PTF1 J082340.04+081936.5: A Hot Subdwarf B Star with a Low-mass White Dwarf Companion in an 87-minute Orbit

Creators: Kupfer, Thomas; van Roestel, Jan; Brooks, Jared; Geier, Stephan; Marsh, Tom R.; Groot, Paul J.; Bloemen, Steven; Prince, Thomas A.; Bellm, Eric; Heber, Ulrich; Bildsten, Lars; Miller, Adam A.; Dyer, Martin J.; Dhillon, Vik S.; Green, Matthew; Irawati, Puji; Laher, Russ; Littlefair, Stuart P.; Shupe, David L.; Steidel, Charles C.; Rattansoon, Somsawat; Pettini, Max

Abstract

We present the discovery of the hot subdwarf B star (sdB) binary PTF1 J082340.04+081936.5. The system has an orbital period of P_(orb) = 87.49668(1) minutes (0.060761584(10) days), making it the second-most compact sdB binary known. The light curve shows ellipsoidal variations. Under the assumption that the sdB primary is synchronized with the orbit, we find a mass of M_(sdB) = 0.45_(-0.07)^(+0.09) M_⊙, a companion white dwarf mass of M_(WD) = 0.46_(-0.09)^(+0.12) M_⊙, and a mass ratio of q = M_(WD)/M_(sdB) = 1.03_(-0.08)^(+0.10). The future evolution was calculated using the MESA stellar evolution code. Adopting a canonical sdB mass of M_(sdB) = 0.47 M_⊙, we find that the sdB still burns helium at the time it will fill its Roche lobe if the orbital period was less than 106 minutes at the exit from the last common envelope (CE) phase. For longer CE exit periods, the sdB will have stopped burning helium and turned into a C/O white dwarf at the time of contact. Comparing the spectroscopically derived log g and T_(eff) with our MESA models, we find that an sdB model with a hydrogen envelope mass of 5 x 10^(-4) M_⊙ matches the measurements at a post-CE age of 94 Myr, corresponding to a post-CE orbital period of 109 minutes, which is close to the limit to start accretion while the sdB is still burning helium.

Additional Information

© 2017. The American Astronomical Society. Received 2016 November 14; revised 2016 December 5; accepted 2016 December 6; published 2017 January 23. 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). T.R.M. acknowledges the support from the Science and Technology Facilities Council (STFC), ST/L00733. This research was partially funded by the Gordon and Betty Moore Foundation through Grant GBMF5076 to Lars Bildsten. This work was supported by the National Science Foundation under grants PHY 11-25915, AST 11-09174, and AST 12-05574. This work was supported in part by the National Science Foundation under Grant No. PHYS-1066293 and the hospitality of the Aspen Center for Physics where parts of this paper was written. We thank the referee for helpful and timely comments. Observations were obtained with the Samuel Oschin Telescope at the Palomar Observatory as part of the PTF project, a scientific collaboration between the California Institute of Technology, Columbia University, Las Cumbres Observatory, the Lawrence Berkeley National Laboratory, the National Energy Research Scientific Computing Center, the University of Oxford, and the Weizmann Institute of Science. 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 Mauna Kea 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 Astrofisica de Canarias. Software: PyRAF (Bellm & Sesar 2016), MAKEE (http://www.astro.caltech.edu/tb/ipac staff/tab/makee/), Gatspy (VanderPlas & Ivezić 2015), 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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