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Le Rôle Des Collisions Avec L'hydrogène

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Le Rôle Des Collisions Avec L'hydrogène Le rôle des collisions avec l’hydrogène dans la determination hors-ETL de l’abondance du fer dans les étoiles froides Rana Ezzeddine To cite this version: Rana Ezzeddine. Le rôle des collisions avec l’hydrogène dans la determination hors-ETL de l’abondance du fer dans les étoiles froides. Astrophysique [astro-ph]. Université Montpellier, 2015. Français. NNT : 2015MONTS147. tel-01990687 HAL Id: tel-01990687 https://tel.archives-ouvertes.fr/tel-01990687 Submitted on 23 Jan 2019 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Délivré par l’ Université de Montpellier Préparée au sein de l’école doctorale I2S Et du Laboratoire Univers et Particules de Montpellier Spécialité: Physique Présentée par EZZEDDINE Rana [email protected] Le rôle des collisions inélastique avec l’hydrogène dans la détermination hors-ETL de l’abondance du fer dans les étoiles froides M. Bertrand Plez Professeur Directeur de thèse M. Marwan Gebran Maître de conférences co-directeur de thèse M. Piercarlo Bonifacio Directeur de recherche CNRS Rapporteur M. Frédéric Thévenin Directeur de recherche CNRS Rapporteur M. Frédéric Paletou Astronome Examinateur M. Matthias Steffen Professeur Examinateur Non-LTE Iron abundance determination in cool stars: The role of Hydrogen collisions Rana Ezzeddine December, 2015 2 Résumé La détermination d’abondances stellaires très précises est un objectif majeur de la plu- part des études spectroscopiques. L’hypothèse de l’équilibre thermodynamique local (ETL), largement utilisée dans les analyses spectroscopiques, est souvent inadéquate pour déter- miner les abondances et les paramètres stellaires, particulièrement pour les étoiles géantes ou pauvres en métaux, où les effets hors-ETL dominent. C’est pourquoi, une modélisation hors-ETL des spectres stellaires est cruciale afin de reproduire les observations et ainsi déterminer avec précision les paramètres stellaires. Cette modélisation hors-ETL nécessite l’utilisation d’un grand jeu de données atomiques, qui ne sont pas toujours connues avec certitude. Les taux de collisions avec l’hydrogène sont une des sources d’incertitudes les plus fortes dans le cas des étoiles froides. Ces taux sont souvent approximés en considérant une approche classique (l’approximation de Drawin), et ceci seulement dans le cas de transi- tions permises liée-liée et d’ionisations. Cette approche classique, tend à surestimer le taux de collision et ne reproduit pas correctement le comportement en fonction de l’énergie de la transition. Je démontre dans cette thèse l’incapacité de l’approximation de Drawin à décrire le taux de collisions avec l’hydrogène. Je présente une nouvelle méthode empirique pour estimer ce taux, par le biais d’ajustement sur des taux quantiques existant pour d’autres éléments. Je montre ensuite que cette méthode d’ajustement quantique (que je nomme QFM) permet de reproduire les résultats des modélisations hors-ETL effectuées avec des taux quantiques existants (cas du Si). J’utilise ensuite cette nouvelle méthode, avec mon propre modèle d’atome de fer, sur des étoiles de référence issues du " Gaia-ESO survey", ce qui me permet de déterminer empiriquement les taux de collision avec l’hydrogène. En partant de paramètres photosphériques non-spectroscopiques, je détermine les abondances en fer de ces étoiles de référence dans le cas ETL comme hors-ETL. Les résultats dans le cas hors-ETL montrent un très bon accord entre les abondances de fer neutre et ionisé avec de faibles écarts raie à raie, particulièrement dans le cas des étoiles pauvres en métaux. Finalement cette méthode est validée par une comparaison avec des calculs quantiques très récents et encore non publiés. 3 Abstract Determination of high precision abundances has and will always be an important goal of all spectroscopic studies. The use of LTE assumption in spectroscopic analyses has been extensively shown in the literature to badly affect the determined abundances and stellar parameters, especially in metal-poor and giant stars which can be subject to large non-LTE effects. Non-LTE modeling of stellar spectra is therefore essential to accurately reproduce the observations and derive stellar abundances. Non-LTE calculations require the input of a bulk of atomic data, which may be subject to uncertainties. In cool stars, hydrogen collisional rates are a major source of uncertainty, which are often approximated using a classical recipe (the Drawin approximation) for allowed bound-bound, and ionization tran- sitions only. This approximation has been shown to overestimate the collisional rates, and does not reproduce the correct behavior with energies. I demonstrate in this dissertation the inability of the Drawin approximation to describe the hydrogen