Chasing discs around O-type (proto)stars

ALMA evidence for an SiO disc and disc wind from G17.64+0.16^★

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: maud@strw.leidenuniv.nl
² INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
³ Instituto de Astrofísica e Ciências do Espa’cco, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
⁴ Centre for Astrophysics, University of Hertfordshire, Hatfield, AL10 9AB, UK
⁵ Kapteyn Astronomical Institute, University of Groningen, The Netherlands
⁶ SRON, Landleven 12, 9747 AD Groningen, The Netherlands
⁷ School of Physics and Astronomy, University of Leeds, Leeds, LS2 9JT, UK
⁸ UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh, EH9 3HJ, UK
⁹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
¹⁰ I. Physikalisches Institut der Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
¹¹ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
¹² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹³ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester, Oxford Road, Manchester M13 9PL, UK
¹⁴ UK ALMA Regional Centre Node, The University of Manchester, Manchester M13 9PL, UK
¹⁵ Universidad Nacional Autónoma de México, Instituto de Radioastronomía y Astrofísica, Morelia, Michoacán, 58089, Mexico
¹⁶ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
¹⁷ Institute of Astronomy and Astrophysics, University of Tübingen, Auf der Morgenstelle 10, 72076 Tübingen, Germany
¹⁸ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str.1, 85748 Garching, Germany
¹⁹ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
²⁰ Indian Institute of Space science and Technology, 695 547, Thiruvananthapuram, India
²¹ European Southern Observatory, Alónso de Cordova 3107, Vitacura, Casilla, 19001, Santiago de Chile, Chile
²² Deutsches SOFIA Institut, Pfaffenwaldring 29, Univrsitüt Stuttgart, 70569 Stuttgart, Germany
²³ Universidad Autonoma de Chile, Av. Pedro Valdivia 425, Santiago de Chile, Chile

Received: 18 July 2018
Accepted: 13 September 2018

Abstract

We present high angular resolution (~0.2″) continuum and molecular emission line Atacama Large Millimeter/sub-millimeter Array (ALMA) observations of G17.64+0.16 in Band 6 (220−230 GHz) taken as part of a campaign in search of circumstellar discs around (proto)-O-stars. At a resolution of ~400 au the main continuum core is essentially unresolved and isolated from other strong and compact emission peaks. We detect SiO (5–4) emission that is marginally resolved and elongated in a direction perpendicular to the large-scale outflow seen in the ¹³ CO (2−1) line using the main ALMA array in conjunction with the Atacama Compact Array (ACA). Morphologically, the SiO appearsto represent a disc-like structure. Using parametric models we show that the position-velocity profile of the SiO is consistent with the Keplerian rotation of a disc around an object between 10 and 30 M_⊙ in mass, only if there is also radial expansion from a separate structure. The radial motion component can be interpreted as a disc wind from the disc surface. Models with a central stellar object mass between 20 and 30 M_⊙ are the most consistent with the stellar luminosity (1 × 10⁵ L_⊙) and indicative of an O-type star. The H30α millimetre recombination line (231.9 GHz) is also detected, but spatially unresolved, and is indicative of a very compact, hot, ionised region co-spatial with the dust continuum core. The broad line-width of the H30α emission (full-width-half-maximum = 81.9 km s⁻¹) is not dominated by pressure-broadening but is consistent with underlying bulk motions. These velocities match those required for shocks to release silicon from dust grains into the gas phase. CH₃ CN and CH₃ OH thermal emission also shows two arc shaped plumes that curve away from the disc plane. Their coincidence with OH maser emission suggests that they could trace the inner working surfaces of a wide-angle wind driven by G17.64 which impacts the diffuse remnant natal cloud before being redirected into the large-scale outflow direction. Accounting for all observables, we suggest that G17.64 is consistent with a O-type young stellar object in the final stages of protostellar assembly, driving a wind, but that has not yet developed into a compact H II region. The existance and detection of the disc in G17.64 is likely related to its isolated and possibly more evolved nature, traits which may underpin discs in similar sources.