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It is shown that the process called radiative diffusivity by Morel & Thevenin (2002, 390, 611) does not exist as they describe it. Their description is based on a confusion between atomic diffusion and turbulent transport (often called turbulent diffusion in the context of particle transport in stars). We evaluate how ions are slowed down by photons and show that this may be neglected in atomic diffusion calculations. In their evolutionary model calculations, Morel & Thevenin (2002) made the ad hoc hypothesis that turbulent diffusion was proportional to radiative viscosity.

About diffusivity, radiative viscosity and particle transport