Thesis title
Constraining astrochemical models of ice formation on the surface of interstellar dust grains using laboratory experiments.
Composition of the jury
- Maria Elisabetta PALUMBO (Research Director, INAF) : Chairperson
- Patrice THEULE (Senior Lecturer, Aix-Marseille University) : Rapporteur
- Stephanie CAZAUX (Associate Professor, Delft University of Technology) : Rapporteur
- Maria Elisabetta PALUMBO (Research Director, INAF) : Examiner
- François Dulieu (Professor, CYU) : Thesis supervisor
- Paola Caselli (Max Planck Institute for Extraterrestrial Physics, Garching, Germany) : Thesis supervisor
- Olli SIPILa (PhD, Max Planck Institute for Extraterrestrial Physics) : Co-supervisor
Abstract
The interstellar medium (ISM) presents a rich chemical complexity. The species that compose it are found both in the gas phase and on dust grains, which act as catalysts in the coldest and densest regions of the ISM, where gas-phase species accrete onto their surfaces to form ice mantles. Light species, such as hydrogen atoms (H), are mobile on grain surfaces and can react with other adsorbed species to produce more complex molecules, including water (H2O), hydrogen peroxide (H2O2), formaldehyde (H2CO), and methanol (CH3OH), among others. The goals of this thesis are to understand the physicochemical processes — diffusion, desorption, kinetics, and morphology — involved in ice formation on interstellar dust grains to constrain astrochemical models which describe chemical processes in the ISM.
This thesis comprises both experimental and theoretical investigations of ice formation on grain surfaces. The experimental work was conducted using the VENUS (VErs de NoUvelles Synthese) apparatus at the LIRA-CY laboratory, CY Cergy Paris Université, and the Paris Observatory — PSL. Species were deposited onto a cold substrate (above 7 K) within an ultra-high-vacuum chamber, and the resulting products were analysed using mass spectrometry and infrared spectroscopy. The theoretical component, performed at the Max Planck Institute for Extraterrestrial Physics (MPE) in Germany, employed the astrochemical code pyRate and a custom code I developed to simulate the experimental conditions of VENUS.
In the first study, I present purely experimental results on the morphology of water ice by investigating its 3 μm stretching-OH band using three deposition methods : background, molecular beam, and direct formation by co-depositing molecular oxygen (O2) with H. Depositing at 10 K, the three methods form amorphous water ice but with different levels of porosity : the background deposition yields porous amorphous ice, while the beam deposition and formed yields compact amorphous ice.
In the second study, I investigate the formation of H2CO and CH3OH through the reactivity of carbon monoxide (CO) and H in the presence of the noble gas neon (Ne). Neon is the fifth most abundant element in the Universe. Because it is chemically inert, it has never been included in astrochemical models. In this study, I show that the presence of Ne significantly inhibits CO reactivity with H between 9 and 12 K, with reductions of up to 82 % and 77 % in the formation of H2CO and CH3OH at 9 K, respectively. I then show that, using pyRate and a pre-stellar core model, Ne condenses onto grains in the central part of the core (< 5000 AU) where CO is completely frozen onto dust grains.
In the third study, I compare my experimental results on the kinetics of H2O and H2O2 formation via the hydrogenation of a half-layer of O2 between 9 and 12 K with simulations performed using the code I developed. I show that regardless of the diffusion model applied (thermal hopping, quantum tunnelling, or an earlier model proposed in the literature), the simulations do not accurately reproduce the abundances of H2O2 and H2O. Reaction-diffusion competition leads to an overestimation of HO2 and OH, which are not observed in experiments, while a tunnelling probability model provides a reasonable estimation for HO2 and OH (when a barrier of 50 K is used on O2 + H), but predicts a reaction barrier for H2O2 + H that is unrealistically low.
