A New Class of Roche Lobe-filling Hot Subdwarf Binaries
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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- July 2020
- DOI:
- 10.3847/2041-8213/aba3c2
- arXiv:
- arXiv:2007.05349
- Bibcode:
- 2020ApJ...898L..25K
- Keywords:
-
- B subdwarf stars;
- Stellar evolution;
- White dwarf stars;
- Compact binary stars;
- Stellar accretion;
- 129;
- 1599;
- 1799;
- 283;
- 1578;
- Astrophysics - Solar and Stellar Astrophysics;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- accepted for publication in ApJL, 11 pages, 5 figures and 1 tables. arXiv admin note: text overlap with arXiv:2002.01485