According to our current understanding, the star formation process starts from the gravitational collapse of a dense region in a molecular cloud. After the initial collapse, a protoplanetary disk is formed from which matter is gradually accreted onto the central protostar. This phase is followed by a gradual dissipation of the envelope after which the protostar becomes optically visible (the so-called classical T Tauri stars; CTTSs), and the gas in the disk starts to dissipate. As this happens, dust particles condensate, ring-like structures and holes appear (the so-called transitional disks; TDs), and planets form.
The quest for understanding the origin of our Solar System is a major research topic in astrophysics and its progress is closely related with the development of new instrumentation. In spite of the great strides made in recent years, many of the details of the star formation process and how the remnants of circumstellar disks give rise to planets are largely unknown. In particular, the first few astronomical units of the disk (R~ 5 au; i.e. within Jupiter’s orbit) hold the key to understand the inital phases of planet formation. In this region, the gaseous disk dynamics is dominated by accretion, which depletes the disk gas. This, in turn, affects the drag of small dust particles within the disk, and has global implications on dust settling and coagulation, and thus on the formation and migration of planets. Photometric or spectroscopic studies can bring general observational constraints to the physics of the inner disk region and its properties, but only interferometry can spatially resolve them. This is because, at a typical distance of the nearest star formation regions, i.e. 100 pc, this region has an angular size of ~50 milliarcsecond (mas), impossible to resolve using standalone optical/IR telescopes.
This project aims at understanding the properties and physical processes ruling solar-mass protoplanetary disks at sub-au scales, and thus to probe disk evolution and the initial conditions of planet formation. Due to the high angular resolution and sensitivity required to achieve this goal, the project is focused on using the new ESO VLT-Interferometer GRAVITY. This instrument operates in the K-band with spectral resolution of up to 4000, and thus it is ideal to perform an unbiased study of the Brγ emission, as a tracer of the hot gas component, in solar-mass YSOs. This line might, in principle, originate in the accreting and outflowing gas material. However, accretion and ejection have very different spatial scales, as well as kinematic signatures. GRAVITY's high angular resolution capabilities and spectral power provide then a unique means to distinguish between the accreting and outflowing gas. For the first time, we will be able to observe (using Guaranteed Time Observations) a large sample of CTTSs, covering a wide range of masses, ages and disk morphologies (i.e. full disks and TDs). To probe where exactly accretion/ejection takes place, its relative contribution to the total line emission, and whether this contribution varies with time gives us important clues regarding the gas dynamics and dispersal mechanisms in the 1-5 au region.
