Etude experimentale et theorique de la contribution de la composante organique refractaire a la phase gazeuse dans l'environnement cometaire

摘要

This Ph.D. work is an experimental and theoretical study of the contribution of the refractory organic component of comets to their gaseous phase. Some gaseous species observed in the coma, such as the formaldehyde (H2CO) and the cyanogen radicals (CN), are not only produced by the nucleus sublimation or the photodissociation of gaseous molecules. They could be produced by the degradation of the refractory organics materials present in cometary grains. The purpose of this Ph.D. work is to test this hypothesis. First, I have measured the quantum yields and the kinetics of production of gaseous species by UV irradiation and heating of solid organic compounds. Then, these data have been incorporated into a model of the cometary environment taking into account such processes. This existing model has also been improved to compare the calculations to recent measurements in comet Hale Bopp. HCN polymers and hexamethylenetetramine (HMT) have been proposed to explain the origin of CN radicals. I have irradiated in the far UV and heated theses compounds in conditions representative of the cometary environment. I show that the HMT is particularly stable under UV irradiation and that it sublimates without fragmentation into smaller gaseous compounds when heated under vacuum. Thus HMT does not seem to be a good candidate as a parent compound of the cometary CN radicals. Thus, I have concentrated my study on the degradation of HCN polymers. I show, by infrared spectroscopy, the production of HCN, CO, CH4 and C2H2 by irradiation of these polymers at 122 and 147 nm and the production of NH3, HCN, HNCO and CO by heating at temperatures ranging from 430 to 580 K. The production kinetics of all these gaseous species have been quantified thanks to the analysis of the temporal evolution of the infrared spectra. However, measurements by laser induced fluorescence (LIF) do not reach the detection limit required to allow the detection of CN radicals supposed to be produced by the degradation of the HCN polymers. Nevertheless, I have modeled the production of CN radical in the cometary environment supposing that their production from HCN polymer degradation is equal to the one measured for HCN. Whereas the column density could not be adequately reproduced, this hypothesis could not be rejected. Its confirmation requires further experimental studies.I have also continued a previous study of the H2CO production in cometary environment, adapting the modeling to the case of comet C/1995 O1 (Hale-Bopp). Indeed in this comet, the H2CO production rates display a much steeper evolution with heliocentric distance than other species of similar volatility, like HCN or H2S. First, I have obtained new experimental data on the thermal degradation of polyoxymethylene (POM) on a large temperature range and for two different polymers. Then, taking into account the POM degradation, I have reproduced the H2CO production rates in this comet and I have shown that their heliocentric evolution is due to the predominance of the thermal degradation of POM for heliocentric distance up to 3,5 UA. This work confirms that the POM degradation could explain the H2CO origin and that, whatever the considered heliocentric distance, the contribution of the refractory organic component to the gaseous phase of comets has to be taken into account.