Three ways to solve the orbit of KIC 11 558 725: A 10-day beaming sdB+WD binary with a pulsating subdwarf

The recently discovered subdwarf B (sdB) pulsator KIC 11 558 725 is one of the 16 pulsating sdB stars detected in the Kepler field. It features a rich g-mode frequency spectrum, with a few low-amplitude p-modes at short periods. This makes it a promising target for a seismic study aiming to constrain the internal structure of this star, and of sdB stars ingeneral. We have obtained ground-based spectroscopic radial-velocity measurements of KIC 11 558 725 based on low-resolution spectra in the Balmer-line region, spanning the 2010 and 2011 observing seasons. From these data we have discovered that KIC 11 558 725 is a binary with period P = 10.05 d, and that the radial-velocity amplitude of the sdB star is 58 km s -1. Consequently the companion of the sdB star has a minimum mass of 0.63 M ?, and is therefore most likely an unseen white dwarf. We analyse the near-continuous 2010-2011 Kepler light curve to reveal the orbital Doppler-beaming effect, giving rise to light variations at the 238 ppm level, which is consistent with the observed spectroscopic orbital radial-velocity amplitude of the subdwarf. We use the strongest 70 pulsation frequencies in the Kepler light curve of the subdwarf as clocks to derive a third consistent measurement of the orbital radial-velocity amplitude, from the orbital light-travel delay. The orbital radius a sdBsini = 11.5 R ? gives rise to a light-travel time delay of 53.6 s, which causes aliasing and lowers the amplitudes of the shortest pulsation frequencies, unless the effect is corrected for. We use our high signal-to-noise average spectra to study the atmospheric parameters of the sdB star, deriving T eff = 27 910 K andlog g = 5.41 dex, and find that carbon, nitrogen and oxygen are underabundant relative to the solar mixture. Furthermore, we analyse the Kepler light curve for its pulsational content and extract more than 160 significant frequencies. We investigate the pulsation frequencies for expected period spacings and rotational splittings. We find period-spacing sequences of spherical-harmonic degrees l = 1 and l = 2, and we associate a large fraction of the g-modes in KIC 11 558 725 with these sequences. From frequency splittings we conclude that the subdwarf is rotating subsynchronously with respect to the orbit. © 2012 ESO.Aarhus Universitet, AU Radboud Universiteit, RU GOA/2008/04 European School of Oncology, ESO KU Leuven National Science Foundation, NSF Spectrum Pharmaceuticals Brussels Instituut voor Milieubeheer Iowa State University, ISU Korean Institute of Criminology, KIC Comisión Sectorial de Investigación Científica, CSIC Association of Canadian Universities for Research in Astronomy, ACURA Seventh Framework Programme, FP7 National Aeronautics and Space Administration, NASA European Research Council, ERC Isaac Newton Trust European Research Council, ERC: 227224 Australian Astronomical Observatory, AAO Physics Department, University of Michigan Secretário de Ciência, Tecnologia e Ensino Superior, Governo do Estado de Parana, SETI: A1 University of Arizona, UA? ?? ? Based on observations obtained by the Kepler spacecraft, the Kitt Peak Mayall Telescope, the Nordic Optical Telescope and the William Herschel Telescope. ?? Tables 1, 4, and 5 are available in electronic form at http://www.aanda.org Telting J. H. 1 Østensen R. H. 2 Baran A. S. 3 Bloemen S. 2 Reed M. D. 3 Oreiro R. 4 Farris L. 3 Ottosen T. A. 5 Aerts C. 2 6 Kawaler S. D. 7 Heber U. 8 Prins S. 2 Green E. M. 9 Kalomeni B. 10 O’Toole S. J. 11 Mullally F. 12 Sanderfer D. T. 12 Smith J. C. 12 Kjeldsen H. 5 1 Nordic Optical Telescope , Apartado 474 , 38700 Santa Cruz de La Palma , Spain 2 Instituut voor Sterrenkunde, KU Leuven , Celestijnenlaan 200D , 3001 Leuven , Belgium 3 Department of Physics, Astronomy, and Materials Science, Missouri State University , Springfield , MO 65804 , USA 4 Instituto de Astrofísica de Andalucía (CSIC) , Glorieta de la Astronomía s/n , 18008 Granada , Spain 5 Department of Physics and Astronomy, Aarhus University , 8000 Aarhus C , Denmark 6 Department of Astrophysics, IMAPP, Radboud University Nijmegen , PO Box 9010 , 6500 GL Nijmegen , The Netherlands 7 Department of