Physics Theses

A first step towards understanding planetary formation is the characterisation of the structure and evolution of protoplanetary discs. Although the large scale disc is understood in some detail, much less is known about the inner 5 au in which the main physical processes take place: accretion, ejection, and planetary formation. Even in the nearest sites of star formation, this region cannot be spatially resolved by stand-alone telescopes; only in recent years have optical and infrared interferometers been able to achieve this, and only in the case of the brightest sources. Hydrogen recombination lines are present in the spectra of young stars. In particular, the Bracket gamma line is the brightest line in the K-band. HD50138 is a target that exhibits bright Bracket gamma and faint high-n Pfund (from level n=24 to n=19) emission. Our results show that the circumstellar environment of the target is complex and shows asymmetries in both the Bracket gamma and the continuum emitting regions. The Bracket gamma is more compact than the continuum emitting region and originates from a wind with a wide opening angle. The origin of the Bracket gamma line is still controversial and it has been found to be associated with both accretion and ejection processes. Thus, more tracers are needed to better constrain the inner gaseous disc properties. Molecules are also present in this part of the disc. In particular, the CO overtone emission at 2.3\,\mic\ is a good tracer of the inner disc properties. This emission is not common and in the case of T Tauri stars it has been observed in only three sources. RW Aur A is a CTTS that exhibits irregular dimming events. It shows CO overtone emission in both the bright and dim states. Modelling this emission with an LTE CO ring in Keplerian rotation, we find that the CO conditions do not change during the dimming events. Moreover, no significant accretion variation is detected between the two states, leading to the conclusion that the dimmings are caused by occulting material very close to the star (within 1 au). The CO ro-vibrational emission at 2.3\,\mic\ observed with the second generation interferometer VLTI/GRAVITY opens a new window on constraining the physical properties of the inner gaseous disc, by spatially resolving the CO emission. 51Oph is a target which shows bright CO overtone emission. By modelling the CO emission, the CO is found to emitt very close to the star from a warm and dense gas. This is the first observational evidence of the physical properties so close to the star (~0.1 au) where there is no dust. This is consistent with previous LTE models of the physical conditions in the dust-free disc.