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Etude dans l'ultraviolet lointain de la composante gazeuse de l'environnement circumstellaire des étoiles Ae/Be de Herbig -- L'hydorgène moléculaire

Abstract : The study of molecular hydrogen is fundamental for a better understanding of the stellar and planetary formation. Molecular hydrogen is the most abundant molecule in the circumstellar environment of young stars, and thus, allows to measure the gaseous content around the stars at different evolutionary stages from the pre-main sequence stage onto the main sequence. In this thesis, I have studied the gaseous component, focusing on molecular hydrogen, in the circumstellar environment of a sample of Herbig Ae/Be stars, observed with the FUSE satellite in the far ultraviolet domain. This sample spans the spectral range from F4 to B2 and includes the main-sequence star Beta-Pictoris. Evidences for several mechanisms of H2 excitation were found, which are clearly correlated to the structure
of the circumstellar environment.

As far as the youngest stars of the sample (earlier than B9 type) are concerned, the excitation diagrams of H2 are well reproduced by a photodissociation regions (PDRs) model. This study shows that the observed gas is likely distributed in circumstellar envelopes, remnant
of the molecular clouds in which the stars were formed.

The group of Ae/B9 stars of the sample, which are known to possess circumstellar disks, is less homogeneous than the Be stars group. For
most of these stars, the lines of sight do not pass through the disks because of the high inclination angles. This implies that the gaseous disk is generally not observed. When circumstellar H2 is observed, the excitation conditions give evidence for the presence of
collisionnally excited medium close to the star. Using a flared disk model and assuming that gas and dust are coupled into the disks I have concluded that the observed gas is not located within the disk. Several scenario can help in interpreting these excitation conditions: the warm/hot H2 could be located in a transitional region between a hot region, such as an extended chromosphere located above the photosphere, and the disk. The other source which may be suspected for the warm/hot H2 is the photoevaporation of the disk due to the FUV stellar radiation.

This confirms the structural differences between Herbig Ae and Herbig Be stars' environments related to their different evolution. This could be explained by the faster evolution of Herbig Be stars which are associated to stronger radiation fields.

These results set strong constrains on the physical conditions of the circumstellar gas. Once combined with futher observations, they will allow to obtain a global picture of both structure and evolution of the CS environment of HAeBes.
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https://tel.archives-ouvertes.fr/tel-00011721
Contributor : Claire Martin-Zaidi <>
Submitted on : Friday, March 3, 2006 - 11:37:21 AM
Last modification on : Thursday, July 25, 2019 - 4:34:12 PM
Long-term archiving on: : Saturday, April 3, 2010 - 10:41:03 PM

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Claire Martin-Zaidi. Etude dans l'ultraviolet lointain de la composante gazeuse de l'environnement circumstellaire des étoiles Ae/Be de Herbig -- L'hydorgène moléculaire. Astrophysique [astro-ph]. Université de Provence - Aix-Marseille I, 2005. Français. ⟨tel-00011721⟩

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