ON THE NEED FOR DEEP-MIXING IN ASYMPTOTIC GIANT BRANCH STARS OF LOW MASS

Abstract

The photospheres of low-mass red giants show CNO isotopic abundances that are\nnot satisfactorily accounted for by canonical stellar models. The same is true\nfor the measurements of these isotopes and of the $^{26}$Al/$^{27}$Al ratio in\npresolar grains of circumstellar origin. Non-convective mixing, occurring\nduring both Red Giant Branch (RGB) and Asymptotic Giant Branch (AGB) stages is\nthe explanation commonly invoked to account for the above evidence. Recently,\nthe need for such mixing phenomena on the AGB was questioned, and chemical\nanomalies usually attributed to them were suggested to be formed in earlier\nphases. We have therefore re-calculated extra-mixing effects in low mass stars\nfor both the RGB and AGB stages, in order to verify the above claims. Our\nresults contradict them; we actually confirm that slow transport below the\nconvective envelope occurs also on the AGB. This is required primarily by the\noxygen isotopic mix and the $^{26}$Al content of presolar oxide grains. Other\npieces of evidence exist, in particular from the isotopic ratios of carbon\nstars of type N, or C(N), in the Galaxy and in the LMC, as well as of SiC\ngrains of AGB origin. We further show that, when extra-mixing occurs in the RGB\nphases of population I stars above about 1.2 $M_{\\odot}$, this consumes $^3$He\nin the envelope, probably preventing the occurrence of thermohaline diffusion\non the AGB. Therefore, we argue that other extra-mixing mechanisms should be\nactive in those final evolutionary phases.\n