The massive hot subdwarf binary LAMOST J065816.72+094343.1

A hot subdwarf in a 7.7 hour orbit with an unseen companion of 1.30 solar masses, close enough to the Chandrasekhar limit that it is unclear whether it is a massive white dwarf or a neutron star.

F. Mattig, B. N. Barlow, D. Liu, M. Dorsch, S. Geier, M. Pritzkuleit, H. Dawson, B. Wang, V. Schaffenroth, T. Kupfer, C. Derbyshire, S. Barocci-Faul

Astronomy & Astrophysics, 2026. Publisher · arXiv:2601.03810

Abstract

Massive short-period binaries involving hot subdwarf stars (sdO/Bs) are rare but important to constraining pathways for binary star evolution. Moreover, some of the most promising candidate progenitor systems leading to Type Ia supernovae (SNe Ia) involve sdO/Bs. LAMOST J065816.72+094343.1 has been identified as such a candidate. To explore the nature and evolutionary future of LAMOST J065816.72+094343.1, we complemented archival spectroscopic data with additional time series spectra and high-resolution spectroscopy of the object. After combining these with photometric data, we determined the orbital parameters of the system and the mass of the companion.

We solved the orbit of the system by analyzing 68 low- and medium-resolution spectra using state-of-the-art mixed local thermodynamic equilibrium (LTE) and non-LTE model atmospheres. Additionally, we gathered nine high-resolution spectra to determine atmospheric parameters and the projected rotational velocity of the sdOB. The inclination angle of the system was constrained assuming tidal synchronization of the sdOB, which was verified via analysis of the ellipsoidal variations in the TESS light curve.

We determine LAMOST J065816.72+094343.1 to be a binary consisting of a massive 0.82 ± 0.17 M_☉ sdOB component with a 1.30^+0.31_−0.26 M_☉ unseen companion. Due to the companion’s mass being very close to the Chandrasekhar mass limit and high for a white dwarf, it is unclear whether it is a white dwarf or a neutron star. We find the system to be in a close orbit, with a period of P = 0.31955193 d and an inclination angle of i = 49.6^+5.2_−4.2 deg. While the exact nature of the companion remains unknown, we determine the system to either lead to a SN Ia or an intermediate mass binary pulsar, potentially after a phase as an intermediate-mass X-ray binary.