Constraints on the Evolution of an Overluminous Blue Supergiant from its Parent Open Cluster, vdBH 245

Abstract In this paper, we constrain the properties of an overluminous blue supergiant star (BSG) in the open cluster vdBH 245. We confirm the cluster membership of a sample of early-to-mid B-type stars using their proper motions, parallaxes, and radial velocities. We measure the stellar parameters through a spectrophotometric analysis of their optical spectra and near-infrared photometry. We correct for the effects of extinction of the cluster’s light and estimate a most likely age range for vdBH 245 of 31.6 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow/> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>13.8</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>13.1</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> Myr using the Binary Population and Spectral Synthesis (or BPASS) Python software hoki and models from the Geneva group. We find a distance of 2.0 ± 0.2 kpc for the cluster, consistent with the value estimated by Gaia of 2.6 ± 0.5 kpc. However, the new age estimate does not agree with previous studies that have found much younger ages. We find that single-star evolution alone is unable to explain the BSG in that age range, even when considering the impact of rotation on stellar evolution. Instead, we find that the overluminous B8–B9 Ia BSG could be interpreted as a merger between two stars with initial masses ranging from 8.5–12.0 M ⊙ and 1.1–10.8 M ⊙ , with a current mass in the range of 13.4–20.6 M ⊙ . The merger would have occurred somewhere between 5.0 and 21.0 Myr after the cluster’s formation. The discovery of such a BSG appearing too young (≈10 Myr in the single-star paradigm) for its population hints at the importance of considering binary interactions to explain the observed population of Galactic BSGs.