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 6 M⊙. 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 12C to form a 13C pocket. The 13C pocket increases the yields of 19F, 23Na, the neutron‐rich Mg and Si isotopes, 60Fe and 31P. The increase in 31P is by factors of ∼4 to 20, depending on the metallicity. Any structural changes caused by the addition of the 13C 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.
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. Keywords
To reach a deeper understanding of the origin of elements in the periodic table, we construct Galactic chemical evolution (GCE) models for all stable elements from C (A = 12) to U (A = 238) from first principles, i.e., using theoretical nucleosynthesis yields and event rates of all chemical enrichment sources. This enables us to predict the origin of elements as a function of time and environment. In the solar neighborhood, we find that stars with initial masses of M > 30M ⊙ can become failed supernovae if there is a significant contribution from hypernovae (HNe) at M ∼ 20–50M ⊙. The contribution to GCE from super-asymptotic giant branch (AGB) stars (with M ∼ 8–10M ⊙ at solar metallicity) is negligible, unless hybrid white dwarfs from low-mass super-AGB stars explode as so-called Type Iax supernovae, or high-mass super-AGB stars explode as electron-capture supernovae (ECSNe). Among neutron-capture elements, the observed abundances of the second (Ba) and third (Pb) peak elements are well reproduced with our updated yields of the slow neutron-capture process (s-process) from AGB stars. The first peak elements (Sr, Y, Zr) are sufficiently produced by ECSNe together with AGB stars. Neutron star mergers can produce rapid neutron-capture process (r-process) elements up to Th and U, but the timescales are too long to explain observations at low metallicities. The observed evolutionary trends, such as for Eu, can well be explained if ∼3% of 25–50M ⊙ HNe are magneto-rotational supernovae producing r-process elements. Along with the solar neighborhood, we also predict the evolutionary trends in the halo, bulge, and thick disk for future comparison with Galactic archeology surveys.
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