Fragmentation of carbon molecules of astrophysical interest with accelerators

De nos jours environ 200 molécules ont été observées dans le milieu interstellaire, environ 75% d'entre elles sont carbonées et 25% sont des hydrocarbures. Les grains de poussières contiennent également du carbone en grande quantité. La présence du carbone dans la majorité des molécules et dans les grains de poussières s'explique par son abondance et sa capacité à former des liaisons. Il est donc crucial de les étudier d'un point de vue astrochimique. Dans une première partie, ce travail présente les mesures des rapports de branchement et des énergies dissipées sous forme d'énergie cinétique dans les fragments neutres lors d'une collision à haute vitesse, des molécules carbonées suivantes : SiC, AlC, AlCH, C ₂ O, CN, CH ₂ et CH. Ces mesures ont été obtenues grâce au dispositif expérimental AGAT. Ce dernier est installé à demeure auprès de l'accélérateur Tandem d'Orsay. Il permet la collision molécule-atome en cinématique inverse et la détection 100% efficace de tous les fragments émis, y compris les neutres. Dans une seconde partie, l'effet de l'ajout ou la correction de nouveaux rapports de branchement d'hydrocarbures est discuté, sur la modélisation de la chimie de deux objets célèbres : la région de photo-dissociation de la nébuleuse de la Tête de Cheval et le nuage moléculaire TMC-1. Ces deux objets sont bien connus pour leur richesse en molécules observées. Ces nouveaux rapports de branchement diminuent les abondances calculées des espèces de la phase gazeuse, déjà trop basses par rapport aux observations. Une hypothèse stipule que des hydrocarbures sont dégagés dans la phase gazeuse via les grains de poussières carbonés. Suite à cette hypothèse, pour la première fois, les effets de l'incorporation de réactions de photo-production d'hydrocarbures par les grains de carbone amorphes hydrogénés ont été étudiés, sur la chimie de la phase gazeuse de la nébuleuse de la Tête de Cheval. L'ajout de ces réactions resserre l'écart entre la modélisation et les observations.Nowadays, about 200 molecules have been observed in the interstellar medium, about 75% are carbon molecules and 25% hydrocarbons. Dust grains also contain carbon in large quantities. The presence of carbon in the majority of molecules of the gaseous phase and in the dust is due to its abundance and its ability to form bonds. So, it is crucial to study the carbon in an astrochemical point of view.In the first part, this work presents measurements of branching ratios and energy dissipated as kinetic energy in neutral fragments which are emitted in a high speed collision, of the following carbon molecules: SiC, AlC, AlCH, C ₂ O, CN, CH ₂ and CH. These measurements were obtained from the experimental setup AGAT. This setup is permanently installed at the Tandem Orsay facility. It allows molecule-atom collisions and 100% effective detection of all the transmitted fragments, including neutral ones.In the second part, the effect of the addition or correction of new hydrocarbon branching ratios is discussed, by modeling of chemistry of two famous objects: the photo-dissociation region of the Horsehead nebula and the molecular cloud TMC-1. Both objects are well known for their numerous observed molecules. These new branching ratios reduce the calculated abundances of species in the gas phase, even though these abundances were already too low compared to the observations. A hypothesis states that hydrocarbons are released into the gas phase via the carbonaceous interstellar dust. Following this assumption, for the first time, the effects of the incorporation of photo-production reactions of hydrocarbons from the hydrogenated amorphous carbons were studied on the chemistry of the gaseous phase of the Horsehead nebula. The addition of these reactions narrows the gap between modeling and observations

UV Photolysis of Hydrogenated Amorphous Carbons of Astrophysical Interest

International audienceIn the gas phase, most of the ionized or neutral molecules detected in the interstellar and circumstellar media contain at least one carbon atom. Carbon chemistry plays thus a dominant role in the understanding of the structure and evolution of the interstellar medium (ISM). One particular zone of interest to observe small carbonaceous radicals and molecules, are the sharp molecular clouds edges exposed to energetic photons. These photon-dominated regions are rich in these hydrocarbons (like CCH, c-C3H2, C4H), and provide tests for the chemistry models in the diffuse to molecular transition. The pure gas phase models generally fail in reproducing the abundance of many of the observed species, and several authors suggest such abundances may arise from the products of the VUV photodissociation of carbonaceous grains or PAHs. Hydrogenated amorphous carbons (a-C:H or HAC), abundantly observed in the ISM, could also be at the origin of many of these small carbonaceous radicals. Experimentally, this work investigates the production and release of hydrocarbons from the VUV photolysis of a-C:H interstellar analogues under ultra-high vacuum. The experimental results are applied to a Photon Dominated Region model to constrain the impact of this release on the observed gas phase species

Semiempirical breakdown curves of C2_2N(+)^{(+)} and C3_3N(+)^{(+)}molecules; application to products branching ratios predictions of physical and chemical processes involving these adducts

We constructed semiempirical breakdown curves (BDC) for C$_2$N, C$_3$N, C$_2$N$^+$ and C$_3$N$^+$ molecules. These BDC, which are energy dependent dissociation branching ratios (BR) curves, were used to predict products branching ratios for various processes leading to the formation of C$_2$N$^{(+)}$ and C$_3$N$^{(+)}$ excited adducts. These processes, of astrochemical interest, are neutral-neutral and ion-molecule reactions, dissociative recombination and charge transfer reactions with He$^+$. Model predictions of BR are compared to the literature data and to reported values in the kinetic database for astrochemistry KIDA. With the new BR values, the C$_n$N abundances in cold cores were simulated

Excitation and fragmentation in high velocity CnN+ - He collisions

We will present measurements and modeling for two aspects of the CnN+ - He collisions (n=1-3, v=2.25 a.u) :cross sections for electronic excitation processes and fragmentation branching ratios for the excited and ionized CnNq+ molecules produced in the collision (q=-1,0,1,2-5).SCOPUS: cp.jinfo:eu-repo/semantics/publishe

Excitation and fragmentation in hight velocity CnN+ - He collisions

info:eu-repo/semantics/nonPublishe