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ELM of ELM-WD: An extremely low mass hot donor star discovered in LAMOST survey

Hailong YuanZhenwei LiZhongrui Bai ...+6 Haotong Zhang
Sep 2022
The Extremely Low Mass White Dwarfs (ELM WDs) and pre-ELM WDs are helium corewhite dwarfs with mass $<\sim 0.3M_{\odot}$. They are formed in close binariesand have lost over half of their initial masses via Common Envelope (CE)ejection or stable Roche Lobe Over Flow (RLOF). Both evolution simulations andobservations show that a lower mass limit for ELM WDs exists at$\approx0.14M_{\odot}$. Here we report the discovery of an extremely low massELM WD, ID70904216 in LAMOST survey, that may be lower than the ELM WD masslimit. Based on LAMOST and P200 spectroscopic observations, ID70904216 showsorbital period $P_{orb} =$ 0.219658 days and radial velocity semi-amplitude$K1=317.33km/s$, which gives the mass function of 0.73$M_{\odot}$, indicatingthe companion is a compact star. The low resolution spectra shows a F type starwith $T_{\rm eff} \sim 7361K$ without emission features. The temperature isconsistent with that derived from SED fitting($7440K$) and multi-color lightcurve solution($7400K$). The optical light curves, in ZTF g, r and i bands andCatalina V band, show ellipsoidal variability with amplitudes $\approx30\%$,suggesting that the visible companion is heavily tidal distorted. Combiningwith the distance from Gaia survey, the WD code modeling estimates that themass of the visible star is $M1=0.08^{+0.06}_{-0.03}M_{\odot}$, and the mass ofthe invisible star is $M2=0.94^{+0.45}_{-0.10}M_{\odot}$. The radius of thevisible donor is $R=0.29\pm0.01R_{\odot}$. The inclination angle is constrainedbetween 60$^{\circ}$ and 90$^{\circ}$. The observations indicate the system isa pre-ELM WD + WD/NS binary system with an extremely low mass hot donor belowthe $0.14M_{\odot}$ theoretical limit.