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 $<\sim 0.3M_{\odot }$. Evolution simulations show that a lower mass limit for ELM WDs exists at $\approx 0.14M_{\odot }$ and no one 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. Multi-epoch spectroscopy shows an orbital period $P_{orb}=$0.219658$\pm 0.000002$ days and a radial velocity semi-amplitude $K1=318.5\pm 3.3km/s$, which gives the mass function of 0.74$M_{\odot }$, indicating the companion is a compact star. The F-type low resolution spectra illustrate no emission features, and the temperature ($\sim 7400K$) is consistent with that from Spectral Energy Distribution fitting and multi-color light curve solution. The optical light curves, in ZTF g, r and i bands and Catalina V band, show ellipsoidal variability with amplitudes $\sim 30\%$, 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 $M1=0.085_{-0.024}^{+0.036}{M}_{\odot }$, and the mass of the invisible component is $M2=0.98_{-0.09}^{+0.16}{M}_{\odot }$. The radius of the visible component is $R1=0.29_{-0.03}^{+0.04}{R}_{\odot }$. The inclination angle is approximately between 60${}^{\circ }$ and 90${}^{\circ }$. 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 $0.14M_{\odot }$ theoretical limit.

18 pages, 10 figures