Orbital properties of an unusually low-mass sdB star in a close binary system with a white dwarf

• Autores: Silvotti, R.; Østensen, R. H.; Bloemen, S.; Telting, J. H.; Heber, U.; Oreiro, R.; Reed, M. D.; Farris, L. E.; O’Toole, S. J.; Lanteri, L.; Degroote, P.; Hu, H.; Baran, A. S.; Hermes, J. J.; Althaus, Leandro Gabriel; Marsh, T. R.; Charpinet, S.; Li, J.; Morris, R. L.; Sanderfer, D. T.
• Año de publicación: 2012
• Idioma: inglés
• Tipo de recurso: artículo
• Materia: Close binaries; KIC 6614501 (estrella); Subdwarfs; White dwarfs
• Institución: Consejo Nacional de Investigaciones Científicas y Técnicas

We have used 605 days of photometric data from the Kepler spacecraft to study KIC 6614501, a close binary system with an orbital period of 0.157 497 47(25) days (3.779 939 h), that consists of a low-mass subdwarf B (sdB) star and a white dwarf (WD). As seen in many other similar systems, the gravitational field of the WD produces an ellipsoidal deformation of the sdB which appears in the light curve as a modulation at two times the orbital frequency. The ellipsoidal deformation of the sdB implies that the system has a maximum inclination of ∼40°, with i ≈ 20° being the most likely. The orbital radial velocity (RV) of the sdB star is high enough to produce a Doppler beaming effect with an amplitude of 432 ± 5 ppm, clearly visible in the folded light curve. The photometric amplitude that we obtain, K1 = 85.8 km s−1, is ∼12 per cent less than the spectroscopic RV amplitude of 97.2 ± 2.0 km s−1. The discrepancy is due to the photometric contamination from a close object at about 5 arcsec north-west of KIC 6614501, which is difficult to remove. The atmospheric parameters of the sdB star, Teff = 23 700 ± 500 K and log g = 5.70 ± 0.10, imply that it is a rare object below the extreme horizontal branch (EHB), similar to HD 188112. The comparison with different evolutionary tracks suggests a mass between ∼0.18 and ∼0.25 M⊙, too low to sustain core helium burning. If the mass was close to 0.18–0.19 M⊙, the star could be already on the final He-core WD cooling track. A higher mass, up to ∼0.25 M⊙, would be compatible with a He-core WD progenitor undergoing a cooling phase in a H-shell flash loop. A third possibility, with a mass between ∼0.32 and ∼0.40 M⊙, cannot be excluded and would imply that the sdB is a ‘normal’ (but with an unusually low mass) EHB star burning He in its core. In all these different scenarios, the system is expected to merge in less than 3.1 Gyr due to gravitational wave radiation.
Fil: Silvotti, R.. INAF. Osservatorio Astrofisico di Torino; Italia
Fil: Østensen, R. H. . Instituut voor Sterrenkunde; Bélgica
Fil: Bloemen, S.. Instituut voor Sterrenkunde; Bélgica
Fil: Telting, J. H.. Nordic Optical Telescope; España
Fil: Heber, U.. Dr. Karl Remeis-Observatory & ECAP, Astronomical Inst; Alemania
Fil: Oreiro, R.. Instituto de Astrofísica de Andalucía; España
Fil: Reed, M. D. . Anglo-Australian Observatory; Australia
Fil: Farris, L. E. . Missouri State University. Department of Physics, Astronomy and Materials Science; Estados Unidos
Fil: O’Toole, S. J. . Anglo-Australian Observatory; Australia
Fil: Lanteri, L.. INAF. Osservatorio Astrofisico di Torino; Italia
Fil: Degroote, P.. Instituut voor Sterrenkunde; Bélgica
Fil: Hu, H.. The Observatories. Institute of Astronomy; Reino Unido
Fil: Baran, A. S. . Missouri State University. Department of Physics, Astronomy and Materials Science; Estados Unidos
Fil: Hermes, J. J. . University of Texas at Austin; Estados Unidos
Fil: Althaus, Leandro Gabriel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico La Plata. Instituto de Astrofísica de La Plata; Argentina. Universidad Nacional de la Plata. Facultad de Ciencias Astronómicas y Geofísicas; Argentina
Fil: Marsh, T. R. . University Of Warwick; Reino Unido
Fil: Charpinet, S.. Université de Toulouse. Laboratoire d’Astrophysique de Toulouse-Tarbes; Francia
Fil: Li, J.. National Aeronautics And Space Administration; Estados Unidos
Fil: Morris, R. L. . National Aeronautics And Space Administration; Estados Unidos
Fil: Sanderfer, D. T. . National Aeronautics And Space Administration; Estados Unidos