2020/07/01 by Thomas Kupfer, Evan B. Bauer, Kevin B. Burdge +30
Physics and Astronomy · #Astrophysical Phenomena and Observations #Astrophysics and Star Formation Studies #Binary number #Black-body radiation #Effective temperature #Light curve #Population #Roche lobe #Stellar, planetary, and galactic studies #Subdwarf #Surface gravity #White dwarf #astro-ph.HE #astro-ph.SR
paper · pdf · doi:10.3847/2041-8213/aba3c2
accepted for publication in ApJL, 11 pages, 5 figures and 1 tables. arXiv admin note: text overlap with arXiv:2002.01485
openalex publication_date 2020/07/01 · arxiv created 2020/07/08 · arxiv updated 2020/07/29 · openalex created_date 2020/07/29 · openalex updated_date 2026/08/06
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 eff = 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 M sdOB = 0.41 ± 0.04 M ⊙ and a WD mass of M WD = 0.68 ± 0.05 M ⊙ . The weak eclipses imply a WD blackbody temperature of 63,000 ± 10,000 K and a radius R WD = 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 T eff ≈ 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.