Letter to the Editor

The GRAVITY young stellar object survey

II. First spatially resolved observations of the CO bandhead emission in a high-mass YSO

¹ Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, D02 XF86 Dublin, Ireland
e-mail: alessio@cp.dias.ie
² Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
³ University College Dublin, School of Physics, Belfield, Dublin 4, Ireland
⁴ Department of Space, Earth & Environment, Chalmers University of Technology, 412 93 Gothenburg, Sweden
⁵ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
⁶ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁷ Universidade do Porto – Faculdade de Engenharia, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal
⁸ CENTRA, Instituto Superior Tecnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal
⁹ European Southern Observatory, 19001 Casilla, Santiago 19, Chile
¹⁰ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
¹¹ Instituto de Astronomía, Universidad Nacional Autónoma de México, Apdo. Postal 70264, 04510 Ciudad de México, Mexico
¹² Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse, 85741 Garching bei München, Germany
¹³ Department of Astronomy, Stockholm University, Oskar Klein Center, 106 91 Stockholm, Sweden
¹⁴ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹⁵ LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, Univ. Paris Diderot, Sorbonne Paris Cité, France
¹⁶ Department of Physics, Le Conte Hall, University of California, Berkeley, CA 94720, USA

Received: 26 January 2020
Accepted: 9 March 2020

Abstract

Context. The inner regions of the discs of high-mass young stellar objects (HMYSOs) are still poorly known due to the small angular scales and the high visual extinction involved.

Aims. We deploy near-infrared spectro-interferometry to probe the inner gaseous disc in HMYSOs and investigate the origin and physical characteristics of the CO bandhead emission (2.3–2.4 μm).

Methods. We present the first GRAVITY/VLTI observations at high spectral (ℛ = 4000) and spatial (mas) resolution of the CO overtone transitions in NGC 2024 IRS 2.

Results. The continuum emission is resolved in all baselines and is slightly asymmetric, displaying small closure phases (≤8°). Our best ellipsoid model provides a disc inclination of 34° ±1°, a disc major axis position angle (PA) of 166° ± 1°, and a disc diameter of 3.99 ± 0.09 mas (or 1.69  ±  0.04 au, at a distance of 423 pc). The small closure phase signals in the continuum are modelled with a skewed rim, originating from a pure inclination effect. For the first time, our observations spatially and spectrally resolve the first four CO bandheads. Changes in visibility, as well as differential and closure phases across the bandheads are detected. Both the size and geometry of the CO-emitting region are determined by fitting a bidimensional Gaussian to the continuum-compensated CO bandhead visibilities. The CO-emitting region has a diameter of 2.74±_0.07^0.08 mas (1.16  ±  0.03 au), and is located in the inner gaseous disc, well within the dusty rim, with inclination and PA matching the dusty disc geometry, which indicates that both dusty and gaseous discs are coplanar. Physical and dynamical gas conditions are inferred by modelling the CO spectrum. Finally, we derive a direct measurement of the stellar mass of M_* ∼ 14.7_−3.6⁺² M_⊙ by combining our interferometric and spectral modelling results.