ALMA reveals a candidate hot and compact disc around the O-type protostar IRAS 16547−4247

Abstract

We present high angular resolution (similar to 0.3 arcsec) submillimeter continuum (0.85 mm) and line observations of the O-type protostar IRAS 16547-4247 carried out with the Atacama Large Millimeter/Submillimeter Array (ALMA). In the 0.85 mm continuum band, the observations revealed two compact sources (with a separation of 2 arcsec), one of them associated with IRAS 16547-4247, and the other one to the west. Both sources are well-resolved angularly, revealing a clumpy structure. On the other hand, the line observations revealed a rich variety of molecular species related to both continuum sources. In particular, we found a large number of S-bearing molecules, such as the rare molecule methyl mercaptan (CH3SH). At scales larger than 10 000 au, molecules (e. g. SO2 or OCS) mostly with low-excitation temperatures in the upper states (E-k less than or similar to 300 K) are present in both millimeter continuum sources, and show a southeast-north-west velocity gradient of 7 kms(-1) over 3 arcsec (165 km s(-1) pc(-1)). We suggest that this gradient probably is produced by the thermal (free-free) jet emerging from this object with a similar orientation at the base. At much smaller scales (about 1000 au), molecules with high-excitation temperatures (E-k >= 500 K) are tracing a rotating structure elongated perpendicular to the orientation of the thermal jet, which we interpret as a candidate disc surrounding IRAS 16547-4247. The dynamical mass corresponding to the velocity gradient of the candidate to disc is about 20M(circle dot), which is consistent with the bolometric luminosity of IRAS 16547-4247.