A New Class of Roche Lobe-filling Hot Subdwarf Binaries

July 2020 • 2020ApJ...898L..25K

Authors • 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.

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.

Links

• SIMBAD http://simbad.u-strasbg.fr/simbo.pl?bibcode=2020ApJ...898L..25K
• PDF https://iopscience.iop.org/article/10.3847/2041-8213/aba3c2/pdf
• PDF https://stacks.iop.org/2041-8205/898/L25/pdf
• PREPRINT http://arxiv.org/abs/2007.05349
• DATA http://gtc.sdc.cab.inta-csic.es:443/gtc/jsp/BibResult.jsp?bib=2020ApJ...898L..25K
• DATA https://archive.gemini.edu/searchform/publication=2020ApJ...898L..25K
• ELECTR https://doi.org/10.3847%2F2041-8213%2Faba3c2