Physics and Astronomy, Iowa State University , Ames , IA 50011 , USA 8 Dr. Remeis Sternwarte Bamberg, Universität Erlangen-Nürnberg , Germany 9 Steward Observatory, University of Arizona , 933 North Cherry Avenue , Tucson , AZ 85721 , USA 10 University of Ege, Department of Astronomy & Space Sciences , 35100 İzmir , Turkey 11 Australian Astronomical Observatory , PO Box 296 , Epping , NSW 1710 , Australia 12 SETI Institute/NASA Ames Research Center , Moffett Field , CA 94035 , USA e-mail: jht@not.iac.es 17 07 2012 17 07 2012 8 2012 544 aa/2012/08 A1 23 4 2012 16 6 2012 © ESO, 2012 2012 ESO The recently discovered subdwarf B (sdB) pulsator KIC?11?558?725 is one of the 16 pulsating sdB stars detected in the Kepler field. It features a rich g -mode frequency spectrum, with a few low-amplitude p -modes at short periods. This makes it a promising target for a seismic study aiming to constrain the internal structure of this star, and of sdB stars ingeneral. We have obtained ground-based spectroscopic radial-velocity measurements of KIC?11?558?725 based on low-resolution spectra in the Balmer-line region, spanning the 2010 and 2011 observing seasons. From these data we have discovered that KIC?11?558?725 is a binary with period P = 10.05?d, and that the radial-velocity amplitude of the sdB star is 58?km?s -1 . Consequently the companion of the sdB star has a minimum mass of 0.63? M ? , and is therefore most likely an unseen white dwarf. We analyse the near-continuous 2010–2011 Kepler light curve to reveal the orbital Doppler-beaming effect, giving rise to light variations at the 238?ppm level, which is consistent with the observed spectroscopic orbital radial-velocity amplitude of the subdwarf. We use the strongest 70 pulsation frequencies in the Kepler light curve of the subdwarf as clocks to derive a third consistent measurement of the orbital radial-velocity amplitude, from the orbital light-travel delay. The orbital radius a sdB sin i ?=?11.5? R ? gives rise to a light-travel time delay of 53.6?s, which causes aliasing and lowers the amplitudes of the shortest pulsation frequencies, unless the effect is corrected for. We use our high signal-to-noise average spectra to study the atmospheric parameters of the sdB star, deriving T eff ?=?27?910?K andlog? g ?=?5.41?dex, and find that carbon, nitrogen and oxygen are underabundant relative to the solar mixture. Furthermore, we analyse the Kepler light curve for its pulsational content and extract more than 160 significant frequencies.We investigate the pulsation frequencies for expected period spacings and rotational splittings. We find period-spacing sequences of spherical-harmonic degrees l = 1 and l = 2, and we associate a large fraction of the g -modes in KIC?11?558?725 with these sequences. From frequency splittings we conclude that the subdwarf is rotating subsynchronously with respect to the orbit. stars: early-type binaries: spectroscopic subdwarfs stars: oscillations stars: individual: KIC11558725 idline A&A 544, A1 (2012) cover_date August 2012 first_month 8 last_month 8 first_year 2012 last_year 2012 Based on observations made with the Nordic Optical Telescope, operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio del Roque de los Muchachos (ORM) of the Instituto de Astrofisica de Canarias, and the William Herschel Telescope and Isaac Newton Telescope also at ORM, operated by the Isaac Newton Group. M.D.R. and L.F. were Visiting Astronomers to the Kitt Peak National Observatory, National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA) under cooperative agreement with the National Science Foundation. M.D.R. and L.F. were supported by the Missouri Space Grant Consortium funded by NASA. The authors gratefully acknowledge the Kepler team and all who have contributed to enabling the mission. Funding for the Kepler Mission is provided by NASA’s Science Mission Directorate. The research leading to these results has received funding from the European Research Council under the European Community’s Seventh Framework Programme (FP7/2007–2013)/ERC grant agreement No. 227224 ( prosperity ), as well as from the Research Council of KU Leuven grant agreement GOA/2008/04. J.H.T. cordially thanks the Instituut voor Sterrenkunde, KU Leuven, for its hospitality. -