The GRAVITY young stellar object survey

IV. The CO overtone emission in 51 Oph at sub-au scales

¹ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
² Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, D02, XF86 Dublin, Ireland
³ School of Physics, University College Dublin, Belfield, Dublin 4, Ireland
⁴ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
⁵ Department of Space, Earth & Environment, Chalmers University of Technology, 412 93 Gothenburg, Sweden
⁶ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937, Köln, Germany
⁷ Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal
⁸ CENTRA, Centro de Astrofísica e Gravitação, Instituto Superior Técnico, Avenida Rovisco Pais 1, 1049 Lisboa, Portugal
⁹ Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse, 85741 Garching bei München, Germany
¹⁰ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, 92195 Meudon, France
¹¹ European Southern Observatory, Casilla 19001, Santiago 19, Chile
¹² Max-Planck-Institute for Radio Astronomy, Auf dem Hügel 69, 53121 Bonn, Germany
¹³ Department of Physics, Le Conte Hall, University of California, Berkeley, CA 94720, USA
¹⁴ Unidad Mixta Internacional Franco-Chilena de Astronomía (CNRS UMI 3386), Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 33, Las Condes, Santiago, Chile
¹⁵ Sterrewacht Leiden, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands
¹⁶ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹⁷ Universidade de Lisboa – Faculdade de Ciências, Campo Grande, 1749-016 Lisboa, Portugal
¹⁸ Instituto de Astronomía, Universidad Nacional Autónoma de México, Apdo. Postal 70264, Ciudad de México 04510, México
¹⁹ Institute of Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA

^★ Corresponding author; e-mail: maria.koutoulaki@eso.org

Received: 22 March 2020
Accepted: 20 October 2020

Abstract

Context. 51 Oph is a Herbig Ae/Be star that exhibits strong near-infrared CO ro-vibrational emission at 2.3 μm, most likely originating in the innermost regions of a circumstellar disc.

Aims. We aim to obtain the physical and geometrical properties of the system by spatially resolving the circumstellar environment of the inner gaseous disc.

Methods. We used the second-generation Very Large Telescope Interferometer instrument GRAVITY to spatially resolve the continuum and the CO overtone emission. We obtained data over 12 baselines with the auxiliary telescopes and derive visibilities, and the differential and closure phases as a function of wavelength. We used a simple local thermal equilibrium ring model of the CO emission to reproduce the spectrum and CO line displacements.

Results. Our interferometric data show that the star is marginally resolved at our spatial resolution, with a radius of ~10.58 ± 2.65R_⊙. The K-band continuum emission from the disc is inclined by 63° ± 1°, with a position angle of 116° ± 1°, and 4 ± 0.8 mas (0.5 ± 0.1 au) across. The visibilities increase within the CO line emission, indicating that the CO is emitted within the dust-sublimation radius. By modelling the CO bandhead spectrum, we derive that the CO is emitted from a hot (T = 1900–2800 K) and dense (N_CO = (0.9–9) × 10²¹ cm⁻²) gas. The analysis of the CO line displacement with respect to the continuum allows us to infer that the CO is emitted from a region 0.10 ± 0.02 au across, well within the dust-sublimation radius. The inclination and position angle of the CO line emitting region is consistent with that of the dusty disc.

Conclusions. Our spatially resolved interferometric observations confirm the CO ro-vibrational emission within the dust-free region of the inner disc. Conventional disc models exclude the presence of CO in the dust-depleted regions of Herbig AeBe stars. Ad hoc models of the innermost disc regions, that can compute the properties of the dust-free inner disc, are therefore required.