A new class of Roche lobe–filling hot subdwarf binaries
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Authors
Kupfer, Thomas
Bauer, Evan B.
Burdge, Kevin B.
Roestel, Jan van
Bellm, Eric C.
Fuller, Jim
Hermes, J.J.
Marsh, Thomas R.
Bildsten, Lars
Kulkarni, Shrinivas R.
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Thomas Kupfer, Evan B Bauer, Kevin B Burdge, Jan van Roestel, Eric C Bellm, Jim Fuller, J.J. Hermes, Thomas R Marsh, Lars Bildsten, Shrinivas R Kulkarni, E.S. Phinney, Thomas A Prince, Paula Szkody, Yuhan Yao, Andreas Irrgang, Ulrich Heber, David Schneider, Vik S Dhillon, Gabriel Murawski, Andrew J Drake, Dmitry A Duev, Michael Feeney, Matthew J Graham, Russ R Laher, S.P. Littlefair, A.A. Mahabal, Frank J Masci, Michael Porter, Dan Reiley, Hector Rodriguez, Ben Rusholme, David L Shupe, Maayane T Soumagnac. "A New Class of Roche Lobe–filling Hot Subdwarf Binaries." The Astrophysical Journal, Volume 898, Issue 1, pp. L25 - L25. https://doi.org/10.3847/2041-8213/aba3c2
Abstract
We present the discovery of the second binary with a Roche lobe–filling hot subdwarf transferring mass to a white dwarf (WD) companion. This 56 minute binary was discovered using data from the Zwicky Transient Facility. Spectroscopic observations reveal an He-sdOB star with an effective temperature of Teff = 33,700 ± 1000 K and a surface gravity of log(g) = 5.54 ± 0.11. The GTC+HiPERCAM light curve is dominated by the ellipsoidal deformation of the He-sdOB star and shows an eclipse of the He-sdOB by an accretion disk as well as a weak eclipse of the WD. We infer a He-sdOB mass of MsdOB = 0.41 ± 0.04 M⊙ and a WD mass of MWD = 0.68 ± 0.05 M⊙. The weak eclipses imply a WD blackbody temperature of 63,000 ± 10,000 K and a radius RWD = 0.0148 ± 0.0020 R⊙ as expected for a WD of such high temperature. The He-sdOB star is likely undergoing hydrogen shell burning and will continue transferring mass for ≈1 Myr at a rate of 10−9 M⊙ yr−1, which is consistent with the high WD temperature. The hot subdwarf will then turn into a WD and the system will merge in ≈30 Myr. We suggest that Galactic reddening could bias discoveries toward preferentially finding Roche lobe–filling systems during the short-lived shell-burning phase. Studies using reddening-corrected samples should reveal a large population of helium core–burning hot subdwarfs with Teff ≈ 25,000 K in binaries of 60–90 minutes with WDs. Though not yet in contact, these binaries would eventually come into contact through gravitational-wave emission and explode as a subluminous thermonuclear supernova or evolve into a massive single WD.
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