A Runaway Red Supergiant in M31

TL;DR

This paper reports the discovery of a runaway red supergiant star in M31 with a high peculiar velocity, providing insights into stellar dynamics and evolution of massive stars outside the Milky Way.

Contribution

The first identification of a runaway red supergiant in M31 based on its velocity discrepancy, expanding understanding of evolved star kinematics in external galaxies.

Findings

Star has a velocity discrepancy of 300 km/s with M31's kinematics.

Star's velocity suggests it may escape M31's disk.

Star's properties are consistent with a 12-15 solar mass RSG.

Abstract

A significant percentage of OB stars are runaways, so we should expect a similar percentage of their evolved descendants to also be runaways. However, recognizing such stars presents its own set of challenges, as these older, more evolved stars will have drifted further from their birthplace, and thus their velocities might not be obviously peculiar. Several Galactic red supergiants (RSGs) have been described as likely runaways, based upon the existence of bow shocks, including Betelgeuse. Here we announce the discovery of a runaway RSG in M31, based upon a 300 km s$^{-1}$ discrepancy with M31's kinematics. The star is found about 21' (4.6 kpc) from the plane of the disk, but this separation is consistent with its velocity and likely age ($\sim$10 Myr). The star, J004330.06+405258.4, is an M2 I, with $M_V=-5.7$, $\log L/L_\odot$=4.76, an effective temperature of 3700 K, and an inferred mass of 12-15$M_\odot$. The star may be a high-mass analog of the hypervelocity stars, given that its peculiar space velocity is probably 400-450 km s$^{-1}$, comparable to the escape speed from M31's disk.