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Physico-chimie des disques protoplanétaires

Abstract : Planetary formation takes place in the protoplanetary disks of gas and dust surrounding young stars. Nevertheless the overall properties of these disks are not yet well constrained by current observations. Observations in the millimeter domain bring constrains about the gas and dust composition of the disks. The chemistry of proto-planetary disks is thought to be dominated by two major processes: photo-dissociation in a warm upper layer (illuminated by the ultraviolet radiation from the central star and the ISRF), and depletion on dust grains in the disk mid-plane. During my PhD thesis I studied the effect of several parameters (like the grain size distribution, the stellar UV intensity or the gas-to-dust ratio) on the molecular abundance distribution in the disks. I used IRAM-PdBI observations of several molecules : CO observations in two disks surounding Herbig Ae stars (CQ\,Tau and MWC\,758) and the two molecules affected by dissociation processes CN and HCN in three disks surounding two T-Tauri stars and one Herbig Ae star. I compared the observational data with the output of a chemistry code (a modified version of the \cpdr\ Meudon code). I studied more precisely the gas dissipation problem looking at the CO distribution in two low mass disks surrounding Herbig Ae stars. I showed that the mean CO abundance in those two sources is low despite disk temperatures above the CO condensation temperature. Modeling the chemistry, I find that photo-dissociation of CO is a viable mechanism to explain its low abundance, especially as grain growth processes increase its efficiency. A modification of the gas-to-dust ratio is not mandatory to explain the low CO abundance. Thus, CO is not a direct tracer of the gas-to-dust ratio in those low mass disks. Moreover, we find that the temperature of large grains can be low enough to prevent CO from being released from the grain surfaces. For the other molecules, we find low excitation temperatures for CN and HCN. These observations suggest that those molecules are located in a region closer to the disk mid-plane than models including depletion on grains predict, and that the disk mid-plane is very cold (at least the T-Tauri ones).
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Contributor : Edwige Chapillon <>
Submitted on : Wednesday, December 22, 2010 - 2:16:44 PM
Last modification on : Monday, November 23, 2020 - 2:38:10 PM
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E. Chapillon. Physico-chimie des disques protoplanétaires. Astrophysique [astro-ph]. Université Joseph-Fourier - Grenoble I, 2008. Français. ⟨tel-00549741⟩

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