Amanda Karakas

Reaction Rate Uncertainties and the Production of 19F in Asymptotic Giant Branch Stars

We present nucleosynthesis calculations and the resulting 19F stellar yields for a large set of models with different masses and metallicity. During the asymptotic giant branch (AGB) phase, 19F is produced as a consequence of nucleosynthesis occurring during the convective thermal pulses and also during the interpulse periods if protons from the envelope are partially mixed in the top layers of the He intershell (partial mixing zone). We find that the production of fluorine depends on the temperature of the convective pulses, the amount of primary 12C mixed into the envelope by third dredge-up, and the extent of the partial mixing zone. Then we perform a detailed analysis of the reaction rates involved in the production of 19F and the effects of their uncertainties. We find that the major uncertainties are associated with the 14C(α, γ)18O and 19F(α, p)22Ne reaction rates. For these two reactions we present new estimates of the rates and their uncertainties. In both cases the revised rates are lower than previous estimates. The effect of the inclusion of the partial mixing zone on the production of fluorine strongly depends on the very uncertain 14C(α, γ)18O reaction rate. The importance of the partial mixing zone is reduced when using our estimate for this rate. Overall, rate uncertainties result in uncertainties in the fluorine production of about 50% in stellar models with mass 3 M☉ and of about a factor of 7 in stellar models of mass 5 M☉. This larger effect at high masses is due to the high uncertainties of the 19F(α, p)22Ne reaction rate. Taking into account both the uncertainties related to the partial mixing zone and those related to nuclear reactions, the highest values of 19F enhancements observed in AGB stars are not matched by the models. This is a problem that will have to be revised by providing a better understanding of the formation and nucleosynthesis in the partial mixing zone, as well as in relation to reducing the uncertainties of the 14C(α, γ)18O reaction rate. At the same time, the possible effect of cool bottom processing at the base of the convective envelope should be included in the computation of AGB nucleosynthesis. This process could, in principle, help to match the highest 19F abundances observed by decreasing the C/O ratio at the surface of the star, while leaving the 19F abundance unchanged.

Silicon and Carbon Isotopic Ratios in AGB Stars: SiC Grain Data, Models, and the Galactic Evolution of the Si Isotopes

PresolarSiCgrainsofthemainstream,Y,andZtypearebelievedtocomefromcarbonstars.WecomparedtheirCand Si isotopicratios withtheoretical modelsfor theenvelopecompositions of AGB stars.Two setsof models (FRANEC and Monash) use a range of stellar masses (1.5–5M� ) and metallicities, different prescriptions for mass loss, and two sets of neutron-capture cross sections for the Si isotopes. They predict that the shifts in Si isotopic ratios and the increase of 12 C/ 13 C in the envelope during third dredge-up are higher for higher stellar mass, lower metallicity, and lower mass-loss rate. Because the 22 Ne neutron source dominates Si nucleosynthesis, the effect of the 13 C source is negligible. Comparison of the model predictions with grain data confirms an AGB origin for these grains, with Yand Z grains having originated in stars with lower than solar metallicity. The Si isotopic ratios of the Z grains favor the Si cross sections by Guber et al. over those by Bao et al. The 12 C/ 13 C ratios of low-metallicity models are much higher than those found in Z grains, and cool bottom processing must be invoked to explain the grains’ C isotopic ratios. By combining Z grain Si data with the models, we determined the evolution of the 29 Si/ 28 Si ratio in the Galaxy as function of metallicity Z .A tZ <0:01 this ratio rises much faster than current Galactic evolution models predict and suggests an early source of the heavy Si isotopes not considered in these models. Subject headingg dust, extinction — Galaxy: evolution — nuclear reactions, nucleosynthesis, abundances — stars: AGB and post-AGB — stars: carbon

Nucleosynthesis Predictions for Intermediate-Mass Asymptotic Giant Branch Stars: Comparison to Observations of Type I Planetary Nebulae

