Environnement circumstellaire des étoiles jeunesEnvironnement circumstellaire des étoiles jeunesCircumstellar environment of young stars;
Résumé
The close surroundings of young low mass stars hold a multitude of physical phenomena related to star formation. This thesis presents a set of works on theoretical, experimental, and observational issues connected with these phenomena. After a description of the properties of T Tauri, FU Orionis and Ae/Be Herbig stars, with a particular emphasis on their accretion disks, I study the vertical structure of such disks which results from the radiative transfer and the hydrostatic equilibrium. The energy dissipation comes from both the viscous friction of disk particles accreting onto the star and from the absorption of the stellar radiation. A disk ``chromosphere'' is shown to result from the grazing stellar radiation. In the following I study the possibility of detecting directly the circumstellar features (disk, binarity, planets, jets,...) thanks to the high angular resolution techniques (adaptive optics and interferometry). I then present the prototype of a coronagraph at high spatial resolution that I designed, modelized, built and tested for that kind of observations. Finally I describe the observations of the young stellar system Z Canis Majoris, which I obtained at the diffraction limit of the 3.6 meter ESO telescope in the near infrared. This object is shown to be composed of a binary system in addition to an elongated disk-like structure perpendicular to the known jet and illuminated not by the central source but by the infrared companion. [A copy of this thesis (which is mostly in french) can be obtained in binary mode by ftp. There is a file 'these_malbet.tar' in the directory '/pub/publications/' at the FTP node 'gag.observ-gr.fr (IP 130.190.200.11)'. By doing 'tar -xvf these_malbet.tar', you create a directory 'these_malbet/' where there is a 'readme' which gives all information.]