2017MNRAS.471.1888F

We report the discovery of HR 5455 as an ancient field blue straggler that upon Roche lobe overflow from its evolved companion, now a white dwarf, has been rejuvenated to a M_BS = 1.32 ± 0.05 M_☉ bright F6V source with a purported age of τ ≃ 1.7 Gyr. The unequivocal evidence for HR 5455 being instead a τ ≃ 12 Gyr old Population II star arises from its low iron-to-magnesium abundance ratio, and is supported by its kinematics, the lack of lithium and a companion mass M_WD = 0.49 ± 0.09 M_☉ from the P = 1086 ± 77 d Hipparcos astrometric orbit, which is in perfect agreement with the Rappaport et al. period-white dwarf mass relation. Like the F7V star 49 Lib that we recently disclosed as an ancient field blue straggler in a similar context, HR 5455 is also a very metal-rich source at [Mg/H] = +0.19 and [Fe/H] = -0.10. Unlike 49 Lib, however, only the white dwarf progenitor mass is directly constrained to M_{WD,progenitor} = 1.06 ± 0.02 M_☉. In case of a conservative mass transfer, which in analogy to the 49 Lib system appears likely, a blue straggler progenitor mass M_{BS,progenitor} = 0.75 M_☉ would follow.