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s-process nucleosynthesis in low-mass AGB stars by the $^{13}$C($α$,n)$^{16}$O neutron source

Inma Domínguez, Carlos Abia, Maurizio Busso, Oscar Straniero, Sara Palmerini

Abstract

In this review we outline our knowledge on slow neutron captures, concentrating on its main part occurring during the final stages of stellar evolution for low or intermediate-mass stars when they evolve during the Asymptotic Giant Branch, or AGB, stars. We focus our attention on how, in this field, studies passed from a first era of inquiries based on nuclear systematics, to numerical nucleosynthesis computations performed in stellar codes. We then discuss how these last were forced, by observational constraints, to almost abandon, for the synthesis of nuclei between Sr and Pb, the rather naturally activated Ne22 neutron source (operating efficiently at T > 30 keV, and producing a neutron density N_n > 5 10^8 cm^-3). This implied considering the alternative reaction 13C(alpha,n)16O, that can be activated locally after each of the TDU. The mentioned crucial reaction occurs at T< 8 keV, in the time intervals separating two subsequent thermal pulses (TP). The layers where 13C(alpha,n)16O operates are characterized by a radiative equilibrium and their low temperature also yields low values for the neutron density (N_n < 10^7 cm^-3).

s-process nucleosynthesis in low-mass AGB stars by the $^{13}$C($α$,n)$^{16}$O neutron source

Abstract

In this review we outline our knowledge on slow neutron captures, concentrating on its main part occurring during the final stages of stellar evolution for low or intermediate-mass stars when they evolve during the Asymptotic Giant Branch, or AGB, stars. We focus our attention on how, in this field, studies passed from a first era of inquiries based on nuclear systematics, to numerical nucleosynthesis computations performed in stellar codes. We then discuss how these last were forced, by observational constraints, to almost abandon, for the synthesis of nuclei between Sr and Pb, the rather naturally activated Ne22 neutron source (operating efficiently at T > 30 keV, and producing a neutron density N_n > 5 10^8 cm^-3). This implied considering the alternative reaction 13C(alpha,n)16O, that can be activated locally after each of the TDU. The mentioned crucial reaction occurs at T< 8 keV, in the time intervals separating two subsequent thermal pulses (TP). The layers where 13C(alpha,n)16O operates are characterized by a radiative equilibrium and their low temperature also yields low values for the neutron density (N_n < 10^7 cm^-3).
Paper Structure (13 sections, 21 equations, 8 figures)

This paper contains 13 sections, 21 equations, 8 figures.

Table of Contents

  1. Introduction
  2. The phenomenological approach
  3. Early nucleosynthesis models for AGB stars
  4. The 13C pocket
  5. The role of convection
  6. Rotation
  7. Gravity waves
  8. Magnetic circulation
  9. Crucial observational data
  10. Normal AGB star observations
  11. Post-AGB stars
  12. Pre-solar grains
  13. Conclusions

Figures (8)

  • Figure 1: Lower panel: the recurrent occurrence of thermonuclear instabilities in the He-shell luminosity of a low-mass AGB star ($M=2$ M$_\odot$). Upper panel: the corresponding variation of the H-shell luminosity. Note the sudden increase of the nuclear energy released by the He burning (up to $10^8$ L$_\odot$), causing the expansion of the more external layer that becomes progressively cooler, until the H-burning shell dies down. At that time, the convective envelope penetrates inward, moving the ashes of the He burning upward (see Fig. \ref{['fig4']})
  • Figure 2: A sketch of two successive thermal instabilities (thermal pulses), developing convective zones that mix the whole He-rich mantel. Note: a) that the second convective zone partially overlaps the region mixed by the previous one and b) the inward penetration of the convective envelope after each thermal pulse (the third dredge-up).
  • Figure 3: The evolution of the positions in mass of the bottom of the convective envelope (M$_{\rm CEB}$), of the H-burning shell (M$_{\rm H}$) and of the He burning shell (M$_{\rm He}$), in models of 2 M$_\odot$, for 3 different metallicity, specifically: solar, i.e [Fe/H] = 0, (top panel), 1/20th solar, i.e. [Fe/H] = -0.03 (middle panel) and 1/200 solar, i.e. [Fe/H] = -0.003 (bottom panel). Note the inward penetration of the convective envelope (TDU) after each thermal pulse and the forward displacement of the He-burning shell marking the activation of the $^{13}$C($\alpha$,n)$^{16}$O in the inter-shell zone during the inter-pulse period (more details in cris11).
  • Figure 4: The region around the boundary of the convective envelope during the 6th TDU episode of the 2 M$_\odot$ with solar composition (the same model shown in the upper panel of Fig, \ref{['fig5']}). Upper panel: chemical composition in the transition region between the convective envelope and the radiative He-rich zone. Lower panel: the exponential decline of the convective velocity and the pressure gradient. Here the $\beta$ parameter in equation \ref{['vexpo']} was set to 0.1. Adapted from straniero_2006NuPhA.
  • Figure 5: The $^{13}$C pocket that forms during the inter-pulse that follows the first fully developed TDU episode of the 2 M$_\odot$ model in Fig. \ref{['fig5']} (upper panel). The mass fraction of the main chemical species are shown. A well developed $^{13}$C pocket starts at $m\sim0.552$ M$_\odot$ and it extends outward for about $10^{-3}$ M$_\odot$. Where more protons were diffused at the time of the TDU (see Fig, \ref{['velco']}), a $^{14}$N pocket, partially overlapping the $^{13}$C pocket, forms. Also, note the presence of a smaller $^{23}$Na pocket.
  • ...and 3 more figures