Interaction and formation of Nitrogen bearing molecules on interstellar dust grains

Axe Chimie interstellaire

Soutenance le lundi 15 octobre 2018 à 14h
Salle Jean François Denisse
Observatoire de Paris à partir
77 Avenue Denfert Rochereau
75014 Paris

Laboratoires co-porteurs
Mots clés

Interstellar medium (ISM), laboratory (laboratory), molecular clouds, solid state reaction, surface reaction

Projet de recherche

I am studying Astrochemistry at LERMA laboratory in the University of Cergy Pontoise. In this document, I will report the results of the project and the activities that I have participated in during my PhD.

From the beginning of my project, I carried out experiments in the LERMA laboratory of the University of Cergy Pontoise, as well as some data reduction from observations in Paris Observatory. After a year I left out the part related to observations, because the work was too fragmented, and I preferred to concentrate on experiments in the laboratory in agreement with my thesis directors.

Our goal is to study the formation of complex organic molecules (COMs) in the interstellar molecular clouds that are the raw material from which stars are born in the Universe. In particular, we want to understand the specific role of nitrogen molecules. Depending on the grain environment and the gas phase (including density, temperature and radiation), the chemical pathways of formation of COMs differ. Consequently, there are many complex pathways and mechanisms that contribute to the abundance and diversity of molecules in the interstellar medium and in the Universe. Laboratory experiments provide fundamental data and feed and constrain astrophysical models that trace the natural formation of COMs.

Our experiments were carried out with the prototypic device called VENUS ("Vers de NoUvelles Synthèses").

Fig 1: VENUS at the LERMA laboratory of the University of Cergy Pontoise in march 2018 .
Fig 1: VENUS at the LERMA laboratory of the University of Cergy Pontoise in march 2018 .

We assembled, calibrated and performed all the experiments throughout the project. Taking into account the temperatures (10-250K) and the conditions of submonolayers, the machine is composed of an ultra-high vacuum chamber, the main chamber, with a base pressure of 10?10 mbar. A four atomic or molecular beam system is collimated and directed towards the grain surface located in the main chamber. The surface is cooled and temperature controlled between 8 and 250K. Experiments are generally performed at 10 K, a typical temperature for dark molecular clouds.

We have two coupled detection modes: infrared spectroscopy and mass spectrometry to record and detect the products of our experiments.

In this context, I concentrated on experiments on the formation of complex organic molecules on grains and in molecular clouds as well as on their desorption. I conducted numerous experiments over three years on molecules such as carbon monoxide (CO), nitrogen (N2), nitric oxide (NO), ammonia (NH3), formaldehyde (H2CO), acetonitrile (CH3CN) and methyl isocyanide (CH3NC).

Thesis title

Etude expérimentale  de la désorption et de la réactivité d'espéces contenant de l'azote sur des grais de poussiéres interstellairee

Thesis abstract

The interstellar medium (ISM) is a key component of galaxies which is the matrix of the formation of stars and planets. It is composed of interstellar gas and dust. The presence of complex organic molecules (COMs) can be understood via reactions in the gas phase. However, the synthesis of some key species needs the intervention of the solid-state reactions on dust grains surface. The aim of this thesis is to understand physical and chemical processes (i.e diffusion, thermal desorption, and reactivity) occurring in the interstellar medium. More specifically, my thesis focuses on the investigation of desorption and reactivity of nitrogen bearing species that lead to COMs by hydrogenation related processes.

All experiments were performed with the VENUS set-up located at the LERMA Cergy laboratory in the University of Cergy Pontoise, France. We proceeded experiments involving some atoms and molecules such as hydrogen, deuterium, oxygen, CO, N2, NO, H2O, H2CO, CH3CN, and CH3NC. Solid state chemical processes have been simulated in the astrophysical context of the formation of COMs such as low pressures, low temperatures, low gas fluxes, and without the addition of external energy. According to the gas-solid coupling processes (i.e adsorption and thermal desorption), COMs are formed via competitive mechanisms, influenced surface substrates (i.e gold surface and water ice substrate), and kinetic conditions.

Articles to come
  1. Segregation effect and reduction of N2 binding energy in CO-N2 systems adsorbed on water ice substrates. (under review) – abstract in annexe.
  2. Experimental and theoretical study of the NO hydrogenation chemical network on interstellar dust grains.
  3. Experimental study of the penetration of oxygen and deuterium atoms into porous water ice. (Submitted end of May)
  4. Effective pathway of formation of prebiotic formamide on interstellar grains.
  5. Experimental study of the chemical network of methyl isocyanide on interstellar dust grains.
Publications
  1. Chaabouni, H., S. Diana, T. Nguyen, and F. Dulieu. 2018. “Thermal Desorption of Formamide and Methylamine from Graphite and Amorphous Water Ice Surfaces.” Astronomy & Astrophysics 612 (April). EDP Sciences: A47. Doi:10.1051/0004- 6361/201731006. Other articles will be submitted. Associated keywords are usually in astrophysics and astrochemistry, interstellar medium (ISM), laboratory (laboratory), molecular clouds, solid state reaction and surface reaction.
Conferences and seminars 
  1. i) “Journée de l'Ecole Doctorale Science et Ingénierie 2016”, 23rd June 2016, Neuville site, University of Cergy Pontoise, France.
  2. “Star formation in Different Environment” conference, 25th – 29th July 2016, Quy Nhon, Vietnam. Contributed poster. 
  3. “Blowing in the wind” conference, 7th – 13th August 2016, Quy Nhon, Vietnam. Contributed talk.
  4. “Complex Organic Molecules in Space”conference, 8th – 10th February 2018, Neuville site in University of Cergy Pontoise, France. Contributed poster.
  5. “Workshop Saint Florent: Processus physico-chimiques d'intérêt astrophysique: Les molécules complexes” 12th – 15th , June 2017, Corse, France.
  6. “Journée de l'Ecole Doctorale Science et Ingénierie 2017 ”, 29th June 2017, Neuville site, University of Cergy Pontoise, France. Contributed poster.
  7. “Pre-biotic Molecules in Space and Origins of Life on Earth ” 19th – 23rd March 2018, Physikzentrum Bad Honnef, Germany. Contributed poster.
  8. “Journée de l'Ecole Doctorale Science et Ingénierie 2018 ”, 14th May 2018, Neuville site, University of Cergy Pontoise, France. Contributed talk.
Awards
  • Price of the best student poster (section Laboratory Astrophysics) in “Pre-biotic Molecules in Space and Origins of Life on Earth ” 19th – 23rd March 2018, Physikzentrum Bad Honnef, Germany.