ALMA observations of the kinematics and chemistry of disc formation
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.