A Protostellar Jet Emanating from a Hypercompact HII Region

TL;DR

This study provides radio observations of a high-mass protostellar jet from a hypercompact HII region, revealing proper motions and shock structures that suggest disk accretion and jet activity similar to low-mass protostars.

Contribution

First direct evidence of a collimated protostellar jet from a hypercompact HII region with proper motion analysis of radio lobes.

Findings

Outer lobes show proper motions indicating velocities of 520 and 390 km/s.

Outer lobes exhibit bow shock morphology.

Jet activity occurs despite ionizing radiation from the hypercompact HII region.

Abstract

We present radio continuum observations of the high-mass young stellar object (HMYSO) G345.4938+01.4677 made using the Australia Telescope Compact Array (ATCA) at 5, 9, 17, and 19 GHz. These observations provide definite evidence that the outer and inner pairs of radio lobes consist of shock ionized material being excited by an underlying collimated and fast protostellar jet emanating from a hypercompact HII region. By comparing with images taken 6 yr earlier at 5 and 9 GHz using the same telescope, we assess the proper motions of the radio sources. The outer West and East lobes exhibit proper motions of $64\pm12$ and $48\pm13$ milliarcsec yr$^{-1}$, indicating velocities projected in the plane of the sky and receding from G345.4938+01.4677 of $520$ and $390$ km s$^{-1}$, respectively. The internal radio lobes also display proper motion signals consistently receding from the HMYSO, with magnitudes of $17\pm11$ and $35\pm10$ milliarcsec yr$^{-1}$ for the inner West and East lobes, respectively. The morphology of the outer West lobe is that of a detached bow shock. At 17 and 19 GHz, the outer East lobe displays an arcuate morphology also suggesting a bow shock. These results show that disk accretion and jet acceleration --- possibly occurring in a very similar way compared with low-mass protostars --- is taking place in G345.4938+01.4677 despite the presence of ionizing radiation and the associated hypercompact HII region.