The First Ultracompact Roche Lobe-Filling Hot Subdwarf Binary

March 2020 • 2020ApJ...891...45K

Authors • Kupfer, Thomas • Bauer, Evan B. • Marsh, Thomas R. • van Roestel, Jan • Bellm, Eric C. • Burdge, Kevin B. • Coughlin, Michael W. • Fuller, Jim • Hermes, JJ • Bildsten, Lars • Kulkarni, Shrinivas R. • Prince, Thomas A. • Szkody, Paula • Dhillon, Vik S. • Murawski, Gabriel • Burruss, Rick • Dekany, Richard • Delacroix, Alex • Drake, Andrew J. • Duev, Dmitry A. • Feeney, Michael • Graham, Matthew J. • Kaplan, David L. • Laher, Russ R. • Littlefair, S. P. • Masci, Frank J. • Riddle, Reed • Rusholme, Ben • Serabyn, Eugene • Smith, Roger M. • Shupe, David L. • Soumagnac, Maayane T.

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 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 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.

Links

• SIMBAD http://simbad.u-strasbg.fr/simbo.pl?bibcode=2020ApJ...891...45K
• PDF https://iopscience.iop.org/article/10.3847/1538-4357/ab72ff/pdf
• PDF https://stacks.iop.org/0004-637X/891/45/pdf
• PREPRINT http://arxiv.org/abs/2002.01485
• DATA https://archive.stsci.edu/mastbibref.php?bibcode=2020ApJ...891...45K
• ELECTR https://doi.org/10.3847%2F1538-4357%2Fab72ff