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

Creators: 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.

Abstract

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⊙ with a WD companion (M_(WD) = 0.72 ± 0.17 M⊙). 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⊙. 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⊙, 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.

Additional Information

© 2017 The American Astronomical Society. Received 2017 August 11; revised 2017 October 12; accepted 2017 October 18; published 2017 December 7. This work was supported by the GROWTH project funded by the National Science Foundation under grant No. 1545949. J.v.R. acknowledges support by the Netherlands Research School of Astronomy (NOVA) and the foundation for Fundamental Research on Matter (FOM). This research is funded in part by the Gordon and Betty Moore Foundation through Grant GBMF5076 and was also supported by the National Science Foundation under grant PHY 11-25915. We acknowledge stimulating workshops at Sky House where these ideas germinated. T.R.M. acknowledge the support from the Science and Technology Facilities Council (STFC), ST/L00733. S.G. is supported by the Deutsche Forschungsgemeinschaft (DFG) through grant GE2506/9-1. D.K. acknowledges financial support from the National Research Foundation of South Africa. P.J.G. acknowledges support from NOVA for the original OmegaWhite observations and hospitality of the Kavli Institute for Theoretical Physics. Armagh Observatory is core funded by the Northern Ireland Executive through the Department for Communities. This paper uses observations made at the South African Astronomical Observatory. Based on observations obtained at the European Southern Observatory, proposal 297.D-5010. Some results presented in this paper are based on observations obtained at the Gemini Observatory, proposal ID GS-2016B-FT-9, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the National Research Council (Canada), CONICYT (Chile), Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina), and Ministério da Ciência, Tecnologia e Inovação (Brazil). The authors thank the staff at the ESO Paranal and Gemini South observatories for performing the observations in service mode. Some of the data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. We thank the Swift team for the granting our DDT request. Facilities: Gemini:South (GMOS-S) - , Keck:I (LRIS) - , VLT:Antu (FORS2) - , NTT (ULTRACAM) - , SALT (SALTICAM) - , Radcliffe (SHOC) - . Software: LCURVE (Copperwheat et al. 2010), emcee (Foreman-Mackey et al. 2013), MESA (Paxton et al. 2011, 2013, 2015), FITSB2 (Napiwotzki et al. 2004).

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