The extreme colliding-wind system Apep: resolved imagery of the central binary and dust plume in the infrared

TL;DR

This study provides high-resolution infrared imaging and proper motion analysis of the Apep system, revealing a close Wolf-Rayet binary with a dust plume, and addresses discrepancies in wind speed measurements to improve understanding of colliding-wind binaries.

Contribution

The paper presents the first resolved imagery of Apep's central binary and dust plume, and refines the system's orbital parameters with an improved geometric model.

Findings

Proper motion speed of dust is four times slower than spectroscopic wind speed.

Resolved imaging confirms a close Wolf-Rayet binary at the system's core.

An improved geometric model constrains the binary's orbital parameters.

Abstract

The recent discovery of a spectacular dust plume in the system 2XMM J160050.7-514245 (referred to as "Apep") suggested a physical origin in a colliding-wind binary by way of the "Pinwheel" mechanism. Observational data pointed to a hierarchical triple-star system, however several extreme and unexpected physical properties seem to defy the established physics of such objects. Most notably, a stark discrepancy was found in the observed outflow speed of the gas as measured spectroscopically in the line-of-sight direction compared to the proper motion expansion of the dust in the sky plane. This enigmatic behaviour arises at the wind base within the central Wolf-Rayet binary: a system that has so far remained spatially unresolved. Here we present an updated proper motion study deriving the expansion speed of Apep's dust plume over a two-year baseline that is four times slower than the spectroscopic wind speed, confirming and strengthening the previous finding. We also present the results from high-angular-resolution near-infrared imaging studies of the heart of the system, revealing a close binary with properties matching a Wolf-Rayet colliding-wind system. Based on these new observational constraints, an improved geometric model is presented yielding a close match to the data, constraining the orbital parameters of the Wolf-Rayet binary and lending further support to the anisotropic wind model.