Resolving the heart of the Wolf-Rayet binary Apep with ALMA

Abstract

The Wolf-Rayet (WR) phase is a critical stage in the evolution of massive stars. During this phase, the stars experience a large mass loss in the form of fast winds that are enriched with heavy elements. There are only a few WR binaries in the galaxy, with Apep being a truly exceptional system exhibiting a bright colliding-wind region at radio wavelengths and an impressive dust plume in the infrared. Although the angular separation between the two (WN and WC) components has been indirectly inferred to be about 47 mas, it has yet to be confirmed through direct measurements. We aim to image the Apep system at millimetre wavelengths to determine the position of the WR stars and their mass-loss rates. We observed Apep with ALMA in Bands 6 and 7 in a large baseline configuration, achieving an angular resolution of approximately 20 mas. We used a canonical spherical wind model to convert the measured flux densities into wind mass-loss rates. Towards Apep, we detect point-like emission coming from the WN star and elongated emission most likely from the colliding-wind region; the WC star is not detected. We also detect the O star at 0.7'' that has been associated with the system. Our analysis favours mass-loss rates for the WN and WC components of $\dot{M}_\mathrm{WN}\sim 2\times10^{-5}\,M_\odot \,\mathrm{yr}^{-1}$ and $\dot{M}_\mathrm{WC}\sim10^{-5}\,M_\odot\,\mathrm{yr}^{-1}$, respectively. The mass-loss rates of the WR stars are significantly lower than previously reported, and we examine several factors that may account for this discrepancy. The diffuse emission from the colliding-wind region cannot be accounted for by the synchrotron flux, and thus additional free-free or dust emission is present.