Nucleosynthesis in Asymptotic Giant Branch Stars: Relevance for Galactic Enrichment and Solar System Formation

Abstract

We present a review of nucleosynthesis in AGB stars outlining the development of theoretical models and their relationship to observations. We focus on the new high resolution codes with improved opacities, which recently succeeded in accounting for the third dredge-up. This opens the possibility of understanding low luminosity C stars (enriched in s-elements) as the normal outcome of AGB evolution, characterized by production of 12 C and neutron-rich nuclei in the He intershell and by mass loss from strong stellar winds. Neutron captures in AGB stars are driven by two reactions: 13 C(α,n) 16 O, which provides the bulk of the neutron flux at low neutron densities (N n ≤ 10 7 n/cm 3 ), and 22 Ne(α,n) 25 Mg, which is mildly activated at higher temperatures and mainly affects the production of s-nuclei depending on reaction branchings. The first reaction is now known to occur in the radiative interpulse phase, immediately below the region previously homogenized by third dredge-up. The second reaction occurs during the convective thermal pulses. The resulting nucleosynthesis phenomena are rather complex and rule out any analytical approximation (exponential distribution of neutron fluences). Nucleosynthesis in AGB stars, modeled at different metallicities, account for several observational constraints, coming from a wide spectrum of sources: evolved red giants rich in s-elements, unevolved stars at different metallicities, presolar grains recovered from meteorites, and the abundances of s-process isotopes in the solar system. In particular, a good reproduction of the solar system main component is obtained as a result of Galactic chemical evolution that mixes the outputs of AGB stars of different stellar generations, born with different metallicities and producing different patterns of s-process nuclei. The main solar s-process pattern is thus not considered to be the result of a standard archetypal sprocess occurring in all stars. Concerning the 13 C neutron source, its synthesis requires penetration of small amounts of protons below the convective envelope, where they are captured by the abundant 12 C forming a 13 C-rich pocket. This penetration cannot be modeled in current evolutionary codes, but is treated as a free parameter. Future hydrodynamical studies of time dependent mixing will be required to attack this problem. Evidence of other insufficiencies in the current mixing algorithms is common throughout the evolution of low and intermediate mass stars, as is shown by the inadequacy of stellar models in reproducing the observations of CNO isotopes in red giants and in circumstellar dust grains. These observations require some circulation of matter between the bottom of convective envelopes and regions close to the H-burning shell (cool bottom processing). AGB stars are also discussed in the light of their possible contribution to the inventory of short-lived radioactivities that were found to be alive in the early solar system. We show that the pollution of the protosolar nebula by a close-by AGB st...

Short-lived nuclei in the early Solar System: Possible AGB sources

“…These calculations cover a range of stellar masses and metallicities. These new results are a substantial improvement over those discussed in Wasserburg et al [97] and in Busso et al [84]. The first results of these calculations were outlined by Busso et al [58] and by Gallino et al [201].…”

“…This additional mixing and reaction mechanism fundamentally alters the problem for AGB stars and was discussed by Busso, Gallino & Wasserburg [84] in their review of short-lived nuclei. The result is that 26 Al in AGB stars may effectively be governed by reactions just above the H burning shell due to some transport in a phenomenon called "extra mixing" or Cool Bottom Processing (CBP).…”

Unexpectedly large mass loss during the thermal pulse cycle of the red giant star R Sculptoris

“…The chemical content of the expelled material depends critically on the physical properties of the pulses (e.g., pulse duration and interpulse mass-loss rate). The duration of the pulse limits the time for nucleosynthesis to occur inside the star 18 , while the mass-loss rate between pulses limits the number of thermal pulses an AGB star will experience 17 . These properties will affect the stellar yields of new elements returned to the ISM, as well as eventually lead to the termination of the AGB phase.…”

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

“…The main neutron source is known to be the 13 C(α,n) 16 O reaction, operating at typical energies of 8 keV in radiative conditions, thanks to protons penetration at TDU. This provides a n-density of 10 6 − 10 7 cm −3 [2]. In this scenario, two main sources of uncertainty have so far affected the efficiency of the nrelease: (i) the dependence of the 13 C(α,n) 16 O reaction rate at low energies on a broad resonance near the reaction threshold, and (ii) the difficulties in identifying physical mechanisms for the formation of the 13 C reservoir (or "pocket").…”