The Transformative Journey of HD 93521
  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.
