The Art of Modeling Stellar Mergers and the Case of the B[e] Supergiant
  R4 in the Small Magellanic Cloud
  Most massive stars exchange mass with a companion, leading to evolution which
is altered drastically from that expected of stars in isolation. Such systems
are the result of unusual binary evolution pathways and, as such, may be used
to place stringent constraints on the physics of these interactions. We use the
R4 system's B[e] supergiant, which has been postulated to be the product of a
binary stellar merger, to guide our understanding of such outcomes by comparing
observations of R4 to the results of simulations of mergers performed with the
3d hydrodynamics code FLASH. Our approach tailors the simulation initial
conditions to the observed properties of R4 and implements realistic stellar
profiles generated by the 1d stellar evolution code MESA onto the 3d grid,
resolving the merger inspiral to within $0.02\, R_{\odot}$. We then map the
merger remnant into MESA to track its evolution on the HR diagram over a period
of $10^4$ years. This generates models for a B[e] supergiant with stellar
properties, age, and nebula structure in qualitative agreement with that of the
R4 system. Our calculations provide concrete evidence to support the idea that
R4 was originally a member of a triple system in which the inner binary merged
after its most massive member evolved off the main sequence, producing a new
object that is of similar mass yet significantly more luminous than the A
supergiant companion. The potential applications of the code framework
presented in this paper are wide ranging and can be used to generate models of
a variety of merger stellar remnants.
