The Betelgeuse Project III: Merger Characteristics

TL;DR

This study models the merger of a primary star with a secondary companion to explain Betelgeuse's observed rotation, suggesting such mergers can produce the star's sub-Keplerian equatorial velocity.

Contribution

It provides detailed stellar evolution simulations showing how mergers can account for Betelgeuse's rotation without significantly affecting its core.

Findings

Merger models reproduce Betelgeuse's equatorial velocity within a factor of three.

Mergers of less than 15 solar masses with 1-10 solar mass companions can explain observed rotation.

Inner core remains unaffected by accretion, preserving magnetic field production potential.

Abstract

We previously proposed that Betelgeuse might have been spun up by accreting a companion of about 1 solar mass. Here we explore in more detail the possible systematics of such a merger and a larger range of accreted masses. We use the stellar evolutionary code MESA to add angular momentum to a primary star in core helium burning, core carbon burning, or shell carbon burning. Our models provide a reasonable "natural" explanation for why Betelgeuse has a large, but sub-Keplerian equatorial velocity. They eject sufficient mass and angular momentum in rotationally-induced mass loss to reproduce the observed ratio of the equatorial velocity to escape velocity of Betelgeuse, ~0.23, within a factor of three nearly independent of the primary mass, the secondary mass, and the epoch at which merger occurs. Our models suggest that merger of a primary of somewhat less than 15 solar masses with secondaries of from 1 to 10 solar masses during core helium burning or core carbon burning could yield the equatorial rotational velocity of ~15 km/s attributed to Betelgeuse. For accreting models, a wave of angular momentum is halted at the composition boundary at the edge of the helium core. The inner core is thus not affected by the accretion of the companion in these simulations. Accretion has relatively little effect on the production of magnetic fields in the inner core. Our results do not prove, but do not negate that Betelgeuse might have ingested a companion of several solar masses.