The massive hot subdwarf binary LAMOST J065816.72+094343.1

TL;DR

LAMOST J065816.72+094343.1 is a rare, massive short-period sdOB binary whose unseen companion is constrained to be $M_{comp}=1.30^{+0.31}_{-0.26}\,M_{sun}$ in a $P=0.31955193$ d orbit with $i=49.6^{+5.2}_{-4.2}$ deg. The visible sdOB has $M_{sd}=0.82 \pm 0.17\,M_{sun}$, $T_{eff}=35800 \pm 750$ K, $\log g=5.37 \pm 0.07$, and $v\sin i=37.0 \pm 1.0$ km s$^{-1}$, derived from 68 low-/medium-resolution spectra and 9 high-resolution UVES spectra, combined with SED and Gaia parallax fitting. The data indicate tidal synchronization and ellipsoidal TESS variations, enabling a precise inclination and a robust companion mass estimate, though the compact nature of the companion (WD vs NS) remains ambiguous. Binary evolution modeling and population synthesis suggest the system could evolve into a double WD leading to a SN Ia on timescales longer than the age of the universe, or instead form an intermediate-mass binary pulsar through NS/ONe WD channels; current X-ray limits do not decisively distinguish among these. This system thus provides key constraints on the end-states of massive sdOB binaries and the diversity of possible SN Ia progenitors and IMBP pathways.

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 LAMOSTJ065816.72+094343.1 to be a binary consisting of a massive $0.82 \pm 0.17 \mathrm{M}_{\odot}$ sdOB component with a $1.30^{+0.31}_{-0.26} \mathrm{M}_{\odot}$ 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 $P=0.31955193 \mathrm{d}$ and an inclination angle of $i = 49.6^{+5.2}_{-4.2} \mathrm{deg}$. 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.

Figures (7)

• Figure 1: Phased RV curve and phased and binned TESS light curve for J0658. Both curves are phased to the same period, $P$, and zero point, $t_0$ (see Tab. \ref{['tab:Atmospheric Parameters']}). For the RV curve, the plots show individual RV measurements from the LAMOST survey (salmon), the SOAR Goodman spectrograph (green; 2004SPIE.5492..331C), the ALFOSC spectrograph (light blue), and the UVES spectrograph (ocher). The light curve shows 118287 TESS data points from TESS sectors 33 and 87 that were phased and then binned into 175 bins. Both curves show their respective best-fitting models. For the RV curve, the fit was obtained via the MCMC method, and for the TESS light curve it was obtained using the LCURVE code.
• Figure 2: Results of the stellar evolution and BPS simulations. Blue points mark initial configurations producing wide, detached double-WD binaries; red points reach $M_\mathrm{ch}$, triggering an SN Ia or accretion-induced collapse. The background shading shows BPS population densities by companion type, and the orange diamond marks the adopted masses for J0658.
• Figure 3: Stacked and re-binned (50000 bins) UVES spectra for J0658. The black line represents the spectral data, the red line the best-fitting model spectrum. The gap between the end of the UVES blue arm and red arm ($4500\text{Å} - 5700\text{Å}$) and sections of the spectrum without any absorption lines are not shown.
• Figure 4: Spectral energy distribution of J0658. Valid fluxes from all available photometric surveys are shown. The best-fitting synthetic spectral model is overlaid as a gray line. Horizontal error bars represent the filter width at tenth maximum. (Data publications: Pan-STARRS1 (yellow): 2016arXiv161205560C; 2MASS (salmon): 2003tmc..book.....C; SkyMapper (green): 2019PASA...36...33O; and Gaia G/BP/RP (blue) and Gaia XP (light blue): 2022yCat.1355....0G).
• Figure 5: Posterior distributions of the free parameters in the light curve fit, obtained via MCMC sampling with 10 000 000 samples of the TESS light curve of J0658.