2025ApJ...985..180L

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${}_{-13.8}^{+13.1}$ 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.