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

TL;DR

This work uses JWST/MIRI and VLT/VISIR observations to resolve four concentric dust shells around the colliding-wind Wolf-Rayet binary Apep, revealing highly regular substructures and a stable expansion history. By measuring shell spacing, expansion speeds, and the multi-wavelength, spatially resolved SEDs, the authors infer an orbital period of about 193 years and a dust-formation history extending ~700 years, while also deriving a radial temperature profile compatible with amorphous carbon dust in radiative equilibrium. Radiative-transfer modelling and dynamical constraints place the distance to Apep around 4.6 kpc and suggest a combined luminosity on the order of several hundred thousand solar luminosities, though non-spherical wind geometry or distance effects can influence the wind-speed discrepancy. The findings demonstrate how resolved imaging across wavelengths can tightly constrain dust formation conditions, wind dynamics, and system geometry in dusty colliding-wind binaries, with broader implications for understanding dust production in massive star environments.

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.

Figures (10)

• Figure 1: Colour image of Apep created from JWST/MIRI observations with the F770W, F1500W and F2550W filters. North is oriented towards the top of the page. A colour image of the inner dust shell observed with VLT/VISIR in the 2024 epoch is shown in the inset, where tick marks are separated by 5 arcsec. A distance of 2.4 kpc is assumed for the linear displacement axes.
• Figure 2: Top: MIRI F770W image of Apep with the azimuthal range of shell radius measured in this study indicated by dashed lines, and the direction orthogonal to the line of nodes and its uncertainty range White2025 indicated by dotted lines. Middle: F1500W image with points indicating examples of shell location detections. Bottom: F2550W image labelled with the E1, E2 and E3 features and apertures used to extract photometry.
• Figure 3: Left: Apep's inter-shell spacing as a function of position angle. The position angle in the horizontal axis corresponds to the angle measured counterclockwise from north in Fig. \ref{['fig:filters']} (left panel), in which the azimuthal range with data points are labelled with arcs. Right: the expansion speed of Apep's inner dust shell as a function of position angle. The two curves correspond to the best-fit sinusoidal model with one and two periods over the full azimuthal range respectively. The vertical lines and shaded regions correspond to the line of nodes, apastron and the velocity vector at apastron and their uncertainties White2025.
• Figure 4: The difference image of the inner shell of Apep obseverved by VISIR with the B11.7 filter between 2016 and 2024. Over-emission in 2024 compared to 2016 is shown in purple and under-emission is shown in orange. The difference image suggests a consistent and significant expansion of the dust structure which we measured to be $90 \pm 4$ mas yr$^{-1}$.
• Figure 5: The radial temperature profile of dust in Apep measured from its spatially resolved SED. The thicker error bars are derived by fitting SED models to individual features. The thinner error bars show the adjusted uncertainties upon enforcing the condition that the measurements sample the same temperature profile. The best-fit power law profile with a power-law index of -0.31 is shown, along with the blackbody equilibrium temperature, an amorphous carbon equilibrium temperature profile based on RADMC-3D simulations and an amorphous carbon approximation with a power law index of -0.38 for comparison.