The OmegaWhite Survey for Short-period Variable Stars. V. Discovery of an Ultracompact Hot Subdwarf Binary with a Compact Companion in a 44-minute Orbit

Abstract

We report the discovery of the ultracompact hot subdwarf (sdOB) binary OW J074106.0-294811.0 with an orbital period of {P}<SUB>{orb</SUB>}=44.66279+/- 1.16× {10}<SUP>-4</SUP> minutes, making it the most compact hot subdwarf binary known. Spectroscopic observations using the VLT, Gemini and Keck telescopes revealed a He-sdOB primary with an intermediate helium abundance, {T}<SUB>{eff</SUB>} = 39 400+/- 500 K and {log}g = 5.74 ± 0.09. High signal-to-noise ratio light curves show strong ellipsoidal modulation resulting in a derived sdOB mass {M}<SUB>{sdOB</SUB>}=0.23+/- 0.12 {M}<SUB>⊙ </SUB> with a WD companion ({M}<SUB>{WD</SUB>}=0.72+/- 0.17 {M}<SUB>⊙ </SUB>). The mass ratio was found to be q={M}<SUB>{sdOB</SUB>}/{M}<SUB>{WD</SUB>}=0.32+/- 0.10. The derived mass for the He-sdOB is inconsistent with the canonical mass for hot subdwarfs of ≈ 0.47 {M}<SUB>⊙ </SUB>. To put constraints on the structure and evolutionary history of the sdOB star we compared the derived {T}<SUB>{eff</SUB>}, {log}g, and sdOB mass to evolutionary tracks of helium stars and helium white dwarfs calculated with Modules for Experiments in Stellar Astrophysics (MESA). We find that the best-fitting model is a helium white dwarf with a mass of 0.320 {M}<SUB>⊙ </SUB>, which left the common envelope ≈ 1.1 {Myr} ago, which is consistent with the observations. As a helium white dwarf with a massive white dwarf companion, the object will reach contact in 17.6 Myr at an orbital period of 5 minutes. Depending on the spin-orbit synchronization timescale the object will either merge to form an R CrB star or end up as a stably accreting AM CVn-type system with a helium white dwarf donor.