collisional rates. I intro- duce a new method to estimate these rates based on fitting the existing quantum rates of several elements. I show that this quantum fitting method (QFM) performs well in non-LTE calculations when detailed quantum rates are not available, and is able to reproduce the results obtained with the quantum rates (the case of Si). I test the newly proposed method, with a complete iron model atom that I developed, on a reference set of stars from the "Gaia-ESO survey". Starting from well determined non-spectroscopic atmospheric parame- ters, I determine 1D, non-LTE, and LTE iron abundances for this set of stars. The non-LTE results show excellent agreement between Fe I and Fe II abundances and small line-by-line dispersions, especially for the metal-poor stars. The method is validated upon comparison with new preliminary Fe I+H quantum calculations, whose fits show an excellent agreement with ours. 4 “And you run and you run to catch up with the sun but it’s sinking. Racing around to come up behind you again. The sun is the same in a relative way, but you’re older. Shorter of breath and one day closer to death." — Pink Floyd 5 Acknowledgements First and foremost I would like to thank my Ph.D. advisors Bertrand Plez and Marwan Gebran. They helped me realize my dream of becoming a researcher and an Astrophysicist. And for that I am immensely thankful. Bertrand taught me how to think critically and not to "go with the flow", and not be afraid to take risks and try new ideas. He was supportive and caring. I appreciate all his contributions of time, ideas, and funding and opportunities to make my Ph.D. experience productive. The enthusiasm he has for his research was contagious and motivational for me, even during tough times in the Ph.D. pursuit. Marwan was the reason I applied for this Ph.D. scholarship and where I stand today. He helped me forever change my life and for that I am greatly indebted. He provided the support and positivity to keep going in the good and tough times all throughout his acquain- tance. His immense patience and scientific knowledge and advices were very important to the completion of this work. One of the most important people contributing to this thesis that I would equally like to thank is Thibault Merle (Brussels). He generously provided me with so much knowledge on non-LTE calculations, the radiative transfer code MULTI, atomic data and models and much much more. Without his help, this thesis would have been much more difficult to accomplish. His respectfulness, kindness and enthusiasm for science made the work all the more interesting. It was a pleasure working and collaborating with you these past three years. I thank the members of the jury who accepted to asses my work: Frédéric Thévenin, Piercarlo Bonifacio, Frédéric Paletou and Matthias Steffen. Their time and valuable com- ments are highly appreciated. In addition to my advisors and collaborators, I am greatly thankful to all the colleagues of LUPM 1 (permanent and non-permanent, administration and technicians). Thank you, first of all, for the directors and co-directors of the lab throughout the past three years: Fabrice Feinstein, Bertrand Plez, Denis Puy, Agnes Lèbre and Georges Vasileiadis for their support and help in solving all encountered administrative problems. A special thank you for the AS 2 team members who have been my surrogate family throughout the last three years when I was away from mine: Fabrice Martins, Ana Palacios, Eric Josselin, Dhabia Tahlbi, Yohann Scribano, Henri Reboul, Nicolas Mauron, Gérard Jasniewicz, Olivier Richard, and Julien Morin (his holy figure for Julianism ©) for their immense support, kindness, knowledge, guidance and very good sense of humour. I was very happy to be part of your team for the past three years. A very big thank you goes equally to all the fellow Ph.D. students and postdocs with whome I have spent time at LUPM and outside it throughout these past three years and left a pleasant impact on my life. The great times spent together and your friendship means a lot to me! Thank you for Pierre Ghesquiere, Louis Amard, Anthony Hervé, Lola Faletti, Diane Fernandez, Dimitri Douchin, Morgan Deal, Thibaut Desgardin, Nigel Maxted, Johanna Itam-Pasquet, Marco Padovani, Manal Yassine, Benjamin Tessore (pourquoi t’es malheureux?) and Stephano Magni for all your support. Thank you for the administration members of the lab and all their help and support: Carole Prevot, Amel Chennouf-Salhi, Sylvianne Colaiocco, Lydie Le-Clainche, Christophe 1Laboratoire Univers et Particules de Montpellier 2Astrophysique Stellaire Mercier and all the others. Also I wish to thank the computer technicians for all their help (for the countless number of times my laptop/s decided to stop functioning properly): Nicolas Clementin, Samuel Viscapi, and Claude Zurbach.
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