Reaction rates for the s-process neutron source 22Ne + α

The 22Ne(α,n)25Mg reaction is an important source of neutrons for the s-process. In massive stars responsible for the weak component of the s-process, 22Ne(α,n)25Mg is the dominant source of neutrons, both during core helium burning and in carbon-shell burning. For the main s-process component produced in asymptotic giant branch (AGB) stars, the 13C(α,n)16O reaction is the dominant source of neutrons operating during the interpulse period, with the 22Ne+α source affecting mainly the s-process branchings during a thermal pulse. Rate uncertainties in the competing 22Ne(α,n)25Mg and 22Ne(α,γ)26Mg reactions result in large variations of s-process nucleosynthesis. Here, we present up-to-date and statistically rigorous 22Ne+α reaction rates using recent experimental results and Monte Carlo sampling. Our new rates are used in postprocessing nucleosynthesis calculations both for massive stars and AGB stars. We demonstrate that the nucleosynthesis uncertainties arising from the new rates are dramatically reduced in comparison to previously published results, but several ambiguities in the present data must still be addressed. Recommendations for further study to resolve these issues are provided

HR4049: signature of nova nucleosynthesis ?

The post-Asymptotic Giant Branch (AGB) star HR4049 is in an eccentric binary system with a relatively short period probably surrounded by a dusty circumbinary disk. Extremely anomalous oxygen isotopic ratios, O16/O17 ~ O16/O18 ~ 7, have been measured from CO_2 molecules likely residing in the disk. Such a composition cannot be explained in the framework of AGB and post-AGB evolution while it can be qualitatively associated with the nucleosynthesis occurring during nova outbursts. We discuss nova models, the presence of a white dwarf companion to HR4049 and possible scenarios for the dynamical evolution of this binary system. Circumbinary disks in which mixing occurs between red-giant and nova material may also be invoked as the site of formation of some rare types of meteoritic presolar grains.Comment: 4 pages, 2 figures, submitted for the proceedings of the 8th Nuclei in the Cosmos symposium (Vancouver, Canada, 19-23 July 2004

Isotopic composition of presolar spinel grain OC2: Constraining intermediate-mass asymptotic giant branch models

We analyze the O, Mg, Al, Cr and Fe compositions predicted by detailed models of AGB stars of different masses and metallicities and discuss them in the light of the precise measurements of the composition of a single extraordinary presolar spinel grain, named OC2. Large excesses of the heavy Mg isotopes are present in this grain and thus an origin from an intermediate-mass (IM) asymptotic giant branch (AGB) star was previously proposed for it. Our IM-AGB models with temperatures at the base of the convective envelope ≃ 80 - 85 million degrees produce a good match to the composition of OC2 within the uncertainties related to reaction rates. This solution is possible if, in particular, we take the lower limit and the upper limit for the 16O(p,γ )17F and the 17O(p,α) 14N reaction rates, respectively

Kr Isotopic Compositions in Stardust SiC grains and AGB Winds

Krypton (Kr) is a heavy noble gas that does not chemically react and hence does not condense into dust. However, it is found in trace amounts inside stardust silicon carbide (SiC) grains in meteorites, which are believed to have condensed in the C-rich envelopes of low-mass asymptotic giant branch (AGB) stars. The measured isotopic composition of Kr clearly reveals the signature of the s (slow neutron-capture) process. It is likely that Kr is ionised and implanted in stardust SiC grains via stellar winds in two different evolutionary phases: one during the AGB phase in small grains showing low 86Kr/82Kr, and another during the post-AGB phase in large grains showing high 86Kr/82Kr ratios. The low 86Kr/82Kr ratios observed in stardust SiC grains can be explained by model predictions of AGB winds. On the other hand, to explain the high 86Kr/82Kr ratios we need to look at the material in the winds of the post-AGB phase. We present Kr isotopic compositions predicted by s-process AGB-star models of different masses and metallicities, and compare them to data from stardust SiC grains. We find that to match the high 86Kr/82Kr ratios observed in the large grains, a proton ingestion during the thermal pulse (TP) may be required. We also find that the 84Kr(n,γ)85Kr neutron-capture cross section should to be lower than the current estimate in order for our models to match the pure s-process value

Author Correction: The messy death of a multiple star system and the resulting planetary nebula as observed by JWST

