Element abundance patterns in stars indicate fission of nuclei heavier than uranium

Abstract: The heaviest chemical elements are naturally produced by the rapid neutron-capture process (r-process) during neutron star mergers or supernovae. The r-process production of elements heavier than uranium (transuranic nuclei) is poorly understood and inaccessible to experiments, so must be extrapolated using nucleosynthesis models. We examine element abundances in a sample of stars that are enhanced in r-process elements. The abundances of elements Ru, Rh, Pd, and Ag (atomic numbers Z = 44 to 47, mass numbers A = 99 to 110) correlate with those of heavier elements (63 <= Z <= 78, A > 150). There is no correlation for neighboring elements (34 <= Z <= 42 and 48 <= Z <= 62). We interpret this as evidence that fission fragments of transuranic nuclei contribute to the abundances. Our results indicate that neutron-rich nuclei with mass numbers >260 are produced in r-process events.

• The paper identifies fission of transuranic nuclei as a significant contributor to observed heavy element abundance patterns.
• It employs a standardized sample of 42 Milky Way stars with metallicities from −3.57 to −0.99 to correlate element abundances with [Eu/Fe] ratios.
• The findings challenge existing two-component r-process models, urging refined nucleosynthesis models that incorporate comprehensive fission yield data.

Analysis of Element Abundance Patterns as Indicators of Fission in Nuclei Heavier than Uranium

The study by Roederer et al. scrutinizes the nucleosynthetic origins of heavy elements and raises significant insights into the r-process nucleosynthesis, particularly in environments resulting from neutron star mergers or specific supernovae. It examines star samples in the Milky Way enriched in heavy r-process elements to investigate correlations between the abundances of certain elements, positing that fission processes of transuranic nuclei might be a contributing factor to observed abundance patterns.

Methodology and Stellar