ELM of ELM-WD: An Extremely-low-mass Hot Star Discovered in LAMOST Survey

Abstract

The extremely-low-mass white dwarfs (ELM WDs) and pre-ELM WDs are helium-core white dwarfs with mass <∼ 0.3M ⊙. Evolution simulations show that a lower mass limit for ELM WDs exists at ≈0.14M ⊙ and no ELM WD is proposed by observation to be less massive than that. Here we report the discovery of a binary system, LAMOST J224040.77-020732.8 (J2240 in short), which consists of a very low-mass hot star and a compact companion. Multiepoch spectroscopy shows an orbital period P orb = 0.219658 ± 0.000002 days and a radial-velocity semiamplitude K1 = 318.5 ± 3.3 km s−1, which gives the mass function of 0.74M ⊙, indicating the companion is a compact star. The F-type low-resolution spectra illustrate no emission features, and the temperature (∼7400 K) is consistent with that from spectral energy distribution fitting and multicolor light-curve solution. The optical light curves, in ZTF g, r, and i bands and the Catalina V band, show ellipsoidal variability with amplitudes of ∼30%, suggesting that the visible component is heavily tidally distorted. Combining the distance from Gaia survey, the ZTF light curves are modeled with Wilson–Devinney code and the result shows that the mass of the visible component is , and the mass of the invisible component is . The radius of the visible component is . The inclination angle is approximately between 60° and 90°. The observations indicate the system is most likely a pre-ELM WD + WD/NS binary, and the mass of pre-ELM is possibly lower than the 0.14M ⊙ theoretical limit.