Physics – Optics
Scientific paper
Dec 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt........89a&link_type=abstract
PhD Thesis, University Paris VII, France Conducted between the Observatoire de Grenoble, France, and the Max Planck Institut für
Physics
Optics
1
Scientific paper
In generally accepted scenario of star formation, stars form in an inside-out collapse of the parent molecular cloud and grow up in this dusty environment. Undoubtedly the interaction of the central object with its immediate surroundings is of prime importance to determine the final physical properties of the star. Therefore the study of the star formation process would not be complete without the study of the circumstellar environment from a few stellar radii (inner accretion disk, boundary layer) up to hundreds (if not thousands) of AU (observations of mm-radiation and polarised light from disks, infalling dust envelope). The study of the close circumstellar environment few tens of AU implies the use of HAR techniques. Therefore we used the MPE near-infrared speckle camera SHARP (System for High Angular Resolution Pictures) at the ESO NTT (New Technology Telescope of 3.5m equiped with active optics). This camera has been designed to give diffraction limited images at 2.2 microns (K band), when used at this telescope. Therewith we studied the circumstellar environment of YSOs in search of extended matter or disks. The sources were selected to be highly polarised at optical wavelength, which is interpreted in the frame of scattering (Mie theory) by the presence of asymmetrically distributed matter in the near environment of the source. Over 28 YSOs observed, we found 9 binaries, 3 multiple systems and 4 extended sources from which one is a binary. Some of the results were known but not at our resolution (in average 0.2''). The purpose of the observations was to find an explanation for the high degree of polarisation. Individual results for each multiple source are analysed as well as the relative importance of the different physical polarization mechanism (synchrotron effect, scattering by small dust grains or the so called 'Davis-Greenstein' mechanism). As for most sources the degree of polarisation at NIR wavelength is not known and in order to complete the study of these objects, we made two-dimensional speckle polarimetry measurements. The set-up used and the observational mode are largely presented. This experiment gives polarisation with an up to now never achieved resolution (diffraction limit in K). Some results, among others on the infrared nebula of Chamaeleon, are discussed. To get high quality results, good data are not enough: the reconstruction of the image is extremly important. To avoid the known problem of phase wrapping, we developped a new method based on the integration of the phasors. This method is presented with its tests and results on real data. Two new methods are resulting from this work: an observational one (two-dimensional speckle polarimetry) and an image reconstruction one (reconstruction of the phasors). The first one is still requiring some work and tests to be pushed to its limits. From the observational point of view, the disparity of the sample do not allow to draw general conclusions. However some new exciting results on some of the sources point out and require further observations (detailed information also given in this work) to be completed.
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