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

Research output: Contribution to journalArticlepeer-review


  • S M Christiansen
  • F Grundahl
  • R G Izzard
  • T M Tauris
  • E L Sandquist
  • D A Vandenberg
  • J Jessen-hansen
  • T Arentoft
  • H Bruntt
  • S Frandsen
  • J A Orosz
  • G A Feiden
  • R Mathieu
  • A Geller
  • M Shetrone
  • N Ryde
  • D Stello
  • I Platais
  • S Meibom

Colleges, School and Institutes

External organisations

  • Aarhus Universitet
  • University of Birmingham
  • University of Surrey
  • Department of Astronomy, San Diego State University, San Diego, CA 92182, USA
  • University of Victoria
  • University of Uppsala
  • University of Wisconsin-Madison
  • University of Texas, McDonald Observatory, HC75 Box 1337-L Fort Davis, TX 79734, USA
  • Lunds Universitet
  • University of New South Wales
  • Johns Hopkins University
  • Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
  • Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany
  • Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
  • University of Sydney


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 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, Rs∼0.864R 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 over-massive 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
Early online date18 Sep 2018
Publication statusE-pub ahead of print - 18 Sep 2018


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