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 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 e and a WD mass of M WD =0.68±0.05 M e. The weak eclipses imply a WD blackbody temperature of 63,000±10,000 K and a radius R WD =0.0148±0.0020 R e 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 e 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 reddeningcorrected 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.

Year 1 of the ZTF high-cadence Galactic plane survey: strategy, goals, and early results on new single-mode hot subdwarf B-star pulsators

“…8; P orb = 39 min; Kupfer et al 2020a). In a follow-up search, we discovered ZTF J2055, a second member of that class (Kupfer et al 2020b). Both objects are only a few hundred million years old and members of the Galactic thin disc population, which explains why these objects have not been seen before and shows the value of a high-cadence Galactic plane survey.…”

New Variable Hot Subdwarf Stars Identified from Anomalous Gaia Flux Errors, Observed by TESS, and Classified via Fourier Diagnostics

“…This rapid, 39-min binary consists of an sdOB transferring mass onto a WD. It, along with the He-sdOB+WD binary ZTF J205515.98+465106.5 (Kupfer et al 2020a), represent the only two members of this newly discovered class of Roche lobe-filling hot subdwarfs. Through the emission of gravitational waves, these binaries should eventually come into contact and either explode as an underluminous thermonuclear supernova, or evolve into a single, massive white dwarf.…”

HiPERCAM: a quintuple-beam, high-speed optical imager on the 10.4-m Gran Telescopio Canarias

“…HiPERCAM saw first light on the GTC in February 2018, and it has since been in use for 13 observing runs totalling ∼70 nights (although some of these nights were shared with other instruments or partly lost due to weather). The first tranche of scientific papers based on HiPERCAM data have now been published (Rebassa-Mansergas et al 2019, Nieder et al 2019, Paice et al 2019, Parsons et al 2020, Kupfer et al 2020a, Paice et al 2021, Burdge et al 2020, Kupfer et al 2020b, Montes et al 2021, van Roestel et al 2021. In this section, we summarise the performance of HiPERCAM on the GTC.…”