First Ultracompact Roche Lobe–Filling Hot Subdwarf Binary

Creators: Kupfer, Thomas; Bauer, Evan B.; Marsh, Thomas R.; van Roestel, Jan; Bellm, Eric C.; Burdge, Kevin B.; Coughlin, Michael W.; Fuller, Jim; Hermes, JJ; Bildsten, Lars; Kulkarni, Shrinivas R.; Prince, Thomas A.; Szkody, Paula; Dhillon, Vik S.; Murawski, Gabriel; Burruss, Rick; Dekany, Richard; Delacroix, Alex; Drake, Andrew J.; Duev, Dmitry A.; Feeney, Michael; Graham, Matthew J.; Kaplan, David L.; Laher, Russ R.; Littlefair, S. P.; Masci, Frank J.; Riddle, Reed; Rusholme, Ben; Serabyn, Eugene; Smith, Roger M.; Shupe, David L.; Soumagnac, Maayane T.

Abstract

We report the discovery of the first short-period binary in which a hot subdwarf star (sdOB) filled its Roche lobe and started mass transfer to its companion. The object was discovered as part of a dedicated high-cadence survey of the Galactic plane named the Zwicky Transient Facility and exhibits a period of P = 39.3401(1) minutes, making it the most compact hot subdwarf binary currently known. Spectroscopic observations are consistent with an intermediate He-sdOB star with an effective temperature of T_(eff) = 42,400 ± 300 K and a surface gravity of log(g) = 5.77 ± 0.05. A high signal-to-noise ratio GTC+HiPERCAM light curve is dominated by the ellipsoidal deformation of the sdOB star and an eclipse of the sdOB by an accretion disk. We infer a low-mass hot subdwarf donor with a mass M_(sdOB) = 0.337 ± 0.015 M⊙ and a white dwarf accretor with a mass M_(WD) = 0.545 ± 0.020 M⊙. Theoretical binary modeling indicates the hot subdwarf formed during a common envelope phase when a 2.5–2.8 M⊙ star lost its envelope when crossing the Hertzsprung gap. To match its current P_(orb), T_(eff), log(g), and masses, we estimate a post–common envelope period of P_(orb) ≈ 150 minutes and find that the sdOB star is currently undergoing hydrogen shell burning. We estimate that the hot subdwarf will become a white dwarf with a thick helium layer of ≈0.1 M⊙, merge with its carbon/oxygen white dwarf companion after ≈17 Myr, and presumably explode as a thermonuclear supernova or form an R CrB star.

Additional Information

© 2020 The American Astronomical Society. Received 2020 January 15; revised 2020 January 30; accepted 2020 February 3; published 2020 March 3. Based on observations obtained with the Samuel Oschin Telescope 48 inch at the Palomar Observatory as part of the Zwicky Transient Facility project. The ZTF is supported by the National Science Foundation under grant No. AST-1440341 and a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Center at Stockholm University, the University of Maryland, the University of Washington, Deutsches Elektronen-Synchrotron and Humboldt University, Los Alamos National Laboratories, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, and Lawrence Berkeley National Laboratories. Operations are conducted by COO, IPAC, and UW. 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. Some results presented in this paper are based on observations made with the WHT, operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Institutio de Astrofísica de Canarias. Based on observations made with the Gran Telescopio Canarias (GTC), installed at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias on the island of La Palma. The KPED team thanks the National Science Foundation and the National Optical Astronomical Observatory for making the Kitt Peak 2.1 m telescope available. The KPED team thanks the National Science Foundation, the National Optical Astronomical Observatory, and the Murty family for support in the building and operation of KPED. This research was supported in part by the National Science Foundation through grant ACI-1663688 and at the KITP by grant PHY-1748958. This research benefited from interactions that were funded by the Gordon and Betty Moore Foundation through grant GBMF5076. We acknowledge the use of the Center for Scientific Computing, supported by the California NanoSystems Institute and the Materials Research Science and Engineering Center (MRSEC) at UC Santa Barbara, through NSF DMR 1720256 and NSF CNS 1725797. HiPERCAM and VSD are funded by the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013) under ERC-2013-ADG grant agreement No. 340040 (HiPERCAM). We thank Brad Barlow, Josiah Schwab, and Stephan Geier for helpful conversations. M.C. is supported by the David and Ellen Lee Postdoctoral Fellowship at the California Institute of Technology. T.R.M. was supported by a grant from the United Kingdom's Science and Technology Facilities Council. P.S. acknowledges support from NSF grant AST-1514737. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC; https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. This work benefited from a workshop held at DARK in 2019 July that was funded by the Danish National Research Foundation (DNRF132). We thank Josiah Schwab for his efforts in organizing this. Facilities: PO:1.2 m (ZTF) - , Hale (DBSP) - , Keck:I (LRIS) - , ING:Herschel (ISIS) - , GTC (HiPERCAM). - Software: Lpipe (Perley 2019), PyRAF (Bellm & Sesar 2016), Gatspy (VanderPlas & Ivezić 2015; Vanderplas 2015), FITSB2 (Napiwotzki et al. 2004), LCURVE (Copperwheat et al. 2010), emcee (Foreman-Mackey et al. 2013), MESA (Paxton et al. 2011, 2013, 2015, 2018, 2019), Matplotlib (Hunter 2007), Astropy (Astropy Collaboration et al. 2013, 2018), Numpy (Oliphant 2015).

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