Impact of AGB stars on the chemical evolution of neutron-capture elements

TL;DR

The paper surveys four decades of Galactic Chemical Evolution research to explain how s-process nucleosynthesis in AGB stars shapes the Milky Way's neutron-capture element abundances. It emphasizes non-IRA, metallicity-dependent yields, and the interplay between main s-process in low-mass AGB stars and an early, faster r-process, with additional insights from rotating massive stars and, more recently, magneto-hydrodynamic mixing that forms the $^{13}C$ pocket. Key findings include the dominance of AGB-produced Ba on long timescales, the necessity of an early r-process to account for metal-poor stars, and the utility of combining precise stellar yields with detailed SF histories to reproduce observed trends across Galactic components. The work demonstrates that refining AGB yields (including MHD-induced mixing) and integrating them into GCE models provides strong constraints on nucleosynthesis sites and the Galaxy's formation history, illustrating the power of Galactic archaeology for nuclear astrophysics.

Abstract

In this paper we discuss the impact of the s-process nucleosynthesis in Asymptotic Giant Branch stars on the enrichment of heavy elements. We review the main steps made on this subject in the last 40 years and discuss the importance of modelling the evolution of the abundances of such elements in our Milky Way. From the comparison between model results and observations, we can impose strong constraints on stellar nucleosynthesis as well as on the evolution of the Milky Way.

Figures (8)

• Figure S1: Observed behaviour of the ratios of Y and Ba relative to Fe together with the expected trend for purely secondary elements ([s/Fe]) relative to Fe. Image reproduced with permission from Truran81, copyright by the authors.
• Figure S2: [Ba/Fe] vs [Fe/H] in the top panel and [Ba/Eu] vs [Fe/H] in the bottom panel. The short dashed line describes the halo, the long dashed shows the thick disk and solid line is for the thin disk. Observational data are from Gratton94 (filled squares); Woolf95 (open squares); Francois96 (pentagons); McWilliam95 and McWilliam98 (circles); Norris97 (filled triangles); Jehin99 (filled tilted triangles; Mashonkina99 (open triangles). Thin dotted lines connect stars with different abundance determinations. Image reproduced with permission from Travaglio99, copyright by the authors.
• Figure S3: [Ba/Eu] vs [Fe/H]. Data (filled squares) and lower limits (open square) by Franc07; in blue observational data from Burris00Fulbright00Koch02Honda04Mashonkina00Mashonkina01. The solid line is the prediction of the best model 1, short dashed line and the long dashed line are the predictions of max and min models, able to include most the observational data for [Ba/Fe] vs [Fe/H]. Image reproduced with permission from Cesc06, copyright by the authors.
• Figure S4: Average [Ba/Fe] as a function of stellar age for the subsample of clusters whose analysis is based on dwarfs (filled circles and inverted triangles) compared with the abundance pattern of disk stars (open triangles) by Bensby05. Filled triangles represent abundance measurements that probably need NLTE (Non Local Thermodynamc Equilibrium) corrections Baratella21 The model results are shown for two set of yields: (a) standard yields Travaglio99Busso01, long-dashed curve; and (b) enhanced s-process yields, dot-dashed curve. Both models show a peak at old ages due to the r-process from massive stars. Image reproduced with permission from Dorazi09, copyright by the authors.
• Figure S5: Comparison between observed and predicted [Ba/Fe] ratios vs. [Fe/H]. The black dots, track, and shadowed area are the observations (sources listed in Table 1); dark blue line is model with magneto-rotationally driven SNe Thielemann11 as r-process sources; lighter blue line is the model with neutron star mergers as r-process source with variations in the time delay (from darker to lighter) with $\tau$ = 0, 1, 10, and 100 Myr. Image reproduced with permission from Rizzuti19, copyright by the authors.