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Hydrodynamical simulations of stellar interactions require stable models ofstars as initial conditions. Such initial models, however, are difficult toconstruct for giant stars because of the wide range in spatial scales of thehydrostatic equilibrium and in dynamical timescales between the core and theenvelope of the giant. They are needed for, e.g., modeling the common envelopephase where a giant envelope encompasses both the giant core and a companionstar. Here, we present a new method of approximating and reconstructing giantprofiles from a stellar evolution code to produce stable models formulti-dimensional hydrodynamical simulations. We determine typical stellarstratification profiles with the 1D stellar evolution code MESA. After anappropriate mapping, hydrodynamical simulations are conducted using themoving-mesh code AREPO. The giant profiles are approximated by replacing thecore of the giant with a point mass and by constructing a suitable continuationof the profile to the center. Different reconstruction methods are tested thatcan specifically control the convective behaviour of the model. After mappingto a grid, a relaxation procedure that includes damping of spurious velocitiesyields stable models in three-dimensional hydrodynamical simulations. Initiallyconvectively stable configurations lead to stable hydrodynamical models whilefor stratifications that are convectively unstable in the stellar evolutioncode, simulations recover the convective behaviour of the initial model andshow large convective plumes with Mach numbers up to 0.8. Examples are shownfor a $2M_\odot$ red giant and a $0.67M_\odot$ asymptotic giant branch star. Adetailed analysis shows that the improved method reliably provides stablemodels of giant envelopes that can be used as initial conditions for subsequenthydrodynamical simulations of stellar interactions involving giant stars.

Constructing stable 3D hydrodynamical models of giant stars