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 Evolutionary Map of the Universe (EMU) observations of the star itself, we determine a spectral index of, indicating thermal emission. We propose that the shell and star both exhibit thermal emission, supported by the its appearance in near-infrared and H 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, 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 -45.3 5.4 km s and 22.8 4.7 km s, 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 km s. We also use these proper motion (PM) to determine the shell's origin, estimate an age of yr, and determine its average expansion velocity to be km s. 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 yr, and use this to determine a lower-limit on ionising photons of.