Brogaard, K., Christiansen, S. M., Grundahl, F., Miglio, A., Izzard, R. G., Tauris, T. M., Sandquist, E. L., VandenBerg, D. A., Jessen-Hansen, J., Arentoft, T., Bruntt, H., Frandsen, S., Orosz, J. A., Feiden, G. A., Mathieu, R., Geller, A., Shetrone, M., Ryde, N., Stello, D., Platais, I. and Meibom, Søren
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_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.
