Chasing discs around O-type (proto)stars: Evidence from ALMA observations

¹ INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
e-mail: cesa@arcetri.astro.it
² I. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, 50937 Köln, Germany
³ School of Physics and Astronomy, University of Leeds, West Yorkshire, Leeds LS2 9JT, UK
⁴ ALLEGRO/Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, the Netherlands
⁵ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁶ Kapteyn Astronomical Institute, University of Groningen, 9700 AV, Groningen, The Netherlands
⁷ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁸ Hamburger Sternwarte, Universität Hamburg, 21029 Hamburg, Germany
⁹ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester, Oxford Road, Manchester M13 9PL, UK
¹⁰ UK ALMA Regional Centre Node, The University of Manchester, Manchester M13 9PL, UK
¹¹ Instituto de Radioastronomía y Astrofísica, UNAM, Apdo. Postal 3–72 (Xangari), 58089 Morelia, Michoacán, México
¹² Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
¹³ School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
¹⁴ UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
¹⁵ Institut für Astronomie und Astrophysik, Universität Tübingen, Auf der Morgenstelle 10, 72076 Tübingen, Germany
¹⁶ Centre for Astrophysics, University of Hertfordshire, College Lane, Hatfield, AL10 9AB, UK
¹⁷ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150–762 Porto, Portugal
¹⁸ School of Physics and Astronomy, University of Leeds, West Yorkshire, Leeds LS2 9JT, UK
¹⁹ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85741 Garching, Germany
²⁰ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
²¹ SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD, Groningen, The Netherlands
²² Indian Institute of Space science and Technology, 695 547 Thiruvananthapuram, India
²³ Dublin Institute of Advanced Studies, Fitzwilliam Place 31, Dublin 2, Ireland
²⁴ SOFIA Science Center, Deutsches SOFIA Institut, NASA Ames Research Center, Moffett Field, CA, 94035, USA

Received: 2 December 2016
Accepted: 30 January 2017

Abstract

Context. Circumstellar discs around massive stars could mediate the accretion onto the star from the infalling envelope, and could minimize the effects of radiation pressure. Despite such a crucial role, only a few convincing candidates have been provided for discs around deeply embedded O-type (proto)stars.

Aims. In order to establish whether disc-mediated accretion is the formation mechanism for the most massive stars, we have searched for circumstellar, rotating discs around a limited sample of six luminous (>10⁵L_⊙) young stellar objects. These objects were selected on the basis of their IR and radio properties in order to maximize the likelihood of association with disc+jet systems.

Methods. We used ALMA with ~0.̋2 resolution to observe a large number of molecular lines typical of hot molecular cores. In this paper we limit our analysis to two disc tracers (methyl cyanide, CH₃CN, and its isotopologue, ¹³CH₃CN), and an outflow tracer (silicon monoxide, SiO).

Results. We reveal many cores, although their number depends dramatically on the target. We focus on the cores that present prominent molecular line emission. In six of these a velocity gradient is seen across the core,three of which show evidence of Keplerian-like rotation. The SiO data reveal clear but poorly collimated bipolar outflow signatures towards two objects only. This can be explained if real jets are rare (perhaps short-lived) in very massive objects and/or if stellar multiplicity significantly affects the outflow structure.For all cores with velocity gradients, the velocity field is analysed through position–velocity plots to establish whether the gas is undergoing rotation with ν_rot ∝ R^{− α}, as expected for Keplerian-like discs.

Conclusions. Our results suggest that in three objects we are observing rotation in circumstellar discs, with three more tentative cases, and one core where no evidence for rotation is found. In all cases but one, we find that the gas mass is less than the mass of any embedded O-type star, consistent with the (putative) discs undergoing Keplerian-like rotation. With the caveat of low number statistics, we conclude that the disc detection rate could be sensitive to the evolutionary stage of the young stellar object. In young, deeply embedded sources, the evidence for discs could be weak because of confusion with the surrounding envelope, while in the most evolved sources the molecular component of the disc could have already been dispersed. Only in those objects that are at an intermediate stage of the evolution would the molecular disc be sufficiently prominent and relatively less embedded to be detectable by mm/submm observations.