The first ultracompact Roche lobe-filling hot subdwarf binary

Abstract

We report the discovery of the first short period binary in which a hot subdwarf star (sdOB) fills its Roche lobe and started mass transfer to its companion. The object was discovered as part of a dedicated high-cadence survey of the Galactic Plane named the Zwicky Transient Facility and exhibits a period of $P_{\mathrm{orb}}=39.3401(1)$ min, making it the most compact hot subdwarf binary currently known. Spectroscopic observations are consistent with an intermediate He-sdOB star with an effective temperature of $T_{\mathrm{eff}}=42,400\pm 300$ K and a surface gravity of $\log (g)=5.77\pm 0.05$. A high-signal-to noise GTC+HiPERCAM light curve is dominated by the ellipsoidal deformation of the sdOB star and an eclipse of the sdOB by an accretion disk. We infer a low-mass hot subdwarf donor with a mass $M_{\mathrm{sdOB}}=0.337\pm 0.015$ M${}_{\odot }$ and a white dwarf accretor with a mass $M_{\mathrm{WD}}=0.545\pm 0.020$ M${}_{\odot }$. Theoretical binary modeling indicates the hot subdwarf formed during a common envelope phase when a $2.5-2.8$ M${}_{\odot }$ star lost its envelope when crossing the Hertzsprung Gap. To match its current $P_{\mathrm{orb}}$, $T_{\mathrm{eff}}$, $\log (g)$, and masses, we estimate a post-common envelope period of $P_{\mathrm{orb}}\approx 150$ min, and find the sdOB star is currently undergoing hydrogen shell burning. We estimate that the hot subdwarf will become a white dwarf with a thick helium layer of $\approx 0.1$ M${}_{\odot }$ and will merge with its carbon/oxygen white dwarf companion after $\approx 17$ Myr and presumably explode as a thermonuclear supernova or form an R CrB star.

accepted for publication in ApJ, 21 pages, 12 figures and 4 tables