Kupfer, Thomas; Bauer, Evan B.; Burdge, Kevin B.; Roestel, Jan van; Bellm, Eric C.; Fuller, Jim; Hermes, JJ; Marsh, Thomas R.; Bildsten, Lars; Kulkarni, Shrinivas R.; Phinney, E. S.; Prince, Thomas A.; Szkody, Paula; Yao, Yuhan; Irrgang, Andreas; Heber, Ulrich; Schneider, David; Dhillon, Vik S.; Murawski, Gabriel; Drake, Andrew J.; Duev, Dmitry A.; Feeney, Michael; Graham, Matthew J.; Laher, Russ R.; Littlefair, S. P.; Mahabal, A. A.; Masci, Frank J.; Porter, Michael; Reiley, Dan; Rodriguez, Hector; Rusholme, Ben; Shupe, David L.; Soumagnac, Maayane T.
The Astrophysical Journal
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.
The study of binary stars is essential to stellar astrophysics. A large number of stars form and evolve within binary systems. Therefore, their study is fundamental to understand stellar and galactic evolution. Particularly relevant is that binary systems are still the best source of precise stellar mass and radius measurements. Research lines
Black holes, neutron stars, white dwarfs and their local environment
Accreting black-holes and neutron stars in X-ray binaries provide an ideal laboratory for exploring the physics of compact objects, yielding not only confirmation of the existence of stellar mass black holes via dynamical mass measurements, but also the best opportunity for probing high-gravity environments and the physics of accretion; the most