Improvements to stellar structure models, based on a grid of 3D convection simulations – II. Calibrating the mixing-length formulation

Abstract

We perform a calibration of the mixing length of convection in stellar\nstructure models against realistic 3D radiation-coupled hydrodynamics (RHD)\nsimulations of convection in stellar surface layers, determining the adiabat\ndeep in convective stellar envelopes.\n The mixing-length parameter $\\alpha$ is calibrated by matching averages of\nthe 3D simulations to 1D stellar envelope models, ensuring identical atomic\nphysics in the two cases. This is done for a previously published grid of\nsolar-metallicity convection simulations, covering from 4200 K to 6900 K on the\nmain sequence, and 4300-5000 K for giants with logg=2.2.\n Our calibration results in an $\\alpha$ varying from 1.6 for the warmest\ndwarf, which is just cool enough to admit a convective envelope, and up to 2.05\nfor the coolest dwarfs in our grid. In between these is a triangular plateau of\n$\\alpha$ ~ 1.76. The Sun is located on this plateau and has seen little change\nduring its evolution so far. When stars ascend the giant branch, they largely\ndo so along tracks of constant $\\alpha$, with $\\alpha$ decreasing with\nincreasing mass.\n