Speaker
Description
We present an analysis of the compact HII-region (G012.883–00.285) using VLA L-band continuum imaging from 1996 and MeerKAT L-band observations from 2018, providing a 22-year interferometric baseline. A systemic velocity of VLSR=35.705±0.092km/s, derived from C18O emission was used as input to a Bayesian parallax–kinematic model, which favours two high probability distance solutions: D=2.95±0.29kpc(P=0.52,Scutum–Centaurus arm) and D=3.85±0.27kpc(P=0.43,Norma arm). Both values place the source securely within a major spiral arm star-forming environment, consistent with its observed thermal continuum properties. The measured L-band spectral index of α=−0.3 confirms that the radio continuum is dominated by optically thin thermal free–free emission, although this borderline value leaves room for a weak non-thermal component or spatial variations in electron temperature and optical depth across the nebula. After smoothing both epochs to a common 45′′ beam, we measured the source extents using the 3σ isophotal contour. With this method, the MeerKAT continuum structure appears substantially more extended (1.228′×1.095′) than the VLA image (0.95′×0.80′). Interpreted literally, this would imply an apparent radial increase of 15–20′′ over 22 years, corresponding to expansion velocities exceeding 5000km/s, comparable to those of young supernova remnants, and far exceeding the ∼5–10km/s expected for compact HII regions. Given these nonphysical velocities, we conclude that the observed size discrepancy does not represent true dynamical expansion. Although a purely instrumental explanation where the older VLA data fail to recover faint extended emission detected by MeerKAT remains the most conservative interpretation, the striking morphological differences and borderline spectral index suggest a more physically driven scenario. A rapid transient expansion triggered by an accretion burst from the central massive protostar offers a compelling explanation, consistent with recent models of short-timescale “flickering” in compact HII-regions. Alternatively, faint non-thermal background emission from an undetected supernova remnant or pulsar wind nebula may contribute.Further observational work is underway to investigate these possibilities.
| Stream | Science or Engineering |
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