We explore the possibility that the star alpha Orionis (Betelgeuse) is the
outcome of a merger that occurred in a low mass ratio (q = M2/M1 = 0.07 - 0.25)
binary system some time in the past hundreds of thousands of years. To that
goal, we present a simple analytical model to approximate the perturbed
internal structure of a post-merger object following the coalescence of a
secondary in the mass range 1-4 Msun into the envelope of a 15-17 Msun primary.
We then compute the long-term evolution of post-merger objects for a grid of
initial conditions and make predictions about their surface properties for
evolutionary stages that are consistent with the observed location of
Betelgeuse in the Hertzsprung-Russell diagram. We find that if a merger
occurred after the end of the primary's main-sequence phase, while it was
expanding toward becoming a red supergiant star and typically with radius ~200
- 300 Rsun, then it's envelope is spun-up to values which remain in a range
consistent with the Betelgeuse observations for thousands of years of
evolution. We argue that the best scenario that can explain both the fast
rotation of Betelgeuse and its observed large space velocity is one where a
binary was dynamically ejected by its parent cluster a few million years ago
and then subsequently merged. An alternative scenario in which the progenitor
of Betelgeuse was spun up by accretion in a binary and released by the
supernova explosion of the companion requires a finely tuned set of conditions
but cannot be ruled out.Comment: 20 pages, 8 figures, accepted for publication in the Astrophysical
Journa
