A chemically peculiar Be-shell star in a sub-solar metallicity solution for the post-mass-transfer eclipsing binary V658 Car

V658 Car (HD 92406) is a newborn resp. rejuvenated shell star binary system at the age of only 1 Myr after the end of mass transfer. In this renewed study the peculiarities of the Be star are at first by-passed by the determination of the properties of the contracting hot subdwarf precursor, but finally resolved by combining photometric data and radial velocity results with existing stellar models. For the effective temperatures, radii and masses we get about 12900 K, 1.92/2.20 Rsun and 4.49 Msun for the Be star, and about 18400 K, 1.87 Rsun and 0.56 Msun for its companion star. The Be star has a rotational velocity of 336 km/s and is surrounded and dimmed in our view by a large and luminous equatorial decretion disk having a radius of ~ 42 Rsun. According to stellar models these results fit to a surprisingly low metallicity Z of 0.003 and a Teff ~ 5400 K higher than observational expectations for the Be star, which hence should belong to the chemically peculiar stars, in spite of its rapid rotation.

💡 Research Summary

V658 Car (HD 92406) is a post‑mass‑transfer eclipsing binary composed of a newly formed Be‑shell star and a contracting hot subdwarf (hot sub‑dwarf precursor). The system is extremely young, only about 1 Myr after the end of Roche‑lobe overflow, and therefore provides a rare snapshot of a Be star still close to the zero‑age main sequence. Using new TESS photometry, ground‑based UBV light curves, and updated radial‑velocity measurements (K₂ = 102.1 ± 0.2 km s⁻¹), the author performed a comprehensive BM3 (Binary Maker 3) model of the three light sources: the primary Be star, its large equatorial decretion disk, and the hot secondary.

The secondary star’s colour indices yield an effective temperature of 18 400 ± 500 K, an interstellar extinction A_V ≈ 0.01 mag, a radius of 1.87 ± 0.03 R☉ and a mass of 0.558 ± 0.005 M☉. These parameters place it on the evolutionary tracks of post‑mass‑transfer remnants (Iben & Tutukov 1985; Istrate et al. 2016). From the RV semi‑amplitude and the 32.185‑day orbital period, the orbital separation is a ≈ 73 R☉, giving a total system mass of ≈5.05 M☉ and a primary mass of 4.49 ± 0.03 M☉.

The primary Be star is characterised by T_eff ≈ 12 900 ± 35 K, a mean radius of 2.20 R☉ (polar/equatorial radii 1.923/2.202 R☉), and a projected rotational velocity v sin i ≈ 336 ± 6 km s⁻¹. The derived rotational period is 0.332 ± 0.006 days, corresponding to a rotation‑to‑critical ratio v_rot/v_crit ≈ 0.62, comfortably within the range observed for Be stars (Zorec et al. 2016). The star is surrounded by a luminous equatorial decretion disk of mean radius ≈42 R☉, which fills about 98 % of its Roche lobe. The disk contributes roughly 40 % of the V‑band flux at maximum light, acting as a pseudo‑photosphere that dominates the system’s photometric distance (≈1020 pc).

TESS light curves reveal two persistent periodicities: a longer 0.332‑day signal and a shorter ≈0.163‑day signal. The longer period is interpreted as the stellar rotation, modulated by a large hemispherical chemical spot on the surface. The shorter period is attributed to smaller chemical patches located near both magnetic poles, consistent with an oblique rotator model. This indicates the presence of a dipolar magnetic field that sustains surface abundance inhomogeneities despite the star’s rapid rotation.

A striking result is the discrepancy between the observed effective temperature of the Be star and the temperature predicted by stellar‑evolution models at the derived low metallicity (Z ≈ 0.003). Models (Ekström et al. 2008; Georgy et al. 2013) predict T_eff ≈ 19 000 K for a non‑rotating star of similar mass and metallicity, about 5 400 K higher than the observationally derived value. This offset suggests that the primary belongs to the chemically peculiar (CP) class, a rare occurrence for a rapidly rotating Be star. The low metallicity shifts the mass‑luminosity relation and HR‑diagram position, allowing a 4.5 M☉ star to appear cooler and smaller than solar‑metallicity counterparts.

In summary, the paper presents a self‑consistent picture of V658 Car: a freshly rejuvenated Be star rotating near critical speed, encircled by a massive, luminous decretion disk, and exhibiting magnetic‑induced chemical spots that render it chemically peculiar despite its rapid rotation. The combination of low metallicity, strong magnetic field, and disk contribution resolves the temperature discrepancy and provides a valuable case study for the interplay between mass transfer, rotation, magnetism, and chemical peculiarity in massive binary evolution.