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24 April 2024

» arxiv » 2007.5349

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A new class of Roche lobe-filling hot subdwarf binaries
Thomas Kupfer ; Evan B. Bauer ; Kevin B. Burdge ; Jan van Roestel ; Eric C. Bellm ; Jim Fuller ; JJ 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 ;
Date:	8 Jul 2020
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 $T_{ m eff}=33,700pm1000$ K and a surface gravity of $log(g)=5.54pm0.11$. 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 $M_{ m sdOB}=0.41pm0.04$ M$_odot$ and a WD mass of $M_{ m WD}=0.68pm0.05$ M$_odot$. The weak eclipses imply a WD black-body temperature of $63,000pm10,000$ K and a radius $R_{ m WD}=0.0148pm0.0020$ M$_odot$ 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 $approx1$ Myrs at a rate of $10^{-9} M_odot { m yr}^{-1}$ which is consistent with the high WD temperature. The hot subdwarf will then turn into a WD and the system will merge in $approx30$ Myrs. We suggest that Galactic reddening could bias discoveries towards 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 $T_{ m eff}approx25,000$ 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 sub-luminous thermonuclear supernova or evolve into a massive single WD.
Source:	arXiv, 2007.5349
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