Letter to the Editor

Substructures in the Keplerian disc around the O-type (proto-)star G17.64+0.16^⋆

¹ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
e-mail: lmaud@eso.org
² Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
³ INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁴ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
⁵ National Radio Astronomy Observatory, 1003 Lopezville Road, Socorro, NM 87801, USA
⁶ UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
⁷ I. Physikalisches Institut der Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
⁸ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁹ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹⁰ Universidad Nacional Autónoma de México, Instituto de Radioastronomía y Astrofísica, Morelia, Michoacán 58089, Mexico
¹¹ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
¹² School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
¹³ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
¹⁴ Institute of Astronomy and Astrophysics, University of Tübingen, Auf der Morgenstelle 10, 72076 Tübingen, Germany
¹⁵ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str.1, 85748 Garching, Germany
¹⁶ Kapteyn Astronomical Institute, University of Groningen, The Netherlands
¹⁷ SRON, Landleven 12, 9747 AD Groningen, The Netherlands
¹⁸ European Southern Observatory, Alónso de Cordova 3107, Vitacura, Casilla, 19001 Santiago de Chile, Chile

Received: 5 April 2019
Accepted: 16 June 2019

Abstract

We present the highest angular resolution (∼20 × 15 mas–44 × 33 au) Atacama Large Millimeter/sub-millimeter Array (ALMA) observations that are currently possible of the proto-O-star G17.64+0.16 in Band 6. The Cycle 5 observations with baselines out to 16 km probe scales < 50 au and reveal the rotating disc around G17.64+0.16, a massive forming O-type star. The disc has a ring-like enhancement in the dust emission that is especially visible as arc structures to the north and south. The Keplerian kinematics are most prominently seen in the vibrationally excited water line, H₂O 5_5, 0−6_4, 3 ν₂ = 1 (E_u = 3461.9 K). The mass of the central source found by modelling the Keplerian rotation is consistent with 45 ± 10 M_⊙. The H30α (231.9 GHz) radio-recombination line and the SiO (5-4) molecular line were detected at up to the ∼10σ level. The estimated disc mass is 0.6 − 2.6 M_⊙ under the optically thin assumption. Analysis of the Toomre Q parameter in the optically thin regime indicates that the disc stability is highly dependent on temperature. The disc currently appears stable for temperatures > 150 K; this does not preclude that the substructures formed earlier through disc fragmentation.