Stars and planetary system

26Al and 60Fe yields from AGB stars

We present yields for 26Al and 60Fe from asymptotic giant branch (AGB) stars. For AGB stars of masses lower than ≈4 M yields are of the order of only 10−7 M, while for AGB stars of higher masses yields are up to 10−5 M. In these massive AGB stars 26Al is produced via 25Mg(p, γ)26Al reactions when proton captures occur at the base of the convective envelope (hot bottom burning), while 60Fe is produced via the operation of the 59Fe(n, γ)60Fe reaction when high neutron densities result from the activation of the 22Ne(α, n)25Mg neutron source during thermal pulses. Large nuclear and stellar uncertainties are associated with these predictions, ranging from the rate of the 26Al + p reaction to the amount of material carried from the He-rich shell to the convective envelope via the third dredge-up. When compared to the contribution from core-collapse supernovae, the overall contribution of AGB stars to the Galactic inventory of 26Al and 60Fe is insignificant. On the other hand, a massive AGB star may have polluted the early solar system with short lived radioactive nuclei since we obtain a self-consistent match for the abundances of 41Ca, 26Al, 60Fe, and 107Pd using our 6.5 M model. Finally, the interpretation of the 26Al/27Al ratios in the majority of meteoritic stellar grains from low-mass AGB stars is hindered by the three orders of magnitude error bar of the 26Al(p, γ) 27Si reaction. Grains with very high 26Al/27Al ratios may represent evidence for extra-mixing phenomena in AGB stars or for a post-AGB origin

Germanium production in asymptotic giant branch stars: implications for observations of planetary nebulae

Observations of planetary nebulae (PNe) by Sterling, Dinerstein and Bowers have revealed abundances in the neutron-capture element Germanium (Ge) from solar to factors of 3 -- 10 above solar. The enhanced Ge is an indication that the slow-neutron capture process (s process) operated in the parent star during the thermally-pulsing asymptotic giant branch (TP-AGB) phase. We compute the detailed nucleosynthesis of a series of AGB models to estimate the surface enrichment of Ge near the end of the AGB. A partial mixing zone of constant mass is included at the deepest extent of each dredge-up episode, resulting in the formation of a 13C pocket in the top ~1/10th of the He-rich intershell. All of the models show surface increases of [Ge/Fe] less than about 0.5, except the 2.5Msun, Z=0.004 case which produced a factor of 6 enhancement of Ge. Near the tip of the TP-AGB, a couple of extra TPs could occur to account for the composition of the most Ge-enriched PNe. Uncertainties in the theoretical modeling of AGB stellar evolution might account for larger Ge enhancements than we predict here. Alternatively, a possible solution could be provided by the occurrence of a late TP during the post-AGB phase. Difficulties related to spectroscopic abundance estimates also need to be taken into consideration. Further study is required to better assess how the model uncertainties affect the predictions and, consequently, if a late TP should be invoked

Reaction rates for the s-process neutron source Ne-22+alpha

The 22Ne(α, n)25Mg reaction is an important source of neutrons for the s-process. In massive stars responsible for the weak component of the s-process, 22Ne(α, n)25Mg is the dominant source of neutrons, both during core helium burning and in carbon-shell burning. For the main s-process component produced in asymptotic giant branch (AGB) stars, the 13C(α, n)16O reaction is the dominant source of neutrons operating during the interpulse period, with the 22Ne + α source affecting mainly the s-process branchings during a thermal pulse. Rate uncertainties in the competing 22Ne(α, n)25Mg and 22Ne(α, γ)26Mg reactions result in large variations of s-process nucleosynthesis. Here, we present up-to-date and statistically rigorous 22Ne + α reaction rates using recent experimental results and Monte Carlo sampling. Our new rates are used in postprocessing nucleosynthesis calculations both for massive stars and AGB stars. We demonstrate that the nucleosynthesis uncertainties arising from the new rates are dramatically reduced in comparison to previously published results, but several ambiguities in the present data must still be addressed. Recommendations for further study to resolve these issues are provided

AGB nucleosynthesis at low metallicity: What can we learn from Carbon- and s-elements-enhanced metal-poor stars

CEMP-s stars are very metal-poor stars with enhanced abundances of carbon and s-process elements. They form a significant proportion of the very metal-poor stars in the Galactic halo and are mostly observed in binary systems. This suggests that the observed chemical anomalies are due to mass accretion in the past from an asymptotic giant branch (AGB) star. Because CEMP-s stars have hardly evolved since their formation, the study of their observed abundances provides a way to probe our models of AGB nucleosynthesis at low metallicity. To this end we included in our binary evolution model the results of the latest models of AGB nucleosynthesis and we simulated a grid of 100 000 binary stars at metallicity Z = 0.0001 in a wide range of initial masses and separations. We compared our modelled stars with a sample of 60 CEMP-s stars from the SAGA database of metal-poor stars. For each observed CEMP-s star of the sample we found the modelled star that reproduces best the observed abundances. The result of this comparison is that we are able to reproduce simultaneously the observed abundance of the elements affected by AGB nucleosynthesis (e.g. C, Mg, s-elements) for about 60% of the stars in the sample