The First Ultracompact Roche Lobe-Filling Hot Subdwarf Binary

Thomas 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. DuevMichael 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

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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 Teff = 42,400 ± 300 K and a surface gravity of logg = 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 M sdOB = 0.337 ± 0.015 M and a white dwarf accretor with a mass M WD = 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 Porb, Teff, logg, and masses, we estimate a post-common envelope period of Porb ≈ 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.

Original languageEnglish (US)
Article number45
JournalAstrophysical Journal
Issue number1
StatePublished - Mar 1 2020

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