HD 93521 is a massive, rapidly rotating star that is located about 1 kpc above the Galactic disk, and the evolutionary age for its estimated mass is much less than the time-of-flight if it was ejected from the disk. Here we present a re-assessment of both the evolutionary and kinematical timescales for HD 93521. We calculate a time-of-flight of 39 +/- 3 Myr based upon the distance and proper motions from Gaia EDR3 and a summary of radial velocity measurements. We then determine the stellar luminosity using a rotational model combined with the observed spectral energy distribution and distance. A comparison with evolutionary tracks for rotating stars from Brott et al. yields an evolutionary age of about 5 +/- 2 Myr. We propose that the solution to the timescale discrepancy is that HD 93521 is a stellar merger product. It was probably ejected from the Galactic disk as a close binary system of lower mass stars that eventually merged to create the rapidly rotating and single massive star we observe today.
@article{arxiv.2112.08235,
title = {The Transformative Journey of HD 93521},
author = {Douglas R. Gies and Katherine Shepard and Peter Wysocki and Robert Klement},
journal= {arXiv preprint arXiv:2112.08235},
year = {2022}
}