The First Ultracompact Roche Lobe-Filling Hot Subdwarf Binary
Duev, Dmitry A.; Riddle, Reed; Szkody, Paula; Littlefair, S. P.; Rusholme, Ben; Kaplan, David L.; Burdge, Kevin B.; Bellm, Eric C.; Dekany, Richard; Feeney, Michael; Graham, Matthew J.; Kulkarni, Shrinivas R.; Kupfer, Thomas; Laher, Russ R.; Masci, Frank J.; Shupe, David L.; Soumagnac, Maayane T.; van Roestel, Jan; Prince, Thomas A.; Fuller, Jim; Burruss, Rick; Serabyn, Eugene; Coughlin, Michael W.; Bildsten, Lars; Smith, Roger M.; Drake, Andrew J.; Marsh, Thomas R.; Bauer, Evan B.; Hermes, JJ; Dhillon, Vik S.; Murawski, Gabriel; Delacroix, Alex
United States, United Kingdom, Spain, Poland, Israel
Abstract
We report the discovery of the first short-period binary in which a hot subdwarf star (sdOB) filled 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 = 39.3401(1) minutes, 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_{eff} = 42,400 ± 300 K and a surface gravity of log (g) = 5.77 ± 0.05. A high signal-to-noise ratio 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 MsdOB = 0.337 ± 0.015 M_{⊙} and a white dwarf accretor with a mass MWD = 0.545 ± 0.020 M_{⊙}. Theoretical binary modeling indicates the hot subdwarf formed during a common envelope phase when a 2.5-2.8 M_{⊙} star lost its envelope when crossing the Hertzsprung gap. To match its current P_{orb}, T_{eff}, log (g), and masses, we estimate a post-common envelope period of P_{orb} ≈ 150 minutes and find that 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 ≈0.1 M_{⊙}, merge with its carbon/oxygen white dwarf companion after ≈17 Myr, and presumably explode as a thermonuclear supernova or form an R CrB star.