A very young and fast rotating shell star discovered in the eclipsing binary ZTF J200347.63+394429.8

A photometric study in combination with existing stellar models has revealed details of this eclipsing post-mass-transfer binary. The shell star has an equatorial/ polar radius of ~2.60/1.90 Rsun at an equatorial rotational velocity of ~430 km s-1, an effective mean temperature Teff of ~13300 K and a mass of ~3.42 Msun. This former accretor star is surrounded by a large decretion disk of ~47 Rsun. The secondary star is a helium white dwarf precursor with a radius of 0.98 Rsun, a Teff of ~17100 K and a mass of 0.29 Msun. The parameters of this former donor star indicate an age of the binary system of only ~1.8 Myr after the end of mass transfer. The results fit to a sub-solar metallicity of Z = 0.007.

💡 Research Summary

The paper presents a comprehensive photometric and modeling study of the eclipsing binary ZTF J200347.63+394429.8, a system with an orbital period of 44.685 days located in Cygnus at a distance of ~2.94 kpc. Using archival ZTF g‑ and r‑band data together with new UBV observations obtained with remotely‑controlled 17–24 inch telescopes, the authors constructed detailed light curves and fitted them with the Binary Maker 3 (BM 3) code. The resulting model reveals a post‑mass‑transfer configuration consisting of a rapidly rotating primary (the “shell star”) and a low‑mass helium white‑dwarf precursor secondary, surrounded by a massive decretion disk.

Primary (shell star)

• Mass = 3.42 ± 0.05 M☉, equatorial radius = 2.60 ± 0.14 R☉, polar radius = 1.90 ± 0.14 R☉, giving a flattening ratio of 1.37 ± 0.04 (close to the theoretical limit of 1.5).
• Mean effective temperature ≈ 13 300 K (derived from the total surface area and luminosity), while a non‑rotating counterpart would have Teff ≈ 14 750 K.
• Equatorial rotational velocity = 432 ± 9 km s⁻¹, corresponding to a rotation‑to‑orbital velocity ratio W = 0.86 ± 0.03, i.e., 86 % of the critical break‑up speed.
• Bolometric luminosity ≈ 146 L☉, contributing roughly 30 % of the V‑band flux at maximum light.

Decretion disk

• Mean radius ≈ 47 R☉, essentially filling the Roche lobe of the primary.
• The disk causes a prolonged “disk eclipse” lasting 9.3 days, during which it blocks 78–81 % of the system’s flux in the V, B, and U bands.
• The disk’s optical depth varies with orbital phase: when the primary is seen through the dense inner part, the apparent mean Teff is ~8 370 K; through a less dense outer region it rises to ~8 680 K; and when the star is viewed without the disk the temperature reaches ~11 590 K.
• The disk contributes 25 %–28 % of the total light at 550 nm depending on the eclipse phase.

Secondary (He‑WD precursor)

• Mass = 0.290 ± 0.004 M☉, radius = 0.98 ± 0.01 R☉, Teff = 17 140 ± 570 K, bolometric luminosity ≈ 74 L☉.
• These parameters match evolutionary tracks for low‑mass helium white‑dwarf precursors (Driebe et al. 1998; Istrate et al. 2016).

Orbital geometry

• Semi‑major axis = 81.97 R☉, eccentricity = 0.075, argument of periastron = 61°, inclination = 89.37° (nearly edge‑on).
• Primary eclipse is almost total (duration = 14.6 h, missed by only 0.03°), while the secondary eclipse is annular (duration = 13.1 h).
• The mass ratio derived from the light‑curve solution is q = M₂/M₁ = 1/1.80 ± 0.035.

Evolutionary interpretation
The authors propose a conservative Roche‑lobe overflow scenario: the original primary (≈ 2.19 M☉) transferred its hydrogen‑rich envelope to the companion (≈ 1.52 M☉). Mass transfer ceased ~1.8 Myr ago, leaving the accretor spun up to near‑critical rotation and surrounded by a viscous decretion disk, while the donor became a contracting helium white‑dwarf precursor. The system’s metallicity Z ≈ 0.007 (sub‑solar, comparable to the Large Magellanic Cloud) is invoked to explain the unusually high rotation rate, as lower metallicity reduces line‑driven winds and allows stars to retain angular momentum.

Context and significance
Only a handful of non‑degenerate stars have reported projected rotational velocities > 450 km s⁻¹ (e.g., LAMOST J040643.69+542347.8, VFTS 102). This B‑type shell star, with v ≈ 430 km s⁻¹, joins that exclusive group and represents the fastest rotator among B‑type stars known to date. Moreover, the combination of extreme flattening, a massive, optically thick decretion disk, and a very young post‑mass‑transfer age makes ZTF J200347.63+394429.8 a unique laboratory for testing theories of binary mass transfer, angular‑momentum accretion, and disk formation. The system challenges existing models of Be/shell stars by demonstrating that rapid rotation and large‑scale disks can arise immediately after mass transfer, even in a relatively metal‑poor Milky Way environment.

In summary, the paper delivers a detailed observational characterization and evolutionary reconstruction of a rare, ultra‑fast rotating shell star in an eclipsing post‑mass‑transfer binary, providing valuable constraints on stellar rotation, binary interaction, and disk physics.