Background: The competing ${}^{22}\mathrm{Ne}(\alpha ,\gamma ){}^{26}\mathrm{Mg}$ and ${}^{22}\mathrm{Ne}(\alpha ,n){}^{25}\mathrm{Mg}$ reactions control the production of neutrons for the weak $s$ process in massive and asymptotic giant branch (AGB) stars. In both systems, the ratio between the corresponding reaction rates strongly impacts the total neutron budget and strongly influences the final nucleosynthesis. A number of experimental studies have been performed over recent years which necessitate the reevaluation of the ${}^{22}\mathrm{Ne}(\alpha ,\gamma ){}^{26}\mathrm{Mg}$ and ${}^{22}\mathrm{Ne}(\alpha ,n){}^{25}\mathrm{Mg}$ reaction rates. Evaluations of the reaction rates following the collection of new nuclear data presently show differences of up to a factor of 500, resulting in considerable uncertainty in the resulting nucleosynthesis.

Background: Globular clusters are known to exhibit anomalous abundance trends such as the sodium-oxygen anticorrelation. This trend is thought to arise via pollution of the cluster interstellar medium from a previous generation of stars. Intermediate-mass asymptotic giant branch stars undergoing hot bottom burning (HBB) are a prime candidate for producing sodium-rich oxygen-poor material, and then expelling this material via strong stellar winds. The amount of ${}^{23}\mathrm{Na}$ produced in this environment has been shown to be sensitive to uncertainties in the ${}^{22}\mathrm{Ne}(p,\gamma ){}^{23}\mathrm{Na}$ reaction rate. The ${}^{22}\mathrm{Ne}(p,\gamma ){}^{23}\mathrm{Na}$ reaction is also activated in classical nova nucleosynthesis, strongly influencing predicted isotopic abundance ratios in the Na-Al region. Therefore, improved nuclear physics uncertainties for this reaction rate are of critical importance for the identification and classification of pre-solar grains produced by classical novae.

The origin of the proton-rich trans-iron isotopes in the Solar system is still uncertain. Single-degenerate thermonuclear supernovae (SNIa) with n-capture nucleosynthesis seeds assembled in the external layers of the progenitor’s rapidly accreting white dwarf (RAWD) phase may produce these isotopes. We calculate the stellar structure of the accretion phase of five white dwarf (WD) models with initial masses ≥ 0.85 $\, \mathrm{M}_\odot$ using the stellar code mesa The near-surface layers of the 1, 1.26, 1.32 and 1.38 $\, \mathrm{M}_\odot$ models are most representative of the regions in which the bulk of the p nuclei are produced during SNIa explosions, and for these models we also calculate the neutron-capture nucleosynthesis in the external layers. Contrary to previous RAWD models at lower mass, we find that the H-shell flashes are the main site of n-capture nucleosynthesis. We find high neutron densities up to several 1015 cm−3 in the most massive WDs. Through the recurrence of the H-shell flashes these intermediate neutron densities can be sustained effectively for a long time leading to high-neutron exposures with a strong production up to Pb. Both the neutron density and the neutron exposure increase with increasing the mass of the accreting WD. Finally, the SNIa nucleosynthesis is calculated using the obtained abundances as seeds. We obtain solar to supersolar abundances for p-nuclei with A > 96. Our models show that SNIa are a viable p-process production site.

We studied $\alpha$ cluster states in ${}^{26}\mathrm{Mg}$ via the ${}^{22}\mathrm{Ne}({}^6\mathrm{Li},d\gamma ){}^{26}\mathrm{Mg}$ reaction in inverse kinematics at an energy of 7 MeV/nucleon. States between $E_x$ = 4–14 MeV in ${}^{26}\mathrm{Mg}$ were populated and relative $\alpha$ spectroscopic factors were determined. Some of these states correspond to resonances in the Gamow window of the ${}^{22}\mathrm{Ne}(\alpha ,\mathrm{n})$${}^{25}\mathrm{Mg}$ reaction, which is one of the main neutron sources in the astrophysical $s$-process. Using our new ${}^{22}\mathrm{Ne}(\alpha ,\mathrm{n})$${}^{25}\mathrm{Mg}$ and ${}^{22}\mathrm{Ne}(\alpha ,\gamma ){}^{26}\mathrm{Mg}$ reaction rates, we performed new $s$-process calculations for massive stars and asymptotic giant branch stars and compared the resulting abundances with the abundances obtained using other ${}^{22}\mathrm{Ne}+\alpha$ rates from the literature. We observe an impact on the $s$-process abundances up to a factor of three for intermediate-mass AGB stars and up to a factor of ten for massive stars. Additionally, states in ${}^{25}\mathrm{Mg}$ at $E_x <$ 7.5 MeV are identified via the ${}^{22}\mathrm{Ne}({}^6\mathrm{Li},t){}^{25}\mathrm{Mg}$ reaction for the first time. We present the $({}^6\mathrm{Li}, t)$ spectroscopic factors of these states and note similarities to the $(d,p)$ reaction in terms of reaction selectivity.

The slow neutron-capture process (s-process) efficiency in low-mass AGB stars (1.5 < M/M${}_{\odot }$ < 3) critically depends on how mixing processes in stellar interiors are handled, which is still affected by considerable uncertainties. In this work, we compute the evolution and nucleosynthesis of low-mass AGB stars at low metallicities using the MESA stellar evolution code. The combined data set includes models with initial masses M${}_{ini}$/M${}_{\odot }=2$ and 3 for initial metallicities $Z=0.001$ and $0.002$. The nucleosynthesis was calculated for all relevant isotopes by post-processing with the NuGrid mppnp code. Using these models, we show the impact of the uncertainties affecting the main mixing processes on heavy element nucleosynthesis, such as convection and mixing at convective boundaries. We finally compare our theoretical predictions with observed surface abundances on low-metallicity stars. We find that mixing at the interface between the He-intershell and the CO-core has a critical impact on the s-process at low metallicities, and its importance is comparable to convective boundary mixing processes under the convective envelope, which determine the formation and size of the ¹³C-pocket. Additionally, our results indicate that models with very low to no mixing below the He-intershell during thermal pulses, and with a ¹³C-pocket size of at least ∼3 × 10${}^{-4}$ M${}_{\odot }$, are strongly favored in reproducing observations. Online access to complete yield data tables is also provided.