Astrochemical pathways for the synthesis of amino-acids in interstellar clouds

Abstract

There is much evidence to support the exogeneous origin of amino acids on Earth. For instance, one may cite the identification of amino acids in meteoritic samples with isotopic ratios pointing towards a non-terrestrial origin, the enantiomeric excess found in the Murchison meteorite in the same direction as the homochirality problem found on Earth, or the detection of glycine in the coma of comet Churyumov-Gerasimenko and some of its likely precursors in the Interstellar Medium (ISM). However, if we adopt an exogeneous point of view, one can wonder if these amino acids are formed in the initial molecular cloud, during the formation of the solar system in a cold region of the protoplanetary disk, due to a modification due to a perturbation, or via processing within the parent-body. To achieve a deeper insight into this issue, this master thesis focuses on a hypothetical formation of glycine in dense molecular clouds. After a deep bibliographic analysis of the many proposed mechanisms and a selection of the most relevant ones to be considered in Chapter 1, the gas-phase Astrochem code has been used. Its basic functionalities have been explained in Chapter 2. Starting from this modelling tool, the most likely precursors of glycine in a pure gas-phase context have been deeply investigated in Chapter 3 in which we have inferred the relative temporal evolution of the abundances and studied their major contributing paths. This has allowed us to deeply discuss the potential of these routes in such a medium. Moreover, we have been able to compare our results with earlier studies, in which much simpler astrochemical networks have been used. The results for the most abundant molecular species present a good match with respect to previous studies, but do not when more complex species are considered. Finally, in Chapter 4, we have focused on potential network extensions, that are based on our results and discussions in the previous chapter and on theoretical considerations. In a nutshell, this study is offering a deep insight into the behaviour of the simplest precursors of the proposed/selected glycine formation pathways in a pure gas phase context. Furthermore, this work is paving the way to direct long-term scientific perspectives, as discussed in Chapter 6. For instance, one may indeed cite the call for a much comprehensive and scientifically well funded astrochemical code, accompanied by a network based on rigorous kinetic parameters.