Simulating the outer layers of Procyon A: a comparison with the Sun

We compare a new 3D radiation-hydrodynamical simulation of the surface layers of Procyon A to a similar 3D simulation of the surface layers of the Sun. Both simulations include realistic input physics and are performed using the same numerical techniques and computer codes. Convection in the surface layers of Procyon A is very different from the Sun. Compared to the Sun, the atmospheric structure and convective flow in Procyon A exhibit the following characteristics: (1) the highly superadiabatic transition layer (SAL) is located at much shallower optical depth; it is in a dynamically active region, and its outer region is located part of the time in the optically thin atmosphere; (2) the outer region of the SAL moves from an optically thin region to thick region and back again over a time of 20-30 minutes. This motion, which is driven by the granulation, takes place in a time approximately half the turnover time of the largest granules; (3) the peak root mean square velocity in the vertical direction is much larger in Procyon A. The main reason for the radically different radiative-convective behaviour in Procyon A compared to the Sun is the role played by turbulent eddies in determining the overall flow/thermal structure. The turbulent pressure and turbulent kinetic energy can exceed 50 % of the local gas pressure (compared