NUCLEOSYNTHESIS PREDICTIONS FOR INTERMEDIATE-MASS ASYMPTOTIC GIANT BRANCH STARS: COMPARISON TO OBSERVATIONS OF TYPE I PLANETARY NEBULAE (original) (raw)
Related papers
2008
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 ~3-8Msun. 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 13C 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 more s-process elements may help to distinguish between the two neutron sources. We predict that only the most massive models would evolve into Type I PNe, indicating that extra-mixing processes are active in lower-mass stars (3-4Msun), if these stars are to evolve into Type I PNe.
Annual Review of Astronomy and Astrophysics, 1999
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 ? 240 BUSSO s GALLINO s WASSERBURG 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 star may account for concordant abundances of 26 Al, 41 Ca, 60 Fe, and 107 Pd. The AGB star must have undergone a very small neutron exposure, and be of small initial mass (M ≤ 1.5 M ). There is a shortage of 26 Al in such models, that however remains within the large uncertainties of crucial reaction rates. The net 26 Al production problem requires further investigation. Annu. Rev. Astro. Astrophys. 1999.37:239-309. Downloaded from arjournals.annualreviews.org by CALIFORNIA INSTITUTE OF TECHNOLOGY on 09/08/05. For personal use only. Annu. Rev. Astro. Astrophys. 1999.37:239-309. Downloaded from arjournals.annualreviews.org by CALIFORNIA INSTITUTE OF TECHNOLOGY on 09/08/05. For personal use only. ? 246 BUSSO s GALLINO s WASSERBURG Annu. Rev. Astro. Astrophys. 1999.37:239-309. Downloaded from arjournals.annualreviews.org by CALIFORNIA INSTITUTE OF TECHNOLOGY on 09/08/05. For personal use only.
Nucleosynthesis and mixing on the asymptotic giant branch. I - MS and S stars with and without TC
The Astrophysical Journal, 1992
We present the results of s-process nucleosynthesis calculations for Asymptotic Giant Branch (AGB) stars of different metallicities and different initial stellar masses (1.5 and 3 M ⊙ ) and comparisons of them with observational constraints from high resolution spectroscopy of evolved stars over a wide metallicity range. The computations were based on previously published stellar evolutionary models that account for the third dredge up phenomenon occurring late on the AGB. Neutron production is driven by the 13 C(α,n) 16 O reaction during the interpulse periods in a tiny layer in radiative equilibrium at the top of the He-and C-rich shell. The neutron source 13 C is manufactured locally by proton captures on the abundant 12 C; a few protons are assumed to penetrate from the convective envelope into the radiative layer at any third dredge up episode, when a chemical discontinuity is established between the convective envelope and the He-and Crich zone. A weaker neutron release is also guaranteed by the marginal activation of the reaction 22 Ne(α,n) 25 Mg during the convective thermal pulses. Owing to the lack of a consistent model for 13 C formation, the abundance of 13 C burnt per cycle is allowed to vary as a free parameter over a wide interval (a factor of 50). The s-enriched material is subsequently mixed with the envelope by the third dredge up, and the envelope composition is computed after each thermal pulse. We follow the changes in the photospheric abundance of the Ba-peak elements (heavy s, or 'hs') and that of the Zr-peak ones (light s, or 'ls'), whose logarithmic ratio [hs/ls] has often been adopted as an indicator of the s-process efficiency (e.g. of the neutron exposure). Our model predictions for this parameter show a complex trend versus metallicity. Especially noteworthy is the prediction that the flow along the s path at low metallicities drains the Zr-peak and Ba-peak and builds an excess at the doubly-magic 208 Pb, at the termination of the s path. We then discuss the effects on the models of variations in the crucial parameters of the 13 C pocket, finding that they are not critical for interpreting the results.
2001
We present the results of s-process nucleosynthesis calculations for AGB stars of different metallicities and initial masses. The computations were based on previously published stellar evolutionary models that account for the III dredge up phenomenon occurring late on the AGB. Neutron production is driven by the 13C(alpha,n)16O reaction during the interpulse periods in a tiny layer in radiative equilibrium at the top of the He- and C-rich shell. The s-enriched material is subsequently mixed with the envelope by the III dredge up, and the envelope composition is computed after each thermal pulse. We follow the changes in the photospheric abundance of the Ba-peak elements (heavy s, or `hs') and that of the Zr-peak ones (light s, or `ls'), whose logarithmic ratio [hs/ls] has often been adopted as an indicator of the s-process efficiency. The theoretical predictions are compared with published abundances of s elements for Galactic AGB giants of classes MS, S, SC, post-AGB super...
