The GRAVITY young stellar object survey
Authors:   GRAVITY Collaboration, Bouarour Y.-I. , Perraut K. , Ménard F. , Brandner W. , Caratti o Garatti A. , Caselli P. , van Dishoeck E. , Dougados C. , Garcia-Lopez R. , Grellmann R. , Henning T. , Klarmann L. , Labadie L. , Natta A. , Sanchez-Bermudez J. , Thi W.-F. , de Zeeuw P. T. , Amorim A. , Bauböck M. , Benisty M. , Berger J.-P. , Clenet Y. , Coudé du Foresto V. , Duvert G. , Eckart A. , Eisenhauer F. , Eupen F. , Filho M. , Gao F. , Garcia P. , Gendron E. , Genzel R. , Gillessen S. , Jiménez-Rosales A. , Jocou L. , Hippler S. , Horrobin M. , Hubert Z. , Kervella P. , Lacour S. , Le Bouquin J.-B. , Léna P. , Ott T. , Paumard T. , Perrin G. , Pfuhl O. , Rousset G. , Scheithauer S. , Shangguan J. , Stadler J. , Straub O. , Straubmeier C. , Sturm E. , Vincent F. H. , von Fellenberg S. D. , Widmann F. , Wiest M. 
Published: August 2020

Context. Studies of the dust distribution, composition, and evolution of protoplanetary disks provide clues for understanding planet formation. However, little is known about the innermost regions of disks where telluric planets are expected to form.

Aims. We aim constrain the geometry of the inner disk of the T Tauri star RY Lup by combining spectro-photometric data and interferometric observations in the near-infrared (NIR) collected at the Very Large Telescope Interferometer. We use PIONIER data from the ESO archive and GRAVITY data that were obtained in June 2017 with the four 8m telescopes.

Methods. We use a parametric disk model and the 3D radiative transfer code MCFOST to reproduce the spectral energy distribution (SED) and match the interferometric observations. MCFOST produces synthetic SEDs and intensity maps at different wavelengths from which we compute the modeled interferometric visibilities and closure phases through Fourier transform.

Results. To match the SED from the blue to the millimetric range, our model requires a stellar luminosity of 2.5 L⊙, higher than any previously determined values. Such a high value is needed to accommodate the circumstellar extinction caused by the highly inclined disk, which has been neglected in previous studies. While using an effective temperature of 4800 K determined through high-resolution spectroscopy, we derive a stellar radius of 2.29 R⊙. These revised fundamental parameters, when combined with the mass estimates available (in the range 1.3–1.5 M⊙), lead to an age of 0.5–2.0 Ma for RY Lup, in better agreement with the age of the Lupus association than previous determinations. Our disk model (that has a transition disk geometry) nicely reproduces the interferometric GRAVITY data and is in good agreement with the PIONIER ones. We derive an inner rim location at 0.12 au from the central star. This model corresponds to an inclination of the inner disk of 50°, which is in mild tension with previous determinations of a more inclined outer disk from SPHERE (70° in NIR) and ALMA (67 ± 5°) images, but consistent with the inclination determination from the ALMA CO spectra (55 ± 5°). Increasing the inclination of the inner disk to 70° leads to a higher line-of-sight extinction and therefore requires a higher stellar luminosity of 4.65 L⊙ to match the observed flux levels. This luminosity would translate to a stellar radius of 3.13 R⊙, leading to an age of 2–3 Ma, and a stellarmass of about 2 M⊙, in disagreement with the observed dynamical mass estimate of 1.3–1.5 M⊙. Critically, this high-inclination inner disk model also fails to reproduce the visibilities observed with GRAVITY.

Conclusions. The inner dust disk, as traced by the GRAVITY data, is located at a radius in agreement with the dust sublimation radius. An ambiguity remains regarding the respective orientations of the inner and outer disk, coplanar and mildly misaligned, respectively.As our datasets are not contemporary and the star is strongly variable, a deeper investigation will require a dedicated multi-technique observing campaign.