Physics – Optics
Scientific paper
Jul 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000phdt.........4d&link_type=abstract
PhD Thesis, Laboratoire d'Astrophysique, Observatoire de Grenoble, France, 2000.
Physics
Optics
Scientific paper
The high frequency of binary systems among main sequence stars as well as in star-forming regions has been largely documented in the last ten years. This raised the issue of the mechanism responsible for the preferred occurence of multiple systems. Moreover, interactions between a companion and the complex environment of a T Tauri star are only poorly understood. The work conducted during this thesis fits in this framework; the main goals of the thesis are: i) to estimate the binary frequency in various populations of pre-main sequence stars, ii) to study quantitatively the accretion phenomenon in binary T Tauri stars, and iii) to get direct observations and to model circumstellar and circumbinary disks in these systems. Using the Canada-France-Hawaii Telescope adaptive optics system to search for visual binaries, I took part in the observations of several hundreds objects located in various young stellar clusters. Here, I detail the analysis and results concerning two two-million years-old clusters, IC 348 and NGC 6611. When considering all populations studied to date, we find that the visual binary frequency among solar-type stars is the same in all stellar clusters as on the main sequence. Furthermore, this property does not depend on the age of the cluster, indicating that the binary frequency does not evolve after the first million years in these clusters. On the other hand, the extremely young loose star-forming regions display a significatively enhanced binary frequency. The models that best fit these observations are those where the binary frequency resulting from the gravitational collapse is close to 100%. In the densest clusters, this frequency can be subsequently decreased due to the numerous disrupting gravitational encounters between systems. OB stars in NGC 6611 retain significantly more companions in the separation range 200--2000 AU than low-mass objects in closer similarly-aged clusters, when one tries to account for uncompleteness in the observations. I then focus on the accretion phenomenon in binary T Tauri systems using spatially-resolved optical spectroscopy of 125--1400 AU binary systems in Taurus. This approach reveals that the accretion phenomenon lasts over the same time span on both components of a binary system. Furthermore, the comparison of the Hα emission line luminosities suggests that the accretion rate is higher on the primary than on its companion. These results may be explained by the presence of a mass reservoir around the binary; this enveloppe would feed simultaneously both circumstellar disks. I finally present some high-angular resolution images of the GG Tau and UY Aur circumbinary rings and of the HK Tau B and HV Tau C circumstellar disks. These observations were obtained in the optical, the near-infrared and the millimetric ranges, and they allow an accurate description of the environment of these binary systems. I also analyze in details 1 micron polarimetric maps obtained with HST/NICMOS of the two circumbinary disks. In order to estimate the geometrical properties of these disks, as well as their dust grain properties, I performed Monte-Carlo modelling to describe photon multiple scattering and try to reproduce both the observed shape and the polarization level of the disks. This study confirms that the GG Tau circumbinary ring is geometrically thick (its aspect ratio is h/r~0.18), that small dust grains (<1 micron) are still present, and that the dust mass in the ring is at least 10-3 Modot. The UY Aur environment appears much more complex than that of GG Tau: the circumbinary disk, whose inclination is re-estimated to about 60°, coexists with a filament of material which is close to it but appears independent, and several arcs seem to get away from the main disk, one of which getting close to the stars. The scattering modelg reproduces the observations provided that the dust mass in the circumbinary disk is at least 10-3 Modot, in order to account for the non-detection of the back side of the disk from 6000 Å to 2 microns.
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