The destruction of ³He by Rayleigh-Taylor instability on the first giant branch

Abstract

Abstract. Low-mass stars, ∼ 1 − 2 solar masses, near the Main Sequence are efficient at producing 3 He, which they mix into the convective envelope on the giant branch and distribute into the Galaxy by way of envelope loss. This process is so efficient that it is difficult to reconcile the observed cosmic abundance of 3 He with the predictions of Big Bang nucleosynthesis. In this paper we find, by modeling a red giant with a fully three-dimensional hydrodynamic code and a full nucleosynthetic network, that mixing arises in the supposedly stable and radiative zone between the hydrogen-burning shell and the base of the convective envelope. This mixing is due to Rayleigh-Taylor instability within a zone just above the hydrogen-burning shell. In this zone the burning of the 3 He left behind by the retreating convective envelope is predominantly by the reaction 3 He + 3 He → 4 He + 1 H + 1 H, a reaction which, untypically for stellar nuclear reactions, lowers the mean molecular weight, leading to a local minimum. This local minimum leads to Rayleigh-Taylor instability, and turbulent motion is generated which will continue ultimately up into the normal convective envelope. Consequently material from the envelope is dragged down sufficiently close to the burning shell that the 3 He in it is progressively destroyed. Thus we are able to remove the threat that 3 He production in low-mass stars poses to the Big Bang nucleosynthesis of 3 He. Some slow mixing mechanism has long been suspected, that connects the convective envelope of a red giant to the burning shell. It appears to be necessary to account for progressive changes in the 12 C/ 13 C and 14 N/ 12 C ratios on the First Giant Branch. We suggest that these phenomena are also due to the Rayleigh-Taylor-unstable character of the 3 He-burning region.

Intrinsic iron spread and a new metallicity scale for globular clusters

“…Send offprint requests to: E. Carretta, eugenio.carretta@oabo.inaf.it ⋆ Based on observations collected at ESO telescopes under programmes 072.D-507 and 073.D-0211 to also come with differences in the main outcome of the H burning, the He content (Gratton et al 2009;Bragaglia et al in preparation;Prantzos and Charbonnel 2006;Ventura et al 2001). Apart from a few alterations presently well understood in term of an extra-mixing episode after the red giant branch (RGB) bump (Charbonnel 1994(Charbonnel , 1995Charbonnel and Zahn 2007;Eggleton et al 2007) the abundance variations are inherited by currently observed, long lived GC stars from a previous stellar component/generation: both spectroscopic (Gratton et al 2001;Ramirez and Cohen 2002;Carretta et al 2004;Piotto et al 2005) and photometric (e.g. Bedin et al 2004) observations convincingly showed that the observed pattern of chemical composition is present also among unevolved stars on the subgiant branch and the main sequence.…”