On the ancient field blue straggler HR 5455

Abstract

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.