s-Process nucleosynthesis in AGB stars: A Test for stellar evolution
We study the slow neutron capture process (s process) in Asymptotic Giant Branch (AGB) stars using three different stellar evolutionary models computed for a 3 M ⊙ and solar metallicity star. First we investigate the formation and the efficiency of the main neutron source: the 13 C(α,n) 16 O reaction that occurs in radiative conditions. A tiny region rich in 13 C (the 13 C pocket) is created by proton captures on the abundant 12 C in the top layers of the He intershell, the zone between the H shell and the He shell. We parametrically vary the number of protons mixed from the envelope. For high local protons over 12 C number ratio, p/ 12 C ∼ > 0.3, most of the 13 C nuclei produced are further converted by proton capture to 14 N. Besides, 14 N nuclei represent a major neutron poison. We find
Evolution and Nucleosynthesis of Extremely Metal-Poor AGB Stars
Context. Models of primordial and hyper-metal-poor stars that have masses similar to the Sun are known to experience an ingestion of protons into the hot core during the core helium flash phase at the end of their red giant branch evolution. This produces a concurrent secondary flash powered by hydrogen burning that gives rise to further nucleosynthesis in the core. Aims. We aim to model the nucleosynthesis occurring during the proton ingestion event to ascertain if any significant neutron-capture nucleosynthesis occurs. Methods. We perform post-process nucleosynthesis calculations on a one-dimensional stellar evolution calculation of a star with mass 1 M ⊙ and a metallicity of [Fe/H] = −6.5 that suffers a proton ingestion episode. Our network includes 320 nuclear species and 2,366 reactions and treats mixing and burning simultaneously. Results. We find that the mixing and burning of protons into the hot convective core leads to the production of 13 C, which then burns via the 13 C(α,n) 16 O reaction releasing a large number of free neutrons. During the first two years of neutron production the neutron poison 14 N abundance is low, allowing the prodigious production of heavy elements such as strontium, barium, and lead via slow neutron captures (the s process). These nucleosynthetic products are later mixed to the stellar surface and ejected via stellar winds. We compare our results with observations of the hyper-metal-poor halo star HE 1327-2326, which shows a strong Sr overabundance. Conclusions. Our model provides the possibility of self-consistently explaining the Sr overabundance in HE 1327-2326 together with its C, N, and O overabundances (all within a factor of ∼ 4) if the material were heavily diluted, for example, via mass transfer in a wide binary system. The model produces at least 18 times too much Ba than observed, but this may be within the large modelling uncertainties. In this scenario, binary systems of low mass must have formed in the early Universe. If true then this puts constraints on the primordial initial mass function.
Super and Massive AGB Stars: II- Nucleosynthesis and
2013
We have computed detailed evolution and nucleosynthesis models for super and massive AGB stars over the mass range 6.5-9.0 M ⊙ in divisions of 0.5 M ⊙ with metallicities Z=0.02, 0.008 and 0.004. These calculations, in which we find third dredge-up and hot bottom burning, fill the gap between existing low and intermediate-mass AGB star models and high mass star models that become supernovae. For the considered metallicities, the composition of the yields is largely dominated by the thermodynamic conditions at the base of the convective envelope rather than by the pollution arising from third dredge up. We investigate the effects of various uncertainties, related to the mass-loss rate, mixing length parameter, and the treatment of evolution after the envelope instability that develops near the end of the (Super)AGB phase. Varying these parameters alter the yields mainly because of their impact on the amount of third dredge up enrichment, and to a lesser extent on the hot bottom burning conditions. Our models produce significant amounts of 4 He, 7 Li (depending on the mass-loss formulation) 13 C, 14 N, 17 O, 23 Na, 25 Mg, as well the radioactive isotope 26 Al in agreement with previous investigation. In addition our results show enrichment of 22 Ne, 26 Mg and 60 Fe, as well as a substantial increase in our proxy neutron capture species representing all species heavier than iron. These stars may provide important contributions to the Galaxy's inventory of the heavier Mg isotopes, 14 N, 7 Li and 27 Al.
The Astrophysical Journal, 2013
We present a comprehensive abundance analysis of two newly-discovered carbon-enhanced metalpoor (CEMP) stars. HE 2138−3336 is a s-process-rich star with [Fe/H] = −2.79, and has the highest [Pb/Fe] abundance ratio measured thus far, if NLTE corrections are included ([Pb/Fe] = +3.84). HE 2258−6358, with [Fe/H] = −2.67, exhibits enrichments in both s-and r-process elements. These stars were selected from a sample of candidate metal-poor stars from the Hamburg/ESO objective-prism survey, and followed up with medium-resolution (R ∼ 2, 000) spectroscopy with GEM-INI/GMOS. We report here on derived abundances (or limits) for a total of 34 elements in each star, based on high-resolution (R ∼ 30, 000) spectroscopy obtained with Magellan-Clay/MIKE. Our results are compared to predictions from new theoretical AGB nucleosynthesis models of 1.3 M ⊙ with [Fe/H] = −2.5 and −2.8, as well as to a set of AGB models of 1.0 to 6.0 M ⊙ at [Fe/H] = −2.3. The agreement with the model predictions suggests that the neutron-capture material in HE 2138−3336 originated from mass transfer from a binary companion star that previously went through the AGB phase, whereas for HE 2258−6358, an additional process has to be taken into account to explain its abundance pattern. We find that a narrow range of progenitor masses (1.0≤ M(M ⊙ ) ≤1.3) and metallicities (−2.8 ≤ [Fe/H] ≤ −2.5) yield the best agreement with our observed elemental abundance patterns.
s ‐Process Nucleosynthesis in Asymptotic Giant Branch Stars: A Test for Stellar Evolution
The Astrophysical Journal, 2003
We study the slow neutron capture process (s process) in Asymptotic Giant Branch (AGB) stars using three different stellar evolutionary models computed for a 3 M ⊙ and solar metallicity star. First we investigate the formation and the efficiency of the main neutron source: the 13 C(α,n) 16 O reaction that occurs in radiative conditions. A tiny region rich in 13 C (the 13 C pocket) is created by proton captures on the abundant 12 C in the top layers of the He intershell, the zone between the H shell and the He shell. We parametrically vary the number of protons mixed from the envelope. For high local protons over 12 C number ratio, p/ 12 C ∼ > 0.3, most of the 13 C nuclei produced are further converted by proton capture to 14 N. Besides, 14 N nuclei represent a major neutron poison. We find