  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.
