The more we discover about the molecular composition of the interstellar medium (ISM) the more we realise how difficult it is to reproduce the mechanisms behind this complex chemistry. To date, over 175 different molecular species have been detected in the ISM. Many of them are formed in the gas phase, but there is a growing number of species that form more efficiently on grain surfaces during the collapse of star-forming cores. An important issue in mimicking the interstellar medium chemistry is that there are few observational clues about the synthesis of complex organic molecules on grains; experimental work coupled with chemical modelling is therefore essential in order to understand the chemical complexity of the ISM. UCL_CHEM is a computer model that takes into account the gas-grain interactions occurring during this collapse, with the aim of reproducing the observed abundances of molecules in various astronomical environments. The work in this thesis deals with the coupling of UCL_CHEM with the most recent experimental results on the formation in the solid state of various complex organic molecules including methyl formate (see Chapter 3) and ethylene oxide and acetaldehyde (whose chemistry is extensively discussed in Chapters 4 and 5), all which have been the subject of recent astronomical interest. Moreover, important revisions of some reactions occurring in the gas phase have also been made. Despite everything seeming straightforward concerning the interstellar chemistry in the gas phase, there is still a great deal to unearth in this regard. Oxygen, for instance, is an important player in the ISM because it is the most abundant element after hydrogen and helium. Although its chemistry seems well understood, we propose a revised scheme for its reactions with small unsaturated hydrocarbons (see Chapter 6) and we show how the new reaction network affects the molecular abundances of these linear carbon chains. In Chapter 6, we also emphasise the relevance in treating structural isomers as two different species when they show peculiar chemical behaviours. Another key issue in reproducing the interstellar molecular variety concerns the freezeout of species onto dust grain surfaces. In particular, we know little about the constituents of the icy mantles. In Chapter 7, we analyse the case of sulfur-bearing species because the most dominant ice component is still debated.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:626439 |
| Date | January 2013 |
| Creators | Occhiogrosso, A. |
| Contributors | Viti, S. |
| Publisher | University College London (University of London) |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://discovery.ucl.ac.uk/1415490/ |
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