The blue straggler V106 in NGC6791: A prototype progenitor of old single giants masquerading as young

Research output: Contribution to journalArticle


We determine the properties of the binary star V106 in the old open cluster NGC6791. We identify the system to be a blue straggler cluster member by using a combination of groundbased and Kepler photometry and multi-epoch spectroscopy. The properties of the primary component are found to be Mp ~ 1.67M, more massive than the cluster turn-off, with Rp ~ 1.91R and Teff = 7110 ± 100 K. The secondary component is highly oversized and overluminous for its low mass with Ms ~ 0.182M, R ~ 0.864R, and T =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.


  • K. Brogaard
  • S. M. Christiansen
  • F. Grundahl
  • A. Miglio
  • R. G. Izzard
  • T. M. Tauris
  • E. L. Sandquist
  • D. A. Vanden Berg
  • J. Jessen-Hansen
  • T. Arentoft
  • H. Bruntt
  • S. Frandsen
  • J. A. Orosz
  • G. A. Feiden
  • R. Mathieu
  • A. Geller
  • M. Shetrone
  • D. Stello
  • I. Platais
  • S. Meibom
External organisations
  • Aarhus University
  • University of Birmingham
  • University of Surrey
  • University of Bonn
  • Max Planck Institute for Radio Astronomy
  • San Diego State University
  • University of Victoria
  • Uppsala University
  • University of Wisconsin-Madison
  • University of Texas System
  • University of Sydney
  • University of New South Wales
  • Johns Hopkins University
  • Harvard–Smithsonian Center for Astrophysics
Research areas and keywords

Subject classification (UKÄ) – MANDATORY

  • Astronomy, Astrophysics and Cosmology


  • Binaries: close, Blue stragglers, Stars: fundamental parameters, Stars: individual: V106, White dwarfs
Original languageEnglish
Pages (from-to)5062-5072
Number of pages11
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
Publication statusPublished - 2018 Dec 1
Publication categoryResearch