Bibcode
Kupfer, T.; Ramsay, G.; van Roestel, J.; Brooks, J.; MacFarlane, S. A.; Toma, R.; Groot, P. J.; Woudt, P. A.; Bildsten, L.; Marsh, T. R.; Green, M. J.; Breedt, E.; Kilkenny, D.; Freudenthal, J.; Geier, S.; Heber, U.; Bagnulo, S.; Blagorodnova, N.; Buckley, D. A. H.; Dhillon, V. S.; Kulkarni, S. R.; Lunnan, R.; Prince, T. A.
Bibliographical reference
The Astrophysical Journal, Volume 851, Issue 1, article id. 28, 10 pp. (2017).
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12
2017
Journal
Citations
25
Refereed citations
25
Description
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× {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}ȯ with a WD companion
({M}{WD}=0.72+/- 0.17 {M}ȯ ). 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 ≈ 0.47 {M}ȯ . 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}ȯ , 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.
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