The blue straggler V106 in NGC 6791: a prototype progenitor of old single giants masquerading as young

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  • K. Brogaard
  • S. M. Christiansen
  • ,
  • F. Grundahl
  • A. Miglio, School of Physics and Astronomy, University of Birmingham, Edgbaston Park Road, West Midlands, Birmingham, B15 2TT UK 0000-0001-5998-8533
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  • R. G. Izzard, Astrophysics Research Group, Faculty of Engineering and Physical Sciences, University of Surrey, Guildford GU2 7XH, United Kingdom 0000-0003-0378-4843
  • ,
  • T. M. Tauris
  • E. L. Sandquist, Department of Astronomy, San Diego State University, San Diego, CA 92182, USA
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  • D. A. VandenBerg, Department of Physics and Astronomy, University of Victoria, PO Box 1700 STN CSC, Victoria, B.C., V8W 2Y2, Canada
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  • J. Jessen-Hansen
  • T. Arentoft
  • H. Bruntt
  • ,
  • S. Frandsen
  • J. A. Orosz, Department of Astronomy, San Diego State University, San Diego, CA 92182, USA
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  • G. A. Feiden, Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden 0000-0002-2012-7215
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  • R. Mathieu, Department of Astronomy, University of Wisconsin-Madison, Madison, WI 53706, USA
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  • A. Geller, Department of Astronomy, University of Wisconsin-Madison, Madison, WI 53706, USA
  • ,
  • M. Shetrone, McDonald Observatory, University of Texas, HC75 Box 1337-L Fort Davis, TX 79734, USA
  • ,
  • N. Ryde, Department of Astronomy and Theoretical Physics, Lund Observatory, Lund University, Box 43, SE-221 00 Lund, Sweden
  • ,
  • D. Stello
  • I. Platais, Center for Astrophysical Sciences, Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
  • ,
  • S. Meibom, Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
We determine the properties of the binary star V106 in the old open cluster NGC 6791 . We identify the system to be a blue straggler cluster member by using a combination of ground-based and Kepler photometry and multi-epoch spectroscopy. The properties of the primary component are found to be M_p˜ 1.67 M_{⊙ }, more massive than the cluster turn-off, with R_p˜ 1.91 R_{⊙ } and Teff = 7110 ± 100 K. The secondary component is highly oversized and overluminous for its low mass with M_s˜ 0.182 M_{⊙ }, R_s˜ 0.864 R_{⊙ }, and Teff = 6875 ± 200 K. We identify this secondary star as a bloated (proto) extremely low-mass helium white dwarf. These properties of V106 suggest that it represents a typical Algol-paradox system and that it evolved through a mass-transfer phase, which provides insight into its past evolution. We present a detailed binary stellar evolution model for the formation of V106 using the MESA code and find that the mass-transfer phase only ceased about 40 Myr ago. Due to the short orbital period (P = 1.4463 d), another mass-transfer phase is unavoidable once the current primary star evolves towards the red giant phase. We argue that V106 will evolve through a common-envelope phase within the next 100 Myr and merge to become a single overmassive giant. The high mass will make it appear young for its true age, which is revealed by the cluster properties. Therefore, V106 is potentially a prototype progenitor of old field giants masquerading as young.
Original languageEnglish
JournalMonthly Notices of the Royal Astronomical Society
Pages (from-to)5062-5072
Publication statusPublished - 1 Dec 2018

    Research areas

  • binaries: close, blue stragglers, stars: fundamental parameters, stars: individual: V106, white dwarfs

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