O. Trippella

S-PROCESSING in AGB STARS REVISITED. II. ENHANCED 13C PRODUCTION THROUGH MHD-INDUCED MIXING

Slow neutron captures are responsible for the production of about 50% of elements heavier than iron, mainly occurring during the asymptotic giant branch phase of low-mass stars (1 M/M⊙ 3), where the main neutron source is the 13C(α, n)16O reaction. This last reaction is activated from locally produced 13C, formed by partial mixing of hydrogen into the He-rich layers. We present here the first attempt to describe a physical mechanism for the formation of the 13C reservoir, studying the mass circulation induced by magnetic buoyancy without adding new free parameters to those already involved in stellar modeling. Our approach represents the application to the stellar layers relevant for s-processing of recent exact analytical 2D and 3D models for magneto-hydrodynamic processes at the base of convective envelopes in evolved stars in order to promote downflows of envelope material for mass conservation during the occurrence of a dredge-up phenomenon. We find that the proton penetration is characterized by small concentrations, but is extended over a large fractional mass of the He-layers, thus producing 13C reservoirs of several 10−3 M⊙. The ensuing 13C-enriched zone has an almost flat profile, while only a limited production of 14N occurs. In order to verify the effects of our new findings we show how the abundances of the main s-component nuclei can be accounted for in solar proportions and how our large 13C-reservoir allows us to solve a few so far unexplained features in the abundance distribution of post-AGB objects.

Concurrent application of ANC and THM to assess the

The reaction is considered to be the main neutron source responsible for the production of heavy nuclides (from to ) through slow n-capture nucleosynthesis (s-process) at low temperatures during the asymptotic giant branch phase of low-mass stars (, or LMSs). In recent years, several direct and indirect measurements have been carried out to determine the cross section at the energies of astrophysical interest (around ). However, they yield inconsistent results that cause a highly uncertain reaction rate and affect the neutron release in LMSs. In this work we have combined two indirect approaches, the asymptotic normalization coefficient and the Trojan horse method, to unambiguously determine the absolute value of the astrophysical factor. With these, we have determined a very accurate reaction rate to be introduced into astrophysical models of s-process nucleosynthesis in LMSs. Calculations using this recommended rate have shown limited variations in the production of those neutron-rich nuclei (with ) that receive contribution only by slow neutron captures.

^{13}{\rm C}(\alpha,n)^{16}{\rm O}

It has been known for decades that s-process elements from Sr to Pb are produced by Asymptotic Giant Branch stars. However only recently, physically-based mixing mechanisms for the formation of \(^{13}\)C have been proposed. Among them, we aim to verify the robustness of the model of a MHD induced \(^{13}\)C-pocket formation. In doing that we present results of nucleosynthesis models for low mass AGB stars, developed from the MHD scenario, compared with the isotopic abundance ratios of s-elements from presolar Mainstream SiC grains.

absolute cross section at astrophysical energies and possible consequences for neutron production in low-mass AGB stars

We present here the application of a model for a mass circulation mechanism in between the H-burning shell and the base of the convective envelope of low mass AGB stars, aimed at studying the isotopic composition of those presolar grains showing the most extreme levels of 18O depletion and high concentration of 26Mg from the decay of 26Al. The mixing scheme we present is based on a previously suggested magnetic-buoyancy process, already shown to account adequately for the formation of the main neutron source for slow neutron captures in AGB stars. We find that this scenario is also capable of reproducing for the first time the extreme values of the 17O/16O, the 18O/16O, and the 26Al/27Al isotopic ratios found in the mentioned oxide grains, including the highest amounts of 26Al there measured.

s -Processing from MHD-induced mixing and isotopic abundances in presolar SiC grains

I will present a theoretical sensitivity study, carried out with the FUNS evolutionary stellar code, to evaluate the effects induced on the s-process nucleosynthesis by variations of the \(^{13}\)C(\(\alpha \),n)\(^{16}\)O cross section. Some peculiar evolutionary phases, particularly sensitive to this rate, will be discussed in detail.

A Deep Mixing Solution to the Aluminum and Oxygen Isotope Puzzles in Presolar Grains

The present scenario of the s process in AGB stars emphasizes the role of the neutrons released from the 13C(α ,n)16O reaction. This in its turn derives from a local production of 13C in Herich layers, after a penetration of protons from the envelope, whose mechanisms have been so far elusive. We speculate that magnetic buoyancy might be at the origin of the mixing process and present a simple model according to which the mixing (hence the extension of the 13C-rich layer) should involve most of the He-rich buffer above the CO core, i.e. a zone much larger than previously envisaged. This implies a large production of s-elements in the Galaxy, as is indeed suggested by recent observations in open clusters

The Importance of the

Recent improvements in stellar models for intermediate-mass and massive stars are recalled, together with their expectations for the synthesis of radioactive nuclei of lifetime

^13

\tau \lesssim 25

C(

Myr, in order to re-examine the origins of now extinct radioactivities, which were alive in the solar nebula. The Galactic inheritance broadly explains most of them, especially if

\alpha

r