The Origin of Elements from Carbon to Uranium

Abstract: 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_\odot can become failed supernovae if there is a significant contribution from hypernovae (HNe) at M~20-50M_\odot. The contribution to GCE from super asymptotic giant branch (AGB) stars (with M~8-10M_\odot 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, and 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 25-50 M_\odot hypernovae 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 archaeology surveys.

• The paper presents a comprehensive Galactic Chemical Evolution model that integrates multiple stellar events to trace the synthesis of elements from carbon to uranium.
• It evaluates the distinct roles of AGB stars, various supernovae types, and neutron star mergers, emphasizing the impact of failed and magneto-rotational supernovae on observed elemental ratios.
• The findings highlight anomalies in predicted abundances across galactic environments, calling for enhanced simulations and refined nucleosynthesis yields to resolve these discrepancies.

An Analytical Examination of Elemental Origin from Carbon to Uranium

The study titled "The Origin of Elements from Carbon to Uranium" by Chiaki Kobayashi et al. provides a comprehensive model of Galactic Chemical Evolution (GCE), tracing the synthesis and enrichment processes for stable elements ranging from carbon (C) to uranium (U). With a robust theoretical framework, this paper underscores the significance of various astrophysical sites and events in shaping the elemental composition of galaxies, particularly the Milky Way.

Galactic Chemical Evolution Models

The research employs an elaborate GCE model incorporating theoretical nucleosynthesis yields and event rates. By doing so, it evaluates the contributions of diverse stellar phenomena: asymptotic giant branch (AGB) stars, supernovae types Ia, II, and hypernovae (HNe), electron-capture supernovae (ECSNe), neutron star mergers (NSMs), and magneto-rotational supernovae (MRSNe). Each of these processes plays a distinct role in the creation and distribution of stable elements across different metallicities and stellar environments.

Failed Supernovae and Elemental Production

A pivotal aspect of the paper is its focus on failed supernovae—massive stars ($M > 30M_\odot$) that collapse into black holes without ejecting significant heavy elements—challenging existing paradigms concerning stellar life cycles. The incorporation of failed supernovae into the model enhances the understanding of observational data, particularly for [O/Fe] ratios, which align well with NLTE abundance analyses when factoring the reduced core-collapse supernova output at higher stellar masses.

Contribution of Super AGB Stars and ECSNe

The study also explores the role of super AGB stars, whose contributions to GCE are generally negligible unless conditions favor Type Iax or ECSN outcomes, which produce elements under atypical star formation scenarios or in metal-poor environments. Furthermore, ECSNe enrich lighter neutron-capture elements (e.g., Sr, Y, Zr) notably, without requiring the postulation of a distinct LEPP (Lighter Element Primary Process).

Neutron Star Mergers and MRSNe's Role

While NSMs are crucial for producing heavy r-process elements like Th and U, the modeling pinpoints shortcomings in their contribution due to extended timescales that fall short of explaining early enrichment in low-metallicity stars. This inadequacy is supplemented through MRSNe, bridging the observational discrepancy for r-process elemental abundances such as Eu. The paper posits that approximately 3% of $25-50M_\odot$ hypernovae are MRSNe, which satisfactorily accounts for Eu's observed evolution in the galactic environment.

Elemental Anomalies and Observational Discrepancy

Despite the strengths, the models reveal areas needing refinement—such as overproduction of some elements like Ag and underproduction of others, e.g., Au. These discrepancies could hint at inaccuracies in nuclear reaction rates or simplifications within GCE models regarding multi-dimensional stellar processes.

Variability Across Galactic Environments

The study acknowledges spatial metallicity gradients, emphasizing that galactic factors such as star formation rates and outflows significantly affect elemental ratios. The model predicts lower s-process yields in rapidly forming regions, aligning variably with bulge and halo observations, thus requiring localized GCE studies for more granular validation.

In summary, this meticulous study advances the understanding of element synthesis in galaxies, providing a pivotal reference point for further theoretical developments and observational verification. Future research must integrate enhancements such as multi-dimensional simulations and precision in nucleosynthesis yields to fully resolve outstanding discrepancies, ensuring a comprehensive grasp of the cosmic origin and dispersion of elements.