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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
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Number of Authors: 232017 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 851, no 1, article id 28Article in journal (Refereed) Published
Abstract [en]

We report the discovery of the ultracompact hot subdwarf (sdOB) binary OW J074106.0-294811.0 with an orbital period of P-orb = 44.66279 +/- 1.16 x 10(-4) 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-eff = 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-sdOB= 0.23 +/- 0.12 M-circle dot with a WD companion (M-WD = 0.72 +/- 0.17 M-circle dot). The mass ratio was found to be q = M-sdOB/M-WD = 0.32 +/- 0.10. The derived mass for the He-sdOB is inconsistent with the canonical mass for hot subdwarfs of approximate to 0.47 M-circle dot. To put constraints on the structure and evolutionary history of the sdOB star we compared the derived T-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-circle dot, which left the common envelope approximate to 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.

Place, publisher, year, edition, pages
2017. Vol. 851, no 1, article id 28
Keywords [en]
binaries (including multiple): close, stars: individual (OWJ074106.0-294811.0), subdwarfs, white dwarfs
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:su:diva-180134DOI: 10.3847/1538-4357/aa9522ISI: 000425473400010OAI: oai:DiVA.org:su-180134DiVA, id: diva2:1416814
Available from: 2020-03-25 Created: 2020-03-25 Last updated: 2020-03-25Bibliographically approved

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Kupfer, T.Groot, P. J.Woudt, P. A.Lunnan, Ragnhild
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Department of AstronomyThe Oskar Klein Centre for Cosmo Particle Physics (OKC)
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