La deutération dans les protoétoiles de faible masse

Abstract : Despite the low deuterium abundance in the Universe (D/H $\sim$ 1.5$\times$10$^(-5)$), high abundances of deuterated molecules are detected in star forming regions, with a fractionation ((\it i.e.) the ratio of deuterated over main isotopomer) higher than the cosmic abundance of deuterium by several orders of magnitude. Particularly, warm dense gas in hot cores around low-mass protostars is enriched in deuterated species, with even high observed abundances of doubly-deuterated species such as D$_2$CO. These deuterated molecules provide valuable tools to probe the physical conditions occurring during star formation. Deuteration is thought to be driven by the small energy differences between a deuterated species and the normal isotope. Because the temperatures indicated by the fractionation are much lower than the present gas temperatures in hot cores, the observed deuterations are thought to reflect a previous cold phase. Likely these molecules formed during the preceding prestellar core phase -- either in the gas phase or on the grain surface -- and were stored in an ice mantle which evaporated once the YSO heated its environment above the ice sublimation temperature. We study in this thesis the physical and chemical processes leading to the high molecular deuteration observed in low-mass protostellar environments. We present observations of deuterated molecules (namely methanol, formaldehyde and water) both in the gas and in the icy mantles of dust grains in the envelope surrounding such objects. Millimeter observations unveiled a high deuteration of methanol in the gas of the envelope. In particular, triply-deuterated methanol was detected with a fractionation CD$_3$OH/CH$_3$OH $\sim$ 1\% in IRAS16293-2422. The observed fractionations are consistent with the scenario of formation of methanol on dust grain surfaces. Deuterated methanol and formaldehyde were then searched for and detected on a sample of low-mass Class 0 protostars, suggesting that this high deuteration is common in this class of objects. Analysis of the gas-phase water emission in the IRAS16293-2422 envelope leads paradoxically to a fractionation one order of magnitude lower, in agreement with the upper limit on water deuteration in ices, derived by near-infrared observations towards slightly more evolved objects. The last chapter of the thesis presents a grain chemistry model that studies in details water fractionation.
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Bérengère Parise. La deutération dans les protoétoiles de faible masse. Astrophysique [astro-ph]. Université Paul Sabatier - Toulouse III, 2004. Français. ⟨tel-00009303⟩

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