X-Raying a Be star disk: fundamental parameters of the eclipsing binary Be star V658 Car

TL;DR

This paper provides a detailed observational and modeling study of the unique eclipsing Be star V658 Car, revealing its disk structure, stellar parameters, and the presence of a stripped companion, establishing it as a benchmark system for Be star research.

Contribution

It introduces a comprehensive three-component model of V658 Car, combining new observations and ray-tracing simulations to constrain its stellar and disk parameters with unprecedented detail.

Findings

Determined the system as a late-type Be + stripped star binary.

Provided strong spectroscopic evidence for a circumsecondary envelope.

Identified limitations in current models, highlighting the need for more sophisticated approaches.

Abstract

With its two stellar eclipses and two disk attenuations per binary orbit, V658 Carinae stands out as the first clear eclipsing Be + sdOB system. This rare alignment offers a unique opportunity to probe the structure and dynamics of a Be star disk with unprecedented detail. In this study, we present the most comprehensive observational dataset and modeling effort for this system to date, including optical, near-infrared, and ultraviolet spectroscopy, space-based photometry, and optical polarization. Using a new ray-tracing code, we apply a three-component model, consisting of an oblate, rapidly rotating Be star, a symmetric circumstellar disk, and a compact stripped companion, to reproduce the system's light curve, polarization, and spectral features. Our analysis yields precise constraints on the stellar and disk parameters, determining its status as the second known late-type Be + stripped star, and also provides strong spectroscopic evidence for a tenuous circumsecondary envelope. Despite the model's overall success, several key observables, such as the $H_{\alpha}$ equivalent width and the secondary attenuation, remain poorly reproduced, pointing to the need for more sophisticated modelling. In particular, future improvements should incorporate the companion's radiative feedback on the disk and account for asymmetric disk structures expected by the gravitational interaction with the companion. Owing to its unique geometry and rich diagnostics, V658 Car establishes itself as a benchmark system for Be stars (and rapid rotators in general), stripped stars, post-RLOF massive binaries, and circumstellar disk structures.