  We investigate the merger between a 16 solar mass star, on its way to
becoming a red supergiant (RSG), and a 4 solar mass main-sequence companion.
Our study employs three-dimensional hydrodynamic simulations using the
state-of-the-art adaptive mesh refinement code Octo-Tiger. The initially
corotating binary undergoes interaction and mass transfer, resulting in the
accumulation of mass around the companion and its subsequent loss through the
second Lagrangian point (L2). The companion eventually plunges into the
envelope of the primary, leading to its spin-up and subsequent merger with the
helium core. We examine the internal structural properties of the post-merger
star, as well as the merger environment and the outflow driven by the merger.
Our findings reveal the ejection of approximately 0.6 solar mass of material in
an asymmetric and somewhat bipolar outflow. We import the post-merger stellar
structure into the MESA stellar evolution code to model its long-term nuclear
evolution. In certain cases, the post-merger star exhibits persistent rapid
equatorial surface rotation as it evolves in the H-R diagram towards the
observed location of Betelgeuse. These cases demonstrate surface rotation
velocities of a similar magnitude to those observed in Betelgeuse, along with a
chemical composition resembling that of Betelgeuse. In other cases, efficient
rotationally-induced mixing leads to slower surface rotation. This pioneering
study aims to model stellar mergers across critical timescales, encompassing
dynamical, thermal, and nuclear evolutionary stages.

Solar and Stellar Astrophysics