  In order to constrain the evolutionary state of the red supergiant
Betelgeuse, we have produced a suite of models with ZAMS masses from 15 to 25
Msun in intervals of 1 Msun including the effects of rotation. The models were
computed with the stellar evolutionary code MESA. For non-rotating models we
find results that are similar to other work. It is somewhat difficult to find
models that agree within 1 sigma of the observed values of R, Teff and L, but
modestly easy within 3 sigma uncertainty. Incorporating the nominal observed
rotational velocity, ~15 km/s, yields significantly different, and challenging,
constraints. This velocity constraint is only matched when the models first
approach the base of the red supergiant branch (RSB), having crossed the
Hertzsprung gap, but not yet having ascended the RSB and most violate even
generous error bars on R, Teff and L. Models at the tip of the RSB typically
rotate at only ~0.1 km/s, independent of any reasonable choice of initial
rotation. We discuss the possible uncertainties in our modeling and the
observations, including the distance to Betelgeuse, the rotation velocity, and
model parameters. We summarize various options to account for the rotational
velocity and suggest that one possibility is that Betelgeuse merged with a
companion star of about 1 Msun as it ascended the RSB, in the process producing
the ring structure observed at about 7' away. A past coalescence would
complicate attempts to understand the evolutionary history and future of
Betelgeuse.