Type I planetary nebulae (PNe) have high He/H and N/O ratios and are thought to be descendants of stars with initial masses of ∼3–8 M� . These characteristics indicate that the progenitor stars experienced proton-capture nucleosynthesis at the base of the convective envelope, in addition to the slow neutron capture process operating in the He-shell (the s-process). We compare the predicted abundances of elements up to Sr from models of intermediate-mass asymptotic giant branch (AGB) stars to measured abundances in Type I PNe. In particular, we compare predictions and observations for the light trans-iron elements Se and Kr, in order to constrain convective mixing and the s-process in these stars. A partial mixing zone is included in selected models to explore the effect of a 13 C pocket on the s-process yields. The solar-metallicity models produce enrichments of [(Se, Kr)/Fe] 0.6, consistent with Galactic Type I PNe where the observed enhancements are typically 0.3 dex, while lower metallicity models predict larger enrichments of C, N, Se, and Kr. O destruction occurs in the most massive models but it is not efficient enough to account for the 0.3 dex O depletions observed in some Type I PNe. It is not possible to reach firm conclusions regarding the neutron source operating in massive AGB stars from Se and Kr abundances in Type I PNe; abundances for mores-process elements may help to distinguish between the two neutron sources. We predict that only the most massive (M 5 M� ) models would evolve into Type I PNe, indicating that extra-mixing processes are active in lower-mass stars (3–4 M� ), if these stars are to evolve into Type I PNe.

Reaction rate uncertainties and the operation of the NeNa and MgAl chains during HBB in intermediate-mass AGB stars

Context. We test the effect of proton-capture reaction rate uncertainties on the abundances of the Ne, Na, Mg and Al isotopes processed by the NeNa and MgAl chains during hot bottom burning (HBB) in asymptotic giant branch (AGB) stars of intermediate mass between 4a nd 6Mand metallicities between Z = 0.0001 and 0.02. Aims. We provide uncertainty ranges for the AGB stellar yields, for inclusion in galactic chemical evolution models, and indicate which reaction rates are most important and should be better determined. Methods. We use a fast synthetic algorithm based on detailed AGB models. We run a large number of stellar models, varying one reaction per time for a very fine grid of values, as well as all reactions simultaneously. Results. We show that there are uncertainties in the yields of all the Ne, Na, Mg and Al isotopes due to uncertain proton-capture reaction rates. The most uncertain yields are those of 26 Al and 23 Na (variations of two orders of magnitude), 24 Mg and 27 Al (variations of more than one order of magnitude), 20 Ne and 22 Ne (variations between factors 2 and 7). In order to obtain more reliable Ne, Na, Mg and Al yields from IM-AGB stars the rates that require more accurate determination are: 22 Ne(p ,γ ) 23 Na, 23 Na(p ,γ ) 24 Mg, 25 Mg(p ,γ ) 26 Al, 26 Mg(p ,γ ) 27 Al and 26 Al(p ,γ ) 27 Si. Conclusions. Detailed galactic chemical evolution models should be constructed to address the impact of our uncertainty ranges on the observational constraints related to HBB nucleosynthesis, such as globular cluster chemical anomalies.

The chemical evolution of helium in globular clusters : Implications for the self-pollution scenario

We investigate the suggestion that there are stellar populations in some globular clusters with enhanced helium (Y ~ 0.28-0.40) compared to the primordial value. We assume that a previous generation of massive asymptotic giant branch (AGB) stars have polluted the cluster. Two independent sets of AGB yields are used to follow the evolution of helium and CNO using a Salpeter initial mass function (IMF) and two top-heavy IMFs. In no case are we able to produce the postulated large Y ~ 0.35 without violating the observational constraint that the CNO content is nearly constant.

On the origin of fluorine in the Milky Way

The main astrophysical factories of fluorine ( 19 F) are thought to be Type II supernovae, Wolf‐ Rayet stars, and the asymptotic giant branch (AGB) of intermediate-mass stars. We present a model for the chemical evolution of fluorine in the Milky Way using a semi-analytic multizone chemical evolution model. For the first time, we demonstrate quantitatively the impact of fluorine nucleosynthesis in Wolf‐Rayet and AGB stars. The inclusion of these latter two fluorine production sites provides a possible solution to the long-standing discrepancy between model predictions and the fluorine abundances observed in Milky Way giants. Finally, fluorine is discussed as a possible probe of the role of supernovae and intermediate-mass stars in the chemical evolution history of the globular cluster ω Centauri. Ke yw ords: stars: abundances ‐ stars: evolution ‐ galaxies: evolution.

On the asymptotic giant branch star origin of peculiar spinel grain OC2

Microscopic presolar grains extracted from primitive meteorites have extremely anomalous isotopic compositions revealing the stellar origin of these grains. The composition of presolar spinel grain OC2 is different from that of all other presolar spinel grains. Large excesses of the heavy Mg isotopes are present and thus an origin from an intermediate-mass (IM) asymptotic giant branch (AGB) star was previously proposed for this grain. We discuss the isotopic compositions of presolar spinel grain OC2 and compare them to theoretical predictions. We show that the isotopic composition of O, Mg and Al in OC2 could be the signature of an AGB star of IM and metallicity close to solar experiencing hot bottom burning, or of an AGB star of low mass (LM) and low metallicity suffering very efficient cool bottom processing. Large measurement uncertainty in the Fe isotopic composition prevents us from discriminating which model better represents the parent star of OC2. However, the Cr isotopic composition of the grain favors an origin in an IM-AGB star of metallicity close to solar. Our IM-AGB models produce a self-consistent solution to match the composition of OC2 within the uncertainties related to reaction rates. Within this solution we predict that the 16O(p,g)17F and the 17O(p,a)14N reaction rates should be close to their lower and upper limits, respectively. By finding more grains like OC2 and by precisely measuring their Fe and Cr isotopic compositions, it may be possible in the future to derive constraints on massive AGB models from the study of presolar grains.

Germanium Production in Asymptotic Giant Branch Stars: Implications for Observations of Planetary Nebulae

Observations of planetary nebulae (PNe) in the work of Sterling, Dinerstein, & 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 (the 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 approximately one-tenth of the He-rich intershell. All of the models show surface increases of [Ge/Fe] 0.5, except the 2.5 M☉, Z = 0.004 case, which produced a factor of 6 enhancement of Ge. Near the tip of the TP-AGB, a couple of extra thermal pulses (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.

The structure of close binaries in two dimensions

The structure and evolution of close binary stars has been studied using the two-dimensional stellar structure algorithm developed by Deupree. We have calculated a series of solar composition stellar evolution sequences of binary models in which the mass of the two-dimensional model is 8 M☉ with a point-mass 5 M☉ companion. We have also studied the structure of the companion in two dimensions by considering the zero-age main sequence (ZAMS) structure of a 5 M☉ model with an 8 M☉ point-mass companion. This result suggests that treating the 5 M☉ star as a point source for the 8 M☉ evolution is reasonable. In all cases, the binary orbit was assumed to be circular and corotating with the rotation rate of the stars. We considered binary models with three different initial separations, a = 10, 14, and 20 R☉. These models were evolved through central hydrogen burning or until the more massive star expanded to fill its critical potential surface or Roche lobe. The model with a separation of 20 R☉ will be expected to go through case B-type mass transfer during the shell H-burning phase. The 14 R☉ model is expected to go through mass transfer much earlier, near the middle of core hydrogen burning, and the 10 R☉ model is very close to this situation at the ZAMS. The calculations show that evolution of the deep interior quantities is only slightly modified from those of single-star evolution. Describing the model surface as a Roche equipotential is also satisfactory until very close to the time of Roche lobe overflow, when the self-gravity of the model about to lose mass develops a noticeable aspherical component and the surface timescale becomes sufficiently short, so that it is conceivable that the actual surface is not an equipotential.

Abundance Anomalies in NGC6752 - Do AGB Stars Have a Role?

We are in the process of testing a popular theory that the observed abundance anomalies in the Globular Cluster NGC 6752 are due to ‘internal pollution’ from intermediate mass asymptotic giant branch stars. To this end we are using a chemical evolution model incorporating custom-made stellar evolution yields calculated using a detailed stellar evolution code. By tracing the chemical evolution of the intracluster gas, which is polluted by two generations of stars, we are able to test the internal pollution scenario in which the Na- and Al-enhanced ejecta from intermediate mass stars is either accreted onto the surfaces of other stars, or goes toward forming new stars. In this paper we focus mainly on the nucleosynthetic yields of the AGB stars and discuss whether these stars are the source of the observed Na-O anticorrelation. Comparing our preliminary results with observational data suggests that the qualitative theory is not supported by this quantitative study. This study has recently been completed and published in [Fenner, Y., Campbell, S.W., Karakas, A.I., Lattanzio, J.C, Gibson, B.K., 2004, MNRAS, 353, 789]. Details of the stellar models will be in a forthcoming paper [Campbell, S. W., et al. 2004, in prep.].