Characterizing the Post-Red Supergiant binary system AFGL 4106 and its complex nebula with SPHERE/VLT

TL;DR

AFGL 4106 is a massive binary consisting of a post-RSG primary and an active RSG secondary embedded in a complex dusty nebula. Using VLT/SPHERE optical and near-infrared imaging, including polarimetry, the authors resolve the two stars, map the nebula, and perform radiative-transfer modelling with DUSTY to derive stellar and dust properties. The results yield $T_1=6723\pm196$ K, $T_2=3394\pm264$ K; $L_1=(7.9\pm0.18)\times10^4 L_\odot$, $L_2=(3.8\pm0.11)\times10^4 L_\odot$, and radii $R_1=(209\pm12) R_\odot$, $R_2=(567\pm99) R_\odot$, with a dust-shell inner radius of $r_{\rm in}=6.46\times10^{14}$ m. The nebula shows cavities, filaments, and large-scale lobes shaped by winds and possible interactions with the ISM, and hints at an additional unseen companion. Placing the stars on evolutionary tracks suggests a tension with single-star histories, implying past binary interactions or a tertiary object. This resolved view of AFGL 4106 advances understanding of mass loss and nebular shaping in evolved massive binaries and provides robust constraints for future dynamical modelling.

Abstract

Aims: We aim to characterize the physical and morphological properties of the binary system AFGL 4106, composed of two evolved massive stars. Understanding its mass-loss processes and circumstellar environment offers insight into the late stages of stellar evolution in massive binary systems. Methods: We obtained high-angular--resolution, high-contrast imaging using VLT/SPHERE with ZIMPOL (optical) and IRDIS (near-infrared) across multiple filters. We used aperture photometry to extract the spectral energy distributions (SEDs) of each star, and applied radiative transfer modelling to study the system and its surrounding dusty environment. Results: The observations resolve both components of the binary and unveil a complex, dusty nebula featuring asymmetric structures and cavities. SED fitting yields stellar temperatures of T$_1 = 6723\pm196$ K and T$_2 = 3394\pm264$ K, along with bolometric luminosities of L$_1 = (7.9 \pm 0.18) \times 10^4$ L$_\odot$ and L$_2 = (3.8 \pm 0.11) \times 10^4$ L$_\odot$. These values support the classification of the primary as being in a post-red supergiant (post-RSG) phase and the secondary as an active red supergiant (RSG). The luminosity ratio, combined with the inferred radii, indicates that both stars are at close yet distinct stages of their evolution. The binary is surrounded by an extended shell whose asymmetric morphology and large-scale features suggest interaction with the stellar winds and interstellar medium (ISM), and possibly the presence of a third, undetected companion. Conclusions: These observations provide the first resolved view of AFGL 4106's system and its dusty envelope. Our analysis sets constraints on the physical properties and evolutionary status of the system. This work contributes to understanding mass-loss processes in massive binaries and the shaping of nebulae around evolved stars.

Figures (21)

• Figure 1: ZIMPOL Polarized intensity map of AFGL 4106 in filter V.
• Figure 2: Three colours image of AFGL 4106 in Band I_PRIM, J and H. ZIMPOL image is re-scaled to the IRDIS FoV. Color and contrast are enhanced differently in the inner and outer part of the nebula. The red arrow indicates the direction of the Proper Motion of the system.
• Figure 3: ZIMPOL Polarized intensity map of AFGL 4106 in filter I_PRIM.
• Figure 4: IRDIS intensity map of AFGL 4106 in filter J.
• Figure 5: Intensity contours of the AFGL 4106 nebula in the J band are shown in a zoomed-in view in Figure \ref{['fig:contour']}, along with the corresponding eccentricity of these contours in Figure \ref{['fig:ex']}. The top panels present the raw data, while the bottom panels show the residuals after subtracting a linear fit.