A protostellar jet emanating from a hypercompact H _II region

We present radio continuum observations of the high-mass young stellar object (HMYSO) G345.4938+01.4677 obtained 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 H ii region. By comparing with images taken 6 years earlier at 5 and 9 GHz using the same telescope, we assess the proper motions (PMs) of the radio sources. The outer west and east lobes exhibit PMs of 64 ± 12 and 48 ± 13 mas yr⁻¹, indicating velocities projected in the plane of the sky and receding from G345.4938+01.4677 of 520 and 390 ${\text{km s}}^{-1}$, respectively. The internal radio lobes also display PM signals consistently receding from the HMYSO with magnitudes of 17 ± 11 and 35 ± 10 mas 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 H ii region.