What we do and do not know about the s-process

AGB stars are the source for the main component of the $s$-process. Here we discuss both the properties which are reasonably well known and those which still suffer from substantial uncertainties. In the former case, we are fairly sure that the $s$-process contribution from AGB stars comes from masses between about 1 and 3 \msun, and the dominant neutron source is the $^{13}$C$(\alpha$,n)$^{16}$O reaction. In the latter category remains the formation mechanism for the $^{13}$C-pocket. Attempts at including rotation seem to inhibit neutron capture reactions. Explaining the observations seems to require a spread in the size of the $^{13}$C-pocket so some stochastic process, such as rotation, must be involved.Comment: To be published in Nuclear Physics A; Invited Review for "Nuclei in the Cosmos VIII", Vancouver, July 200

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

Cosmological implications of dwarf spheroidal chamical evolution

The chemical properties of dwarf spheroidals in the local group are shown to be inconsistent with star formation being truncated after the reionization epoch (z 8). Enhanced levels of [Ba/Y] in stars in dwarf spheroidals like Sculptor indicate strong s-process production from low-mass stars whose lifetimes are comparable with the duration of the pre-reionization epoch. The chemical evolution of Sculptor is followed using a model with SN II and SN Ia feedback and mass- and metallicity-dependent nucleosynthetic yields for elements from H to Pb. We are unable to reproduce the Ba/Y ratio unless stars formed over an interval long enough for the low-mass stars to pollute the interstellar medium with s-elements. This robust result challenges the suggestion that most of the local group dwarf spheroidals are fossils of reionization and supports the case for large initial dark matter halos

New reaction rate for 16O( p, γ )17F and its influence on the oxygen isotopic ratios in massive AGB stars

The 16O(p, γ )17F reaction rate is revisited with special emphasis on the stellar temperature range of T=60-100 MK, important for hot bottom burning in asymptotic giant branch (AGB) stars. We evaluate existing cross-section data that were obtained since 1958 and, if appropriate, correct published data for systematic errors that were not noticed previously, including the effects of coincidence summing and updated effective stopping powers. The data are interpreted by using two different models of nuclear reactions, that is, a potential model and R-matrix theory. A new astrophysical S factor and recommended thermonuclear reaction rates are presented. As a result of our work, the 16O(p, γ )17F reaction has now the most precisely known rate involving any target nucleus in the mass A> 12 range, with reaction rate errors of about 7% over the entire temperature region of astrophysical interest (T=0.01-2.5 GK). The impact of the present improved reaction rate with its significantly reduced uncertainties on the hot bottom burning in AGB stars is discussed. In contrast to earlier results we find now that there is not clear evidence to date for any stellar grain origin from massive AGB stars

The uncertainties in the 22Ne+α-Capture reaction rates and the production of the heavy magnesium isotopes in asymptotic giant branch stars of intermediate mass

We present new rates for the 22Ne( , n)25Mg and 22Ne( , ) 26Mg reactions, with uncertainties that have been considerably reduced compared to previous estimates, and we study how these new rates affect the production of the heavy magnesium isotopes in models of intermediate-mass asymptotic giant branch (AGB) stars of different initial compositions. All the models have deep third dredge-up, hot bottom burning, and mass loss. Calculations have been performed using the two most commonly used estimates of the 22Ne+ rates as well as the new recommended rates, and with combinations of their upper and lower limits. The main result of the present study is that, with the new rates, uncertainties on the production of isotopes from Mg to P coming from the 22Ne+ -capture rates have been considerably reduced.We have therefore removed one of the important sources of uncertainty to effect models of AGB stars. We have studied the effects of varying the mass-loss rate on nucleosynthesis and discuss other uncertainties related to the physics employed in the computation of stellar structure, such as the modeling of convection, the inclusion of a partial mixing zone, and the definition of convective borders. These uncertainties are found to be much larger than those coming from 22Ne+ -capture rates, when using our new estimates. Much effort is needed to improve the situation for AGB models

New reaction rate for 16O( p, γ )17F and its influence on the oxygen isotopic ratios in massive AGB stars

The 16O(p, γ )17F reaction rate is revisited with special emphasis on the stellar temperature range of T=60-100 MK, important for hot bottom burning in asymptotic giant branch (AGB) stars. We evaluate existing cross-section data that were obtained since 1958 and, if appropriate, correct published data for systematic errors that were not noticed previously, including the effects of coincidence summing and updated effective stopping powers. The data are interpreted by using two different models of nuclear reactions, that is, a potential model and R-matrix theory. A new astrophysical S factor and recommended thermonuclear reaction rates are presented. As a result of our work, the 16O(p, γ )17F reaction has now the most precisely known rate involving any target nucleus in the mass A> 12 range, with reaction rate errors of about 7% over the entire temperature region of astrophysical interest (T=0.01-2.5 GK). The impact of the present improved reaction rate with its significantly reduced uncertainties on the hot bottom burning in AGB stars is discussed. In contrast to earlier results we find now that there is not clear evidence to date for any stellar grain origin from massive AGB stars

NanoSIMS isotopic analysis of small presolar grains : search for Si3N4 grains from AGB stars and Al and Ti isotopic compositions of rare presolar SiC grains

We report isotopic ratio measurements of small SiC and Si3N4grains, with special emphasis on presolar SiC grains of type Z, and new nucleosynthesis models for 26Al/27Al and the Ti isotopic ratios in asymptotic giant branch (AGB) stars. With the NanoSIMS we analyzed 310 SiC grains from Murchison (carbonaceous CM2 chondrite) separate KJB (diameters 0.25-0.45 μm) and 153 SiC grains from KJG (diameters 1.8-3.7 μm), 154 SiC and 23 Si3N4 grains from Indarch (enstatite EH4 chondrite) separate IH6 (diameters 0.25-0.65 μm) for their C and N isotopic compositions, 549 SiC and 142 Si3N4 grains from IH6 for their C and Si isotopic compositions, 13 SiC grains from Murchison and 66 from Indarch for their Al-Mg compositions, and eight SiC grains from Murchison and 10 from Indarch for their Ti isotopic compositions. One of the original objectives of this effort was to compare isotopic analyses with the NanoSIMS with analyses previously obtained with the Cameca IMS 3f ion microprobe. Many of the Si3N4 grains from Indarch have isotopic anomalies but most of these apparently originate from adjacent SiC grains. Only one Si3N4 grain, with 13C and 14N excesses, has a likely AGB origin. The C, N, and Si isotopic data show that the percentage of SiC grains of type Y and Z increase with decreasing grain size (from ∼1% for grains >2 μm to ∼5-7% for grains of 0.5 μm), providing an opportunity for isotopic analyses in these rare grains. Our measurements expand the number of Al-Mg analyses on SiC Z grains from 4 to 23 and the number of Ti analyses on Z grains from 2 to 11. Inferred26Al/27Al ratios of Z grains are in the range found in mainstream and Y grains and do not exceed those predicted by models of AGB nucleosynthesis. Cool bottom processing (CBP) has been invoked to explain the low 12C/13C ratios of Z grains, but this process apparently does not lead to increased 26Al production in the parent stars of these grains. This finding is in contrast to presolar oxide grains where CBP is needed to explain their high 26Al/27Al ratios. The low 46,47,49 Tι/48Τi ratios found in Z grains and their correlation with low 29Si/28Si ratios extend the trend seen in mainstream grains and confirm an origin in low-metallicity AGB stars. The relatively large excesses in 30Si and 50Ti in Z grains are predicted by our models to be the result of increased production of these isotopes by neutron-capture nucleosynthesis in low-metallicity AGB stars. However, the predicted excesses in 50Ti (and 49Ti) are much larger than those found. Even lowering the strength of the 13C pocket cannot solve this discrepancy in a consistent way

Fluorine in carbon-enhanced metal-poor stars: a binary scenario

Aims. A super-solar fluorine abundance was observed in the carbon-enhanced metal-poor (CEMP) star HE 1305+0132 ([F/Fe] = +2.90, [Fe/H] = -2.5). We propose that this observation can be explained using a binary model that involve mass transfer from an asymptotic giant branch (AGB) star companion and, based on this model, we predict F abundances in CEMP stars in general. We discuss wether F can be used to discriminate between the formation histories of most CEMP stars: via binary mass transfer or from the ejecta of fast-rotating massive stars. Methods. We compute AGB yields using different stellar evolution and nucleosynthesis codes to evaluate stellar model uncertainties. We use a simple dilution model to determine the factor by which the AGB yields should be diluted to match the abundances observed in HE 1305+0132. We further employ a binary population synthesis tool to estimate the probability of F-rich CEMP stars. Results. The abundances observed in HE 1305+0132 can be explained if this star accreted 3-11% of the mass lost by its former AGB companion. The primary AGB star should have dredged-up at least 0.2 of material from its He-rich region into the convective envelope via third dredge-up, which corresponds to AGB models of Z 0.0001 and mass 2 . Many AGB model uncertainties, such as the treatment of convective borders and mass loss, require further investigation. We find that in the binary scenario most CEMP stars should also be FEMP stars, that is, have [F/Fe] > +1, while fast-rotating massive stars do not appear to produce fluorine. We conclude that fluorine is a signature of low-mass AGB pollution in CEMP stars, together with elements associated with the slow neutron-capture process