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

Abstract: The chemical evolution of the Universe is governed by the chemical yields from stars, which in turn is determined primarily by the initial stellar mass. Even stars as low as 0.9Msun can, at low metallicity, contribute to the chemical evolution of elements. Stars less massive than about 10Msun 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 ~10Msun 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 10Msun 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.

• The paper presents a comprehensive review of nucleosynthesis in low and intermediate-mass stars, emphasizing key dredge-up events.
• It details the impact of thermal pulses and hot bottom burning on the production of carbon, nitrogen, and s-process elements.
• Model uncertainties in convection, mass loss, and 13C-pocket formation are discussed, outlining challenges in stellar yield predictions.

Nucleosynthesis and Stellar Yields in Low and Intermediate-Mass Stars

The paper by Karakas and Lattanzio provides a comprehensive review of the nucleosynthesis processes occurring in low and intermediate-mass stars, specifically those up to about 10 solar masses. The research encapsulates the intricacies of mixing events and their implications on the chemical enrichment of the interstellar medium (ISM). This review is vital for understanding the stellar contributions to the chemical evolution of galaxies.

Stellar Evolution and Nuclear Processes

The nucleosynthesis journey in low and intermediate-mass stars is marked by several phases of evolution that impact their yield. Starting from the main sequence, stars evolve into giants, undergoing first and second dredge-up processes. These processes bring the products of hydrogen burning to the surface. The most notable phase for nucleosynthesis, however, is the asymptotic giant branch (AGB), characterized by thermal pulses and third dredge-up (TDU) phenomena. During the AGB phase, stars experience thermal instabilities in their helium-burning shells, leading to significant changes in the surface composition due to TDU.

Third Dredge-Up and Hot Bottom Burning

The efficiency of TDU is quantified by the dredge-up parameter (λ), describing how much material from the H-exhausted core is mixed into the envelope. It is pivotal for the creation of carbon stars (C-stars) where carbon production exceeds oxygen on the surface. TDU, combined with hot bottom burning (HBB), where envelopes reach temperatures allowing CNO cycle reactions, contributes to the enrichment of carbon, nitrogen, and heavy s-process elements. HBB is more prominent in massive AGB stars and influences the production of nitrogen and destruction of lithium.

s-Process Nucleosynthesis

AGB stars are a primary site for slow neutron capture process (s-process) nucleosynthesis. They produce a plethora of heavy elements when neutrons are captured by seed nuclei. The paper elaborates on the two main neutron sources: the $^{13}$C($\alpha$,n)$^{16}$O reaction and the $^{22}$Ne($\alpha$,n)$^{25}$Mg reaction, active at distinct stellar masses and conditions. The efficiency and resultant element production from these processes have significant implications for understanding solar and stellar abundances.

Uncertainties and Model Sensitivities

The complexity of stellar interiors mandates a reliance on models fraught with uncertainties, particularly in convection, mass loss, and mixing processes. Convection affects the occurrence and efficiency of TDU, thereby altering yields. Mass loss remains a critical unknown that influences the duration of the AGB phase and subsequent yields. Furthermore, the formation of $^{13}$C pockets—a necessary precursor for forming neutron sources crucial for s-process—is still not well understood due to limitations in one-dimensional models.

Implications and Future Research Directions

The contributions of low and intermediate-mass stars to galactic chemical evolution are indispensable, especially in producing elements like carbon, nitrogen, and s-process elements (e.g., barium, strontium). This research stresses the importance of incorporating AGB yields into chemical evolution models of galaxies to attain a complete picture of elemental abundance distributions. Enhanced models, informed by observations from instruments like the James Webb Space Telescope and advancements in computational astrophysics, are essential for further reducing uncertainties in nucleosynthetic pathways.

In summary, this review emphasizes the need for a deeper understanding of the microphysics governing stellar interiors and the necessity of comprehensive stellar yields encompassing a wide array of metallicities and stellar masses. Enhanced collaboration between observations and model development will anchor future breakthroughs in astrophysical nucleosynthesis.