy nature of a molecular cloud and consider freeze-out in such an environment. Small, quiescent regions of enhanced emission in several molecules (including ammonia and HCO+) have been observed near to Herbig-Haro objects (HHOs) in star forming clouds. It was suggested that these could be due to molecules in small dense clumps being liberated from the dust grain surface by radiation from the shock front. Chemical modelling later proved this theory to be viable, and it was further supported by observational surveys and more detailed modelling of specific regions. In chapter 2 we simulate a dense clump near to an HHO, adapting the chemical code used in the original models to allow the shock front to move past the clump, providing a more realistic description of the effect of the radiation field. Chapter 3 describes how the outputs from these models can be used to simulate observations of part of a molecular cloud made up of small, transient density enhancements irradiated by a passing shock front. We briefly compare our synthetic maps with HCO+ spectra in regions surrounding HHOs. Commonly, researchers use decomposition algorithms on 2D and 3D maps to pick out clumps of emission and evaluate their properties. The mass functions of these objects often appear to emulate the stellar initial mass function, which has led researchers to conclude that the stellar mass is set at a very early stage, prior to the switch on of the protostar. In Chapter 4 we introduce the Gould Belt clouds for which we have HARP CO and SCUBA data (the HARP maps are presented in Appendix B). It is these on which we perform the analysis described in the final 3 Chapters. In Chapter 5 we investigate four popular clumpfinding algorithms, testing them on both synthetic and real (HARP) data, and explore the impact of user defined input parameters on derived properties. We choose one algorithm, with one set of input parameters, and use this to analyse the distribution of CO clumps in five nearby molecular clouds. The results of this study are outlined in Chapter 6. Chapter 7 focuses on the process by which CO freezes-out (depletes) onto the surfaces of dust grains in dark clouds. A single value for the depletion of a particular molecule is difficult to achieve because of its strong dependence on environmental factors and the past evolution of a region. However, we have a consistent data set across a range of environments and so are able to perform a statistical study in which we compare hydrogen densities derived from dust emission with those calculated using the CO maps. We look for missing CO in the gas phase which we then assume to be the result of depletion.

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