A new class of Roche lobe-filling hot subdwarf binaries

Abstract

We present the discovery of the second binary with a Roche lobe-filling hot subdwarf transferring mass to a white dwarf (WD) companion. This 56 minute binary was discovered using data from the Zwicky Transient Facility. Spectroscopic observations reveal an He-sdOB star with an effective temperature of $T_{\mathrm{eff}}=33,700\pm 1000$ K and a surface gravity of $log(g)=5.54\pm 0.11$. The GTC+HiPERCAM light curve is dominated by the ellipsoidal deformation of the He-sdOB star and shows an eclipse of the He-sdOB by an accretion disk as well as a weak eclipse of the WD. We infer a He-sdOB mass of $M_{\mathrm{sdOB}}=0.41\pm 0.04$ M${}_{\odot }$ and a WD mass of $M_{\mathrm{WD}}=0.68\pm 0.05$ M${}_{\odot }$. The weak eclipses imply a WD black-body temperature of $63,000\pm 10,000$ K and a radius $R_{\mathrm{WD}}=0.0148\pm 0.0020$ M${}_{\odot }$ as expected for a WD of such high temperature. The He-sdOB star is likely undergoing hydrogen shell burning and will continue transferring mass for $\approx 1$ Myrs at a rate of $10^{-9}{M}_{\odot }{\mathrm{yr}}^{-1}$ which is consistent with the high WD temperature. The hot subdwarf will then turn into a WD and the system will merge in $\approx 30$ Myrs. We suggest that Galactic reddening could bias discoveries towards preferentially finding Roche lobe-filling systems during the short-lived shell burning phase. Studies using reddening corrected samples should reveal a large population of helium core-burning hot subdwarfs with $T_{\mathrm{eff}}\approx 25,000$ K in binaries of 60-90 minutes with WDs. Though not yet in contact, these binaries would eventually come into contact through gravitational wave emission and explode as a sub-luminous thermonuclear supernova or evolve into a massive single WD.

arXiv commentaccepted for publication in ApJL, 11 pages, 5 figures and 1 tables. arXiv admin note: text overlap with arXiv:2002.01485

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