Amanda I. Karakas

Updated stellar yields from asymptotic giant branch models

An updated grid of stellar yields for low- to intermediate-mass thermally pulsing asymptotic giant branch (AGB) stars is presented. The models cover a range in metallicity Z = 0.02, 0.008, 0.004 and 0.0001, and masses between 1 and 6M ⊙ . New intermediate-mass (M ≥ 3 M ⊙ ) Z = 0.0001 AGB models are also presented, along with a finer mass grid than used in previous studies. The yields are computed using an updated reaction rate network that includes the latest NeNa and MgAl proton capture rates, with the main result that between ∼6 and 30 times less Na is produced by intermediate-mass models with hot bottom burning. In low-mass AGB models, we investigate the effect, on the production of light elements, of including some partial mixing of protons into the intershell region during the deepest extent of each third dredge-up episode. The protons are captured by the abundant 12 C to form a 13 C pocket. The 13 C pocket increases the yields of 19 F, 23 Na, the neutron-rich Mg and Si isotopes, 60 Fe and 31 P. The increase in 31 P is by factors of ∼4 to 20, depending on the metallicity. Any structural changes caused by the addition of the 13 C pocket into the He intershell are ignored. However, the models considered are of low mass and any such feedback is likely to be small. Further study is required to test the accuracy of the yields from the partial-mixing models. For each mass and metallicity, the yields are presented in a tabular form suitable for use in galactic chemical evolution studies or for comparison to the composition of planetary nebulae.

Stellar Models and Yields of Asymptotic Giant Branch Stars

We present stellar yields calculated from detailed models of low and intermediate-mass asymptotic giant branch (AGB) stars. We evolve models with a range of mass from 1 to 6 M⊙, and initial metallicities from solar to 1/200th of the solar metallicity. Each model was evolved from the zero age main sequence to near the end of the thermally pulsing (TP) AGB phase, and through all intermediate phases including the core He-flash for stars initially less massive than 2.5 M⊙. For each mass and metallicity, we provide tables containing structural details of the stellar models during the TP-AGB phase, and tables of the stellar yields for 74 species from hydrogen through to sulfur, and for a small number of iron-group nuclei. All tables are available for download. Our results have many applications including use in population synthesis studies and the chemical evolution of galaxies and stellar systems, and for comparison to the composition of AGB and post-AGB stars and planetary nebulae.

The evolution of isotope ratios in the Milky Way Galaxy

Isotope ratios have opened a new window into the study of the details of stellar evolution, supernovae, and galactic chemical evolution. We present the evolution of the isotope ratios of elemental abundances (from C to Zn) in the solar neighbourhood, bulge, halo, and thick disk, using chemical evolution models with updated yields of Asymptotic Giant Branch (AGB) stars and core-collapse supernovae. The evolutionary history of each element is different owing to the effects of the initial progenitor mass and metallicity on element production. In the bulge and thick disk the star formation timescale is shorter than in the solar neighbourhood, leading to higher [�/Fe] ratios. Likewise, the smaller contribution from Type Ia supernovae in these regions leads to lower [Mn/Fe] ratios. Also in the bulge, the abundances of [(Na, Al, P, Cl, K, Sc, Cu, Zn)/Fe] are higher because of the effect of metallicity on element production from core-collapse supernovae. According to our predictions, it is possible to find metalrich stars ([Fe/H] > 1) that formed in the early Universe as a result of rapid star formation. The chemical enrichment timescale of the halo is longer than in the solar neighbourhood, and consequently the ratios of [(C, F)/Fe] and 12 C/ 13 C are higher owing to a significant contribution from low-mass AGB stars. While the [�/Fe] and [Mn/Fe] ratios are the same as in the solar neighbourhood, the [(Na, Al, P, Cl, K, Sc, Cu, Zn)/Fe] ratios are predicted to be lower. Furthermore, we predict that isotope ratios such as 24 Mg/ 25,26 Mg are larger because of the contribution from low-metallicity supernovae. Using isotopic ratios it is possible to select stars that formed in a system with a low chemical enrichment efficiency such as the satellite galaxies t hat were accreted onto our own Milky Way Galaxy.

The Dawes Review 2: Nucleosynthesis and stellar yields of low-and intermediate-mass single stars

The chemical evolution of the Universe is governed by the chemical yields from stars, which in turn are determined primarily by the initial stellar mass. Even stars as low as 0.9 M ⊙ can, at low metallicity, contribute to the chemical evolution of elements. Stars less massive than about 10 M ⊙ experience recurrent mixing events that can significantly change the surface composition of the envelope, with observed enrichments in carbon, nitrogen, fluorine, and heavy elements synthesized by the slow neutron capture process (the s -process). Low- and intermediate-mass stars release their nucleosynthesis products through stellar outflows or winds, in contrast to massive stars that explode as core-collapse supernovae. Here we review the stellar evolution and nucleosynthesis for single stars up to ~ 10 M ⊙ from the main sequence through to the tip of the asymptotic giant branch (AGB). We include a discussion of the main uncertainties that affect theoretical calculations and review the latest observational data, which are used to constrain uncertain details of the stellar models. We finish with a review of the stellar yields available for stars less massive than about 10 M ⊙ and discuss efforts by various groups to address these issues and provide homogeneous yields for low- and intermediate-mass stars covering a broad range of metallicities.

Quantifying the uncertainties of chemical evolution studies II. Stellar yields

Context. Galactic chemical evolution models are useful tools for interpreting the large body of high-quality observational data on the chemical composition of stars and gas in galaxies that have become available in recent years. Aims. This is the second paper of a series that aims at quantifying the uncertainties in chemical evolution model predictions related to the underlying model assumptions. Specifically, it deals with the uncertainties due to the choice of the stellar yields. Methods. We adopted a widely used model for the chemical evolution of the Galaxy to test the effects of changing the stellar nucleosynthesis prescriptions on the predicted evolution of several chemical species. Up-to-date results from stellar evolutionary models were carefully taken into account. Results. We find that, except for a handful of elements whose nucleosynthesis in stars is well understood by now, large uncertainties still affect model predictions. This is especially true for the majority of the iron-peak elements, but also for much more abundant species such as carbon and nitrogen. The main causes of the mismatch we find among the outputs of different models assuming different stellar yields and among model predictions and observations are (i) the adopted location of the mass cut in models of type II supernova explosions; (ii) the adopted strength and extent of hot bottom burning in models of asymptotic giant branch stars; (iii) the neglection of the effects of rotation on the chemical composition of the stellar surfaces; (iv) the adopted rates of mass loss and of (v) nuclear reactions; and (vi) the different treatments of convection. Conclusions. Our results suggest that it is mandatory to include processes such as hot bottom burning in intermediate-mass stars and rotation in stars of all masses in accurate studies of stellar evolution and nucleosynthesis. In spite of their importance, both these processes still have to be better understood and characterized. As for massive stars, presupernova models computed with mass loss and rotation are available in the literature, but they still wait for a self-consistent coupling with the results of explosive nucleosynthesis computations.

Parameterising the third dredge-up in asymptotic giant branch stars

We present new evolutionary sequences for low and intermediate mass stars (1–6M⊙) for three different metallicities, Z = 0.02, 0.008, and 0.004. We evolve the models from the pre-main sequence to the thermally-pulsing asymptotic giant branch phase. We have two sequences of models for each mass, one which includes mass loss and one without mass loss. Typically 20 or more pulses have been followed for each model, allowing us to calculate the third dredge-up parameter for each case. Using the results from this large and homogeneous set of models, we present an approximate fit for the core mass at the first thermal pulse, Mc1, as well as for the third dredge-up efficiency parameter, λ, and the core mass at the first dredge-up episode, Mcmin, as a function of metallicity and total mass. We also examine the effect of a reduced envelope mass on the value of λ.

Production of Aluminium and the Heavy Magnesium Isotopes in Asymptotic Giant Branch Stars

We investigate the production of aluminium and magnesium in asymptotic giant branch models covering a wide range in mass and composition. We evolve models from the pre-main sequence, through all intermediate stages, to near the end of the thermally-pulsing asymptotic giant branch phase. We then perform detailed nucleosynthesis calculations from which we determine the production of the magnesium and aluminium isotopes as a function of the stellar mass and composition. We present the stellar yields of sodium and the magnesium and aluminium isotopes. We discuss the abundance predictions from the stellar models in reference to abundance anomalies observed in globular cluster stars.

A LARGE C+N+O ABUNDANCE SPREAD IN GIANT STARS OF THE GLOBULAR CLUSTER NGC 1851

Abundances of C, N, and O are determined in four bright red giants that span the known abundance range for light (Na and Al) and s-process (Zr and La) elements in the globular cluster NGC 1851. The abundance sum C+N+O exhibits a range of 0.6 dex, a factor of 4, in contrast to other clusters in which no significant C+N+O spread is found. Such an abundance range offers support for the Cassisi et al. scenario in which the double subgiant branch populations are coeval but with different mixtures of C+N+O abundances. Further, the Na, Al, Zr, and La abundances are correlated with C+N+O, and therefore NGC 1851 is the first cluster to provide strong support for the scenario in which asymptotic giant branch stars are responsible for the globular cluster light element abundance variations.

The remarkable solar twin HIP 56948: a prime target in the quest for other Earths

Context. The Sun shows abundance anomalies relative to most solar twins. If the abundance peculiarities are due to the formation of inner rocky planets, that would mean that only a small fraction of solar type stars may host terrestrial planets. Aims. In this work we study HIP 56948, the best solar twin known to date, to determine with an unparalleled precision how similar it is to the Sun in its physical properties, chemical composition and planet architecture. We explore whether the abundances anomalies may be due to pollution from stellar ejecta or to terrestrial planet formation. Methods. We perform a differential abundance analysis (both in LTE and NLTE) using high resolution (R ~ 100 000) high S/N (600–650) Keck HIRES spectra of the Sun (as reflected from the asteroid Ceres) and HIP 56948. We use precise radial velocity data from the McDonald and Keck observatories to search for planets around this star. Results. We achieve a precision of σ ≲ 0.003 dex for several elements. Including errors in stellar parameters the total uncertainty is as low as σ ≃ 0.005 dex (1%), which is unprecedented in elemental abundance studies. The similarities between HIP 56948 and the Sun are astonishing. HIP 56948 is only 17 ± 7 K hotter than the Sun, and log g, [Fe/H] and microturbulence velocity are only + 0.02 ± 0.02 dex, +0.02 ± 0.01 dex and +0.01 ± 0.01 km s^(-1) higher than solar, respectively. Our precise stellar parameters and a differential isochrone analysis shows that HIP 56948 has a mass of 1.02 ± 0.02 M_⊙ and that it is ~1 Gyr younger than the Sun, as constrained by isochrones, chromospheric activity, Li and rotation. Both stars show a chemical abundance pattern that differs from most solar twins, but the refractory elements (those with condensation temperature T_(cond) ≳ 1000 K) are slightly (~0.01 dex) more depleted in the Sun than in HIP 56948. The trend with T_(cond) in differential abundances (twins − HIP 56948) can be reproduced very well by adding ~3 M_⊕ of a mix of Earth and meteoritic material, to the convection zone of HIP 56948. The element-to-element scatter of the Earth/meteoritic mix for the case of hypothetical rocky planets around HIP 56948 is only 0.0047 dex. From our radial velocity monitoring we find no indications of giant planets interior to or within the habitable zone of HIP 56948. Conclusions. We conclude that HIP 56948 is an excellent candidate to host a planetary system like our own, including the possible presence of inner terrestrial planets. Its striking similarity to the Sun and its mature age makes HIP 56948 a prime target in the quest for other Earths and SETI endeavors.

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 22 Ne(� , n) 25 Mg and 22 Ne(� , � ) 26 Mg 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 22 Ne+� 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 22 Ne+� -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 22 Ne+� -capture rates, when using our new estimates. Much effort is needed to improve the situation for AGB models. Subject headingg nuclear reactions, nucleosynthesis, abundances — stars: AGB and post-AGB — stars: evolution — stars: interiors