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Disclosing the Inner Gaseous Discs of Early Evolved Protostars

Periodic Reporting for period 1 - DIGDEEP (Disclosing the Inner Gaseous Discs of Early Evolved Protostars)

Reporting period: 2016-11-01 to 2018-10-31

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.
The project was organised in several work-packages mainly concerning the gathering of data at the telescope and its reduction and analysis, followed by an investigation of the origin of the HI Brγ line in sub-samples of T Tauri and TDs, and the comparison of the interferometric results with models. The main goals of the project were to:

(a) Investigate the origin of the Brγ line in TTauri and relate it with the presence of winds or accretion.
(b) Investigate the presence of winds and/or close companions in transitional disks (TDs). This later will support the hypothesis of dynamical clearing by planet formation.

The main results of the project can be summarised as follow: No evident signal of close companions has been detected in our sample of TDs. However, the most massive sources of our sample show evidence of extended Brγ line emission that might be associated with the presence of winds in the innermost regions of these disks. In the case of T Tauri sources, the results are less clear. In two of the sources the Brγ line emission is spatially resolved, and through modelling, we find that it is most likely coming from a disk wind, with a small contribution from accretion. In one of the sources, the Brγ line emission size is comparable to that of the magnetospheric accretion region, whereas for three other sources no interferometric signal was detected on the data making the interpretation of the results very difficult.

The dissemination of the results will be carried out in several ways:
1) Publication of results in refereed journals: I lead the first work based on GRAVITY observations of a T Tauri star obtained during the commissioning of the instrument. Another three papers are currently in preparation, and an additional paper summarising the results on the most massive TDs and a big sample of Herbig AeBe stars is foreseen to be submitted for publication in A&A within the next month.
2) Participation in international Conferences and workshops: The high impact results of this project have been presented so far in three international conferences (two as invited review talks).
3) Participation in research institutes and Universities weekly seminars: In total three seminars were conducted within Irish and UK universities.

The exploitation of our results are carried out in the spirit of pursuing scientific excellence, and the transfer of knowledge to the community. More precisely:

1) Further internal research: The results of this project are only the test case of a more ambitious, long term project within the GRAVITY GTO program on YSOs. This project aims at spatially resolving the hot (Brγ) and warm (CO) gas in disks, as well as the dust emission on a large sample of young objects (~100) spanning a wide range of masses, ages and disk properties. This will allow us to investigate for the first time the structure, evolution and dynamics of disks at sub-au scales by probing the gas and dust content simultaneously in a systematic and homogeneous way. This project will provide a long lasting legacy for the community.
2) Collaborative research: The multidisciplinary approach of this project put me in a privilege competitive position to lead international collaborative projects such as the GRAVITY GTO scientific program for young stellar objects, and win time on the latest interferometric instrument VLTI-MATISSE.
The high sensitivity of VLTI-GRAVITY allowed to obtain the first direct observation of an exoplanet using optical interferometry. This work opens the door to new ways of characterising many of the exoplanets known today, and discover new ones, specially those located very close to the central star where coronographics techniques are at a blind.

Finally, during the development of this project it was realised that the same observational techniques developed for this project for interferometric analysis of YSOs could be adapted for high quality imaging of satellites in Geosynchronous Earth Orbit (GEO). Following this idea, I plan to develop a statistical imaging reconstruction method for high fidelity imaging of GEO satellites to identify and characterise these objects, monitor their environment, and performing diagnostic evaluations of these valuable assets.