Atomic diffusion during red giant evolution

¹ LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, France
² Département de Physique, Université de Montréal, Montréal, PQ, H3C 3J7, Canada e-mail: michaudg@astro.umontreal.ca,jacques.richer@umontreal.ca
³ Université Montpellier II - GRAAL, CNRS - UMR 5024, place Eugène Bataillon, 34095 Montpellier, France e-mail: Olivier.Richard@graal.univ-montp2.fr

Received: 14 April 2009
Accepted: 15 December 2009

Abstract

Atomic diffusion has been found to play a role during most stellar evolution stages. Its effect is studied during the relatively rapid red giant (RG) phase to determine the concentration variations it leads to and at what accuracy level it can be safely neglected. A model calculated with atomic diffusion to the helium flash is compared to one calculated without any atomic diffusion and to one calculated with atomic diffusion up to a point on the subgiant branch well past the turnoff but without diffusion thereafter. For stars with a metallicity of , it was found that the mass of the helium core at which the He flash occurs is 0.0026 M larger in the presence of atomic diffusion. The difference decreases to 0.0017 M as metallicity is increased to Z = 0.02. Radiative accelerations are found to play an interesting role around the hydrogen burning shell. The atomic diffusion of ⁴He is also shown to lead to a larger μ inversion than ³He burning. Its potential role in mixing between the burning shell and the surface convection zone is investigated.
Whether one may neglect atomic diffusion during the RG phase depends on the required accuracy. It is not so negligible as one may have expected but still only reduces by about 0.02 dex the luminosity of the RG branch bump. The way it modifies the mass of the core when the flash occurs depends on metallicity.