The blue straggler V106 in NGC6791: A prototype progenitor of old single 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, Aaron M Geller, M. Shetrone, N. Ryde, D. Stello, I. PlataisS. Meibom

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

6 Scopus citations


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.

Original languageEnglish (US)
Pages (from-to)5062-5072
Number of pages11
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
StatePublished - Dec 1 2018


  • Binaries: close
  • Blue stragglers
  • Stars: fundamental parameters
  • Stars: individual: V106
  • White dwarfs

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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