Evolutionary map of the Universe: Detection and analysis of the shell surrounding the runaway Wolf-Rayet star WR16

Abstract

We present the first radio–continuum detection of the circumstellar shell around the well-known WN8 type Wolf-Rayet star WR16 at 943.5 MHz 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 its appearance in near-infrared and H α 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 motion (PM) of WR16 to determine peculiar velocities of the star as Vα(pec) = –45.3 ± 5.4 km s⁻¹ and Vδ(pec) = 22.8 ± 4.7 km s⁻¹, which indicates that the star is moving in a north-west direction, and translates to a peculiar tangential velocity of 50.7 ± 6.9 km s⁻¹. We also use these proper motions 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 Supergiant (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.