The first ultracompact Roche lobe–filling hot subdwarf binary
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Kupfer, Thomas
Bauer, Evan B.
Marsh, Thomas R.
Roestel, Jan van
Bellm, Eric C.
Burdge, Kevin B.
Coughlin, Michael W.
Fuller, Jim
Hermes, J.J.
Bildsten, Lars
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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 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/ab72ff
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.
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