Imaging the closest environment of YSOs with GRAVITY

In this thesis, I explore the innermost region of protoplanetary disks around intermediate and high-mass Young Stellar Objects (YSOs). By means of state-of-the-art image reconstruction techniques at near infrared wavelengths, I reconstructed images of the immediate environment of these sources. Where imaging was not possible, geometrical modelling was used to constrain the structure and dynamics of the innermost disk at milli-arcsecond resolution. Most of the findings of this thesis employ data acquired with the European Southern Observatory (ESO) Very Large Telescope Interferometer (VLTI) instrument GRAVITY. The first science chapter focuses on the study of the hot gas emission surrounding the Herbig Ae star HD 58647 (Bouarour et al. 2023). The objective is to determine the origin of the HI Brackett Gamma line emission and to analyse the kinematics of the hot gas component. For this purpose, I employed spectro-interferometry to reconstruct one of the few images of both the K-band continuum emission and the Brackett Gamma, line emission in a YSO. In the second science chapter, my study focuses on the compact disk of the High-Mass Young Stellar Object (HMYSO) IRAS 13481-6124, particularly focusing on the K-band continuum emission traced by the dust and the Brackett Gamma hot gas emission. For the first time, the disk is spatially resolved at milliarcsecond resolution, revealing substructures likely induced by the presence of a faint companion. Thanks to the spectrally dispersed interferometric observables, we are able to determine the geometry of the hot gas emission traced by the Brackett Gamma line. This analysis enables us to trace both the gaseous disk near the base of IRAS 13481-6124's jet and the collimated wind. In the final chapter, I discuss the outcomes of a survey of HMYSOs conducted at near-infrared wavelengths, using the GRAVITY instrument. Due to their deeply embedded nature and considerable distance, our understanding of the innermost regions of accretion disks around massive stars remains largely unknown, requiring high spatial resolution and sensitivity. In this study, we have successfully spatially resolved and determined the sizes of the dust and hot gas emission from the inner regions of a sample of HMYSOs, employing both high spatial and spectral resolution. Our findings are compared with existing results obtained using similar techniques for HMYSOs and intermediate-mass Herbig Ae/Be stars in both K-band continuum and Brackett Gamma line emission.

The findings detailed in the final two scientific chapters of this thesis significantly increase the sample of studied objects in both number and quality, providing additional observational evidence supporting the notion that the formation of high-mass protostars is a scaled-up version of the processes observed in low-mass counterparts. Moreover, these results suggest that their properties scale proportionally with mass. In view of these insights, we move a step closer to confirming that the process of star formation proceeds in a similar manner regardless of the mass of the star.