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

TL;DR

ALMA observations of IRAS 16547-4247 reveal a candidate hot, compact disk and complex molecular environment around this massive O-type protostar, indicating early disk formation and rich chemistry.

Contribution

This study provides the first high-resolution ALMA imaging of a candidate disk around an O-type protostar, revealing detailed molecular and structural features.

Findings

Detection of a candidate hot, rotating disk at ~1000 AU

Identification of complex S-bearing molecules like CH3SH

Evidence of a thermal jet aligned with molecular velocity gradients

Abstract

We present high angular resolution ($\sim$ 0.3$"$) submillimeter continuum (0.85 mm) and line observations of the O-type protostar IRAS 16547$-$4247 carried out with the Atacama Large Millimeter/Submillimeter Array (ALMA). In the 0.85 mm continuum band, the observations revealed two compact sources (with a separation of 2$"$), one of them associated with IRAS 16547$-$4247, and the other one to the west. Both sources are well resolved angularly, revealing a clumpy structure. On the other hand, the line observations revealed a rich variety of molecular species related to both continuum sources. In particular, we found a large number of S-bearing molecules, such as the rare molecule methyl mercaptan (CH$_3$SH). At scales larger than 10,000 AU, molecules (e.g., SO$_2$ or OCS) mostly with low excitation temperatures in the upper states (E$_k$ $\lesssim$ 300 K) are present in both millimeter continuum sources, and show a southeast-northwest velocity gradient of 7 km s$^{-1}$ over 3$"$ (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$_k$ $\gtrsim$ 500 K) are tracing a rotating structure elongated perpendicular to the orientation of the thermal jet, which we interpret as a candidate disk surrounding IRAS 16547$-$4247. The dynamical mass corresponding to the velocity gradient of the candidate to disk is about 20 M$_\odot$, which is consistent with the bolometric luminosity of IRAS 16547$-$4247.