ALMA observations of the kinematics and chemistry of disc formation

¹ Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5–7, 1350 Copenhagen K, Denmark
e-mail: jolindbe@gmail.com
² Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
³ Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor MI 48109-1042, USA
⁴ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁵ SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV, Groningen, The Netherlands
⁶ Department of Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, 113-0033 Tokyo, Japan

Received: 11 September 2013
Accepted: 4 May 2014

Abstract

Context. The R CrA cloud hosts a handful of Class 0/I low-mass young stellar objects. The chemistry and physics at scales >500 AU in this cloud are dominated by the irradiation from the nearby Herbig Be star R CrA. The luminous large-scale emission makes it necessary to use high-resolution spectral imaging to study the chemistry and dynamics of the inner envelopes and discs of the protostars.

Aims. We aim to better understand the structure of the inner regions of these protostars and, in particular, the interplay between the chemistry and the presence of discs.

Methods. Using Atacama Large Millimeter/submillimeter Array (ALMA) high-resolution spectral imaging interferometry observations, we study the molecular line and dust continuum emission at submillimetre wavelengths.

Results. We detect dust continuum emission from four circumstellar discs around Class 0/I objects within the R CrA cloud. Towards IRS7B we detect C¹⁷O emission showing a rotation curve consistent with a Keplerian disc with a well-defined edge that gives a good estimate for the disc radius at 50 AU. We derive the central object mass to 2.3 M_⊙ and the disc mass to 0.024 M_⊙. The observations are also consistent with a model of material infalling under conservation of angular momentum; however, this model provides a worse fit to the data. We also report a likely detection of faint CH₃OH emission towards this point source, as well as more luminous CH₃OH emission in an outflow orthogonal to the major axis of the C¹⁷O emission.

Conclusions. The faint CH₃OH emission seen towards IRS7B can be explained by a flat density profile of the inner envelope caused by the disc with a radius ≲50 AU. We propose that the regions of the envelopes where complex organic molecules are present in Class 0/I young stellar objects can become quenched as the disc grows.