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${}_{\mathrm{orb}}=44.66279\pm 1.16\times 10^{-4}$ min, 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${}_{\mathrm{eff}}=39400\pm 500$ K and log(g)=$5.74\pm 0.09$. High signal-to-noise ratio lightcurves show strong ellipsoidal modulation resulting in a derived sdOB mass $M_{\mathrm{sdOB}}=0.23\pm 0.12$ M${}_{\odot }$ with a WD companion ($M_{\mathrm{WD}}=0.72\pm 0.17$ M${}_{\odot }$). The mass ratio was found to be $q=M_{\mathrm{sdOB}}/M_{\mathrm{WD}}=0.32\pm 0.10$. The derived mass for the He-sdOB is inconsistent with the canonical mass for hot sbudwarfs of $\approx 0.47$ M${}_{\odot }$. To put constraints on the structure and evolutionary history of the sdOB star we compared the derived T${}_{\mathrm{eff}}$, 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${}_{\odot }$, which left the common envelope $\approx 1.1$ Myr ago, 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 min. 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.

Accepted for publication in ApJ, 13 pages, 7 figures

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