Kupfer, ThomasBauer, Evan B.Marsh, Thomas R.Roestel, Jan vanBellm, Eric C.Burdge, Kevin B.Coughlin, Michael W.Fuller, JimHermes, J.J.Bildsten, LarsKulkarni, Shrinivas R.Prince, Thomas A.Szkody, PaulaDhillon, Vikram S.Murawski, GabrielBurruss, RickDekany, RichardDelacroix, AlexDrake, Andrew J.Duev, Dmitry A.Feeney, MichaelGraham, Matthew J.Kaplan, David L.Laher, Russ R.Littlefair, S.P.Masci, Frank J.Riddle, ReedRusholme, BenSerabyn, EugeneSmith, Roger M.Shupe, David L.Soumagnac, Maayane T.2021-03-112021-03-11Thomas Kupfer, Evan B Bauer, Thomas R Marsh, Jan van Roestel, Eric C Bellm, Kevin B Burdge, Michael W Coughlin, Jim Fuller, J.J. Hermes, Lars Bildsten, Shrinivas R Kulkarni, Thomas A Prince, Paula Szkody, Vik S Dhillon, Gabriel Murawski, Rick Burruss, Richard Dekany, Alex Delacroix, Andrew J Drake, Dmitry A Duev, Michael Feeney, Matthew J Graham, David L Kaplan, Russ R Laher, S.P. Littlefair, Frank J Masci, Reed Riddle, Ben Rusholme, Eugene Serabyn, Roger M Smith, David L Shupe, Maayane T Soumagnac. "The First Ultracompact Roche Lobe–Filling Hot Subdwarf Binary." The Astrophysical Journal, Volume 891, Issue 1, pp. 45 - 45. https://doi.org/10.3847/1538-4357/ab72ff1538-4357https://hdl.handle.net/2144/42245We 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.p. 45en-US© 2020. The American Astronomical Society. All rights reserved.Astronomical and space sciencesAtomic, molecular, nuclear, particle and plasma physicsPhysical chemistry (incl. structural)Astronomy & astrophysicsArticle10.3847/1538-4357/ab72ff553332