HS 2231+2441: an HW Vir system composed by a low-mass white dwarf and a brown dwarf

Abstract

HW Vir systems are rare evolved eclipsing binaries composed by a hot compact star and a low-mass main-sequence star in a close orbit. These systems provide a direct way to measure the fundamental properties, e.g. masses and radii, of their components, hence they are crucial to study the formation of sdB stars and low-mass white dwarfs, the common-envelope phase, and the pre-phase of cataclysmic variables. Here we present a detailed study of HS 2231+2441, an HW Vir type system, by analysing BVR${}_C$I${}_C$ photometry and phase-resolved optical spectroscopy. The spectra of this system, which are dominated by the primary component features, were fitted using NLTE models providing effective temperature Teff = 28500$\pm$500 K, surface gravity log g = 5.40$\pm$0.05 cm s${}^{-2}$, and helium abundance log(n(He)/n(H)) = -2.52$\pm$0.07. Geometrical orbit and physical parameters were derived by modelling simultaneously the photometric and spectroscopic data using the Wilson-Devinney code. We derive two possible solutions for HS 2231+2441 that provide component's masses: M${}_1$ = 0.19 M${}_{\odot }$ and M${}_2$ = 0.036 M${}_{\odot }$ or M${}_1$ = 0.288 M${}_{\odot }$ and M${}_2$ = 0.046 M${}_{\odot }$. Considering the possible evolutionary channels to form a compact hot star, the primary of HS 2231+2441 probably evolved through the red-giant branch scenario and does not have a helium-burning core, which is consistent with a low-mass white dwarf. Both solutions are consistent with a brown dwarf as the secondary.

9 Pages, 9 Figures, 4 tables, accepted for publication in MNRAS

References in corpus (4)

• The ELM Survey. I. A Complete Sample of Extremely Low Mass White Dwarfs
• Hot subdwarf binaries from the MUCHFUSS project - Analysis of 12 new systems and a study of the short-period binary population
• Precise Atmospheric Parameters for the Shortest Period Binary White Dwarfs: Gravitational Waves, Metals, and Pulsations
• An eclipsing post common-envelope system consisting of a pulsating hot subdwarf B star and a brown dwarf companion

Cited by in corpus (5)

• Gaia Data Release 2 Catalogue of Extremely-low Mass White Dwarf Candidates
• A Catalogue of Potential Post Common Envelope Binaries
• Hydrodynamics and survivability during post-main-sequence planetary engulfment
• Eclipse Timing Modeling of Three Post-Common Envelope Binaries: Hybrid Solutions
• Physical Properties of 29 sdB+dM Eclipsing Binaries in Zwicky Transient Facility