The Transformative Journey of HD 93521

TL;DR

This paper re-evaluates the age and origin of the star HD 93521, suggesting it is a merger product ejected from the Galactic disk, based on new kinematic and evolutionary analyses.

Contribution

It provides a revised timescale analysis and proposes a novel origin scenario for HD 93521 as a stellar merger ejected from the Galactic disk.

Findings

Kinematic age of 39 +/- 3 Myr from Gaia data

Evolutionary age of about 5 +/- 2 Myr from stellar models

HD 93521 likely formed from a binary merger scenario

Abstract

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.