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Published online by Cambridge University Press: 10 July 2025
We present the first radio–continuum detection of the circumstellar shell around the well-known WN8 type Wolf-Rayet star WR16 at 943.5MHz using the Australian Square Kilometre Array Pathfinder (ASKAP) Evolutionary Map of the Universe (EMU) survey. At this frequency, the shell has a measured flux density of 72.2±7.2 mJy. Using previous Australia Telescope Compact Array (ATCA) measurements at 2.4, 4.8, and 8.64 GHz, as well as the EMU observations of the star itself, we determine a spectral index of α = +0.74 ± 0.02, indicating thermal emission. We propose that the shell and star both exhibit thermal emission, supported by the its appearance in near-infrared and Hα observations. The latest Gaia parallax is used to determine a distance of 2.28 ± 0.09 kpc. This star is well-known for its surrounding circular nebulosity, and using the distance and an angular diameter of 8′.42, we determine the shell size to be 5.57±0.22 pc. We use the Gaia proper motions (PMs) of WR16 to determine peculiar velocities of the star as Vα(pec) = –45.3±5.4 kms–1 and Vδ(pec) = 22.8±4.7 kms–1, which indicates that the star is moving in a north-west direction, and translates to a peculiar tangential velocity to be 50.7±6.9 kms–1. We also use these PMs to determine the shell’s origin, estimate an age of ∼ 9500 ±1300 yr, and determine its average expansion velocity to be 280 ± 40 km s–1. This average expansion velocity suggests that the previous transitional phase is a Luminous Blue Variable (LBV) phase, rather than a Red Super Giant (RSG) phase. We also use the measured flux at 943.5 MHz to determine a mass-loss rate of 1.753 × 10–5 M⊙ yr–1, and use this to determine a lower-limit on ionising photons of NUV > 1.406 × 1047 s–1.
Deceased on 3rd July 2024