I outline a few features of recent models for the formation of the neutron source
13
C(
α
,n)
16
O in low mass stars (1 ≲ M/M
⊙
≲ 3, LMS ) ascendingfor the second time the Red Giant Branch, generally ...called
Asymptotic Giant Branch
, or
AGB
stars. I also briefly outline the nucleosynthesis results obtained trough them. The mentioned models consider the physical structure below the frequent downward extensions of the convective envelope into the He-intershell (the so-called
third dredge-up
or
TDU
episodes). There, the conditions are such that the occurrence of further mixing is strongly facilitated, due to the minimal temperature gradient. A way to induce proton mixing from the envelope (certainly not the only one) arises whenever the ambient magnetic fields expected for LMS promote the buoyancy of strongly magnetized flux tubes. I review some characteristics of the ensuing mixing episodes, mentioning how different hydrodynamical processes might yield similar effects, thus encouraging stellar physicists to verify in more detail this possibility.
Asymptotic giant branch (AGB) stars are thought to be among the most important sources of fluorine in our Galaxy. While observations and theory agree at close-to-solar metallicity, stellar models ...overestimate fluorine production in comparison to heavy elements at lower metallicities. We present predictions for
19
F abundance for a set of AGB models with various masses and metallicities, in which magnetic buoyancy induces the formation of the
13
C neutron source (the so-called
13
C pocket). In our new models, fluorine is mostly created as a consequence of secondary
14
N nucleosynthesis during convective thermal pulses, with a minor contribution from the
14
N existing in the
13
C pocket zone. As a result, AGB stellar models with magnetic-buoyancyinduced mixing show low
19
F surface abundances which agree with fluorine spectroscopic observations at both low and near-solar metallicity.
The isotopic abundances in presolar SiC grains of AGB origin provide important and precise constraints to those star nucleosynthesis models. By comparing the values of the s-element abundances ...resulting from calculations with the ones measured in these dust grains, it turns out that new measurements of weak-interaction rates in ionized plasmas, as well as of neutron-capture cross sections, are needed, especially in the region near the neutron magic numbers 50 and 82.
Presolar grains and their isotopic compositions provide valuable constraints to AGB star nucleosynthesis. However, there is a sample of O- and Al-rich dust, known as group 2 oxide grains, whose ...origin is difficult to address. On the one hand, the 17O/16O isotopic ratios shown by those grains are similar to the ones observed in low-mass red giant stars. On the other hand, their large 18O depletion and 26Al enrichment are challenging to account for. Two different classes of AGB stars have been proposed as progenitors of this kind of stellar dust: intermediate mass AGBs with hot bottom burning, or low mass AGBs where deep mixing is at play. Our models of low-mass AGB stars with a bottom-up deep mixing are shown to be likely progenitors of group 2 grains, reproducing together the 17O/16O, 18O/16O and 26Al/27Al values found in those grains and being less sensitive to nuclear physics inputs than our intermediate-mass models with hot bottom burning.
Starting from the recognition that radioactive isotopes were present alive in the Early Solar System, inducing composition anomalies from their decay, and through the discovery that other important ...anomalies affected also stable species, we shall discuss how the carriers of these abundance peculiarities were identified in very refractory pre-solar dust grains, formed in circumstellar environments. We shall outline how groups of such grains and subsequently in-dividual single crystals of C-rich or O-rich materials (like, e.g., SiC and Al
2
O
3
) could be analyzed, providing a new tool to verify the composition of stellar winds. This is so especially for AGB stars, which are the primary factories of dust in the Galaxy. For this reason, pristine meteorites open a crucial window on the details of nucleosynthesis processes occurring in such evolved red giants, for both intermediate-mass elements and rare heavy nuclei affected by slow neutron captures (the s-process).
We outline a partial historical summary of the steps through which the nucleosynthesis phenomena induced by
slow
neutron captures (the
s-process
) were clarified, a scientific achievement in which ...Franz Käppeler played a major role. We start by recalling the early phenomenological approach, which yielded a basic understanding of the subject even before models for the parent stellar evolutionary stages were developed. Through such a tool, rough limits for the neutron density and exposure were set, and the crucial fact was understood that more than one nucleosynthesis component is required to account for solar abundances of
s
-process nuclei up to the Pb-Bi region. We then summarize the gradual understanding of the stellar processes actually involved in the production of nuclei from Sr to Pb (the so-called
Main Component
, achieved in the last decade of the past century and occurring in red giants of low and intermediate mass,
M
≲
8 M
⊙
) populating, in the
HR
diagram, the
Asymptotic Giant Branch
or
AGB
region. We conclude by giving some details on more recent research concerning mixing mechanisms inducing the activation of the main neutron source,
13
C(
α
,n)
16
O.
Theoretical predictions as well as experiments performed at storage rings have shown that the lifetimes of β-radionuclides can change significantly as a function of the ionization state. In this ...paper we describe an innovative approach, based on the use of a compact plasma trap to emulate selected stellar-like conditions. It has been proposed within the PANDORA project (Plasmas for Astrophysics, Nuclear Decay Observation and Radiation for Archaeometry) with the aim to measure, for the first time in plasma, nuclear β-decay rates of radionuclides involved in nuclear-astrophysics processes. To achieve this task, a compact magnetic plasma trap has been designed to reach the needed plasma densities, temperatures, and charge-states distributions. A multi-diagnostic setup will monitor, on-line, the plasma parameters, which will be correlated with the decay rate of the radionuclides. The latter will be measured through the detection of the γ-rays emitted by the excited daughter nuclei following the β-decay. An array of 14 HPGe detectors placed around the trap will be used to detect the emitted γ-rays. For the first experimental campaign three isotopes, 176Lu, 134Cs, and 94Nb, were selected as possible physics cases. The newly designed plasma trap will also represent a tool of choice to measure the plasma opacities in a broad spectrum of plasma conditions, experimentally poorly known but that have a great impact on the energy transport and spectroscopic observations of many astrophysical objects. Status and perspectives of the project will be highlighted in the paper.
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