s-PROCESSING IN AGB STARS REVISITED. II. ENHANCED ¹³C PRODUCTION THROUGH MHD-INDUCED MIXING

Abstract

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 13 C(α,n) 16 O reaction. This last is activated from locally-produced 13 C, formed by partial mixing of hydrogen into the He-rich layers. We present here the first attempt at describing a physical mechanism for the formation of the 13 C reservoir, studying the mass circulation induced by magnetic buoyancy and without adding new free parameters to those already involved in stellar modelling.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 extended over a large fractional mass of the He-layers, thus producing 13 C reservoirs of several 10 −3 M ⊙ .The ensuing 13 C-enriched zone has an almost flat profile, while only a limited production of 14 N 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 13 C-reservoir allows us to solve a few so far unexplained features in the abundance distribution of post-AGB objects.

The 13C Neutron Source and s-Processing in AGB Stars

“…Given the very low neutron density, such a change would not introduce any significant variation in the neutron release. Hence the present results are consistent with those in [7].…”

Evolution and Nucleosynthesis of Low and Intermediate-Mass Stars

“…Current solutions have looked at variations in the size of the 13 C pocket [36] or proton-ingestion episodes leading to an intermediate process (i-process), with neutron densities half way between the s− and r-processes [26]. Rotation in AGB stars has also been suggested but there are not yet any models with the masses and metallicities of the observed post-AGB stars [18,31,34].…”

Stellar MHD and Nuclear Physics Coupled Together Solve the Puzzle of Oxide Grain Composition

“…From right to left, the darker the curve, the smaller the value of k. Panels C and D. The evolution on the AGB of the oxygen isotopic ratios 17 and r p from 0.03 to 0.05R ⊙ ). In our approach the mass circulation rateṀ can be derived from the velocity profiles reported in Equation 1 (see [16] for more details). Assuming that the value ofṀ due to buoyancy is constant, the rate of mass penetration into the envelope will depend on: (i) the amount of matter transported by each magnetize structure (tube); and (ii) the fractional area occupied by the tubes at the base of the envelope.…”

“…In particular grains belonging to groups 1 and 2 are are then believed to form in red giant stars because they are characterized by values of 17 O/ 16 O and 18 O/ 16 O respectively larger and smaller than the solar values, which can not be accounted for by the 16 O enrichment typical of explosive nucleosynthesis. However, very small abundances of 18 O and several large excesses of 26 Mg found in those grains are not explained by pure dredge-up phenomena (included in the standard stellar evolutionary models) and [3] firstly suggested the presence of a deep matter circulation as a possible solution to the puzzle of the their isotopic mix.…”

“…Nowadays thanks to updates in CBP models ( [4], [5] and [6]) and high-precision measurements of nuclear reaction cross sections, the whole range of 17 15 O reaction rate by [7] and the 16 O(p,γ) 17 F one by [8] remarked that group 1 grains condensate in the envelope of RGB star of solar composition and with mass ≤ 2M ⊙ [6]. While proton capture cross sections on 17 O and 18 O measured by [9] and [10] decisively contribute to state that AGB stars with mass smaller than 1.5M ⊙ are reliable progenitors for group 2 grains [11].…”