The blue straggler V106 in NGC 6791: a prototype progenitor of old single giants masquerading as young. Issue 4 (18th September 2018) - Brogaard, K
ABSTRACT: 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 $M_{\rm p}\sim 1.67\, \rm M_{\odot }$, more massive than the cluster turn-off, with $R_{\rm p}\sim 1.91\, \rm R_{\odot }$ and T eff = 7110 ± 100 K. The secondary component is highly oversized and overluminous for its low mass with $M_{\rm s}\sim 0.182\, \rm M_{\odot }$, $R_{\rm s}\sim 0.864\, \rm R_{\odot }$, and T eff = 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 $40\, {\rm Myr}$ 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 $100\, {\rm Myr}$ 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.