Stellar yields are a key ingredient in chemical evolution models. Low- and intermediate-mass stars are an integral part of galaxies and help shape their evolution, gas and dust content, as well as their integrated light. Even stars as low as 0.9 M⊙ can, at low metallicity, contribute to the chemical evolution of elements. The days of only considering supernovae are over. However, for low- and intermediate-mass stars to be included, theoretical predictions from stars covering a large range in mass and metallicity need to be calculated.
In this review we have discussed the various mixing processes that affect the surface composition and yields of stars less massive than about 10 M⊙. These recurrent mixing events can significantly change the surface composition of the envelope, with the richest nucleosynthesis occurring during the AGB phase of evolution. AGB stars are observed to show enrichments in C, N, F, and heavy elements synthesised by the s-process. AGB stars release their nucleosynthesis products through stellar outflows or winds, in contrast to massive stars that explode as core-collapse SNe.
Supercomputers have allowed the calculation of stellar yields from detailed (but still single!) AGB models covering large ranges in mass and composition. While significant progress has been made over the past decade, there are still crucial gaps, especially for elements produced by the s-process for all mass and metallicity ranges. This is mostly because many nuclear species (on the order of hundreds) are required to accurately model the s-process and the computational time required is still significant (e.g., months of supercomputer time on a single CPU is required for an intermediate-mass AGB model of low metallicity).
Gaps in our knowledge are also apparent for AGB stars of very low metallicity (e.g., [Fe/H] ≤ -3). More theoretical effort is needed to address these gaps, especially because current and new surveys (e.g., SEGUE, GALAH, APOGEE, and GAIA-ESO) will provide spectra of hundreds of thousands of stars in all regions of our Milky Way Galaxy including in the metal-poor halo. These huge surveys are going to drive dramatic improvements in the reliability of stellar models, by providing data that show inconsistencies and errors in our current understanding. Detailed nucleosynthesis models of AGB stars and SNe at low metallicity will be required in order to disentangle their history or to provide insight into the nature of the Galaxy at the earliest times.
Many significant uncertainties affect the stellar yield calculations, such as convection and mass loss, and these in turn affect the accuracy and reliability of chemical evolution model predictions. Convection has proven to be a persistent problem in one-dimensional stellar evolution calculations. While we have better observations with which to constrain convection and convective borders in AGB models to calibrate any given stellar evolution code, we are only slowly improving our understanding of the physics of convection in stellar interiors (Meakin & Arnett 2007; Arnett, Meakin, & Young 2009; Viallet et al. 2013).
The Spitzer Space Telescope has provided important insight into the nature of mass loss in evolved stars. We have learnt that mass-loss rates are not necessarily smaller at low metallicity owing to the copious dredge-up of primary C. We also presumably have a better understanding of the theory of mass loss, at least for C-rich AGB stars and progress is being made for O-rich AGB stars as well.
Non-standard physics such as rotation and thermohaline mixing are now starting to be included in stellar evolutionary calculations and the first yields are appearing, albeit only for a small number of isotopes. Chemical evolution calculations using these yields show the importance of these physical phenomena on the evolution of light species such as 3He, 7Li, and the C isotopes. Ideally these calculations should be extended to include all species affected by extra mixing.
Where will we be in the next 5 to 20 years? Future effort must go into understanding how convection operates in stellar interiors. This is singly the most important and crucial uncertainty and one that requires multi-dimensional calculations on supercomputers. Advances driven by supercomputers will reveal insights into the nature of 13C pocket formation in low-mass AGB stars as well as help solve the puzzle of the O abundances observed in post-AGB stars (e.g., is there really overshoot into the C-O core?). We still have some way to go to unravel these puzzles!
Supercomputers will also help drive advances in our understanding of rotation and magnetic fields in stellar interiors, as well as non-convective extra mixing processes. While progress has been made in understanding how thermohaline mixing operates in red giant envelopes, we still do not know if thermohaline mixing is efficient in AGB stars. Some form of non-convective mixing is needed to drive changes that we know occur in the envelopes of low-metallicity AGB stars (e.g., low-observed 12C / 13C ratios compared to AGB yields).
The greatest understanding of mass loss from evolved stars will be driven by observations from e.g., ALMA and James Webb Space Telescope (JWST). ALMA is already starting to probe the clumpy nature of mass loss from evolved stars and supergiants. While thermonuclear reaction rates are probably the least of our worries for low- and intermediate-mass stars, we know that some key rates (e.g., those that destroy 23Na and the neutron-producing reactions) are still highly uncertain and can effect stellar yields. New experimental facilities such as the Facility for Rare Isotope Beams being built at the University of Michigan will provide new experimental data.
Stellar yields from populations of binaries covering a range of metallicities are desperately needed. Most stars are in binaries and many will interact. The interactions can lead to dramatic outcomes such as Type Ia SNe, which play an essential role in chemical evolution (and cosmology), but also less energetic outcomes such as novae, symbiotic stars, barium and CH stars, and CEMP stars. Binary evolution will also change the yields from a single stellar population but exactly how still needs to be determined.
In the next 10 years there will be an explosion of new stellar abundance data driven by new surveys and instruments (e.g., the GALAH survey using High Efficiency and Resolution Multi-Element Spectograph (HERMES) on the Anglo-Australian Telescope (AAT), the GAIA-ESO survey, Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST), APOGEE etc.). These data will help answer big questions facing astronomy including how stars evolve and produce elements and how the elements are ejected to enrich the Universe, as well as questions related to the formation and evolution of galaxies. These tremendous investments in astronomical infrastructure will pay the largest dividends when augmented by complementary theoretical and modelling research.
The authors would like to thank the Editors for their patience in waiting for this review and the referee for providing constructive comments on the manuscript. We would also like to thank George Angelou, Harriet Dinerstein, Carolyn Doherty, Cherie Fishlock, Brad Gibson, Falk Herwig, Robert Izzard, Maria Lugaro, Brent Miszalski, David Nataf, and Richard Stancliffe for help in writing this review. A.I.K. is grateful to the ARC for support through a Future Fellowship (FT110100475). This work was partially supported by ARC grants DP120101815, DP1095368, and DP0877317.