Structure and Evolution of Protoplanetary Disks

The scope of this thesis is to investigate the structure and evolution of protoplanetary disk. High resolution observational techniques such as high resolution optical/infrared spectroscopy and infrared interferometry are well suited for this purpose. High resolution spectroscopy allow to resolve the velocity profile of disk emission lines and determine some important parameters such as the disk geometry and the physical conditions of the line emitting region. Infrared interferometry allows to spatially resolve and constraint the disk geometry within the planet forming region. The work presented here aims at contributing to the comprehension of the disk structure and evolution at three different evolutionary stages: 1) the early phase when the system is still (partially) embedded in a remnant of the molecular cloud; 2) the so-called Class II phase (from the classification of Lada 1987). At this stage gas and dust evolve rapidly leading to drastic changes of the disk structure; 3) the transition phase from Class II to Class III when gas and dust are dissipated leaving, eventually, a planetary system. During the early phases of disk evolution the star-disk- envelope system experience powerful instability which are related to rapid enhancement of the mass accretion rate on a timescale of few months. These events are recognizable as so-called FU Orionis outbursts, in which the optical brightness of the system can increase by 4 or more magnitudes. The mass accretion rate increases from 10-7-- 10-8 Myr to 10-3 --10-4 Myr. Statistical studies suggest that young low-mass stars experience several FU Orionis outburst. In late 2003, the young star V1647 Orionis in the L1630 Ori cloud within the Orion B molecular cloud went into outburst. The outburst shares some properties of the FU Orionis outburst. Following spectro-photometric observations confirmed the nature of the outburst as a disk-instability event. We also find, for the first time, probe of a direct link between an accretion event and the ejection of an Herbig-Haro object (HH). During the Class II phase dust coagulation and grain growth occur. This is the first step of planet formation. We applied high resolution optical spectroscopy and infrared interferometry to direct compare gas and dust emission from the disk surface of three protoplanetary disks. This study gives some insight on the relative distribution of gas and dust in disk and on the temporal evolution of the two components. A physical decoupling of gas and dust may occur leading to changes in the relative structure of the two (different scale height) and to rapid settling of dust on the disk midplane. This may increase the dust-to-gas mass ratio in the disk interior and, according to recent simulation, may trigger the formation of planetesimals via gravitational instability. The transition phase from a Class II to a Class III system is characterized by various processes which dissipate the disk material. In particular, viscous accretion and photo-evaporation are very efficient in removing disk material and planet formation is likely in competition with disk dispersion. For this reason, a fundamental quantity is the mass accretion timescale, i.e. the time at which the disk accretion phase ceases. In turn, the time at which the disk accretion phase ceases is a strong constraint on the gas dissipation timescale, relevant for the formation of giant planets. We have observed a number of young stellar clusters of different age aimed at tracing the evolution viscous accretion with time. The preliminary results show that the accretion seems to cease at similar age of the dust dissipation, i.e. within 5 -- 10 Myr.