The formation and evolution of dust in the colliding-wind binary Apep revealed by JWST

TL;DR

This study uses JWST observations to reveal detailed concentric dust shells around the WR+WR binary Apep, providing insights into dust formation, evolution, and the system's distance, with implications for understanding dust contribution in the galaxy.

Contribution

First detailed imaging of nested dust shells around Apep, showing regular structures and dust evolution, and providing new distance constraints for the system.

Findings

Nested concentric dust shells with regular substructure.

Dust temperature evolution consistent with amorphous carbon.

Revised distance estimate for Apep, reducing previous velocity discrepancies.

Abstract

Carbon-rich Wolf-Rayet (WR) stars are significant contributors of carbonaceous dust to the galactic environment, however the mechanisms and conditions for formation and subsequent evolution of dust around these stars remain open questions. Here we present JWST observations of the WR+WR colliding-wind binary Apep which reveal an intricate series of nested concentric dust shells that are abundant in detailed substructure. The striking regularity in these substructures between successive shells suggests an exactly repeating formation mechanism combined with a highly stable outflow that maintains a consistent morphology even after reaching 0.6 pc (assuming a distance of 2.4 kpc) into the interstellar medium. The concentric dust shells show subtle deviations from spherical outflow, which could reflect orbital modulation along the eccentric binary orbit or non-sphericity in the stellar wind. Tracking the evolution of dust across the multi-tiered structure, we measure the dust temperature evolution that can broadly be described assuming an amorphous carbon composition in radiative thermal equilibrium with the central stars. The temperature profile and orbital period place new distance constraints that support Apep being at a greater distance than previously estimated, reducing the line-of-sight and sky-plane wind speed discrepancy previously thought to characterise the system.