Recherche et caractérisation d'exoplanètes à grande séparation autour d'étoiles jeunes de faible masse

More than 20 years ago, the detection of exoplanets created a major revolution in astrophysics. It is, however, only in the mid-2000 that those planets were seen directly, through the light they emit, because they are much fainter than their host star. The few dozen exoplanets that were detected through direct imaging reveal some intriguing aspects of exoplanetary systems architectures and of the nature of low-mass companions. Many direct imaging surveys were undertaken, and new instruments dedicated to high-contrast imaging like GPI or SPHERE are now getting to the point they can detect planets more and more alike to the giant planets in our Solar System.   This thesis is a parallel effort to these surveys. Imaging is exploited under its simplest form, without any sophisticated high-contrast imaging technique, to complete the picture that is starting to emerge. Companions of a few times the mass of Jupiter (MJup) and up were searched in a very distant region around young, low-mass stars. To do so, a deep imaging survey, PSYM-WIDE (Planet Search around Young-associations M dwarfs), was made with Gemini South Observatory with GMOS (Gemini Multi-Object Spectrographs) in two near-infrared filters (i and z). The targets are 95 stars recently identified as probable members of young associations, with ages of less than 200 million years. These young stars have planets that are still contracting and thus are hotter and brighter. The presentation of this survey is the first part of this thesis. In accordance with what could be expected from theory, those companions are really rare. More specifically, according to the survey, 0.84_{-0.66}^{+6.73}% of stars have at least one planetary-mass companion (between 5 and 13MJup) at separations spanning 500 to about 5000 astronomical units (au, 1au being the distance between the Earth and the Sun).   Only one planetary-mass companion (~11MJ) was detected, 2000au from GU Psc, a star with about the third of the mass of the Sun that is a member of the young association (~100--150Myrs) AB Doradus. The detailed study of this unique object constitutes the second part of this thesis. Observations of this companion were made at Gemini North with GNIRS (infrared spectroscopy), at Canada-France-Hawaii Telescope (CFHT; near-infrared photometry) with WIRCam, with the space telescope WISE (mid-infrared photometry) and with the Keck telescope (high-contrast imaging). The comparison of these observations with atmosphere and evolutive models allowed the determination of the physical properties of this unique object. Its spectral type is T3.5, it has a temperature of 1000-1100K and a low surface gravity typical for this young age. Just like all distant planetary-mass companions, it is hard to explain its presence so far from its host with current planetary formation scenarios.  It is, however, a very interesting object, as it is much easier to study that exoplanets that are closer to their host star. The third portion of this thesis exploit this advantage. We present the photometric variability study of GU Psc b, which was observed for 3 sequences of 5-6 consecutive hours with WIRCam, at CFHT. The periodic variation of the luminosity can be explained by the presence of a non-uniform atmosphere. Different portions of the surface are seen as the planet rotates, bringing more luminous and less luminous portions in and out of view. A variability study is thus a unique way of characterizing the atmosphere and rotation period of planets. For GU Psc b, a small, 4+-1% peak-to-peak amplitude variability was detected on the third epoch on a timescale of 6h. Longer observations and/or observations on larger ground-based or space telescopes will allow this result to be confirmed and the rotation period of GU Psc b to be better constrained.

Keyphrases: Exoplanets, Direct Imaging, Low-mass stars, Young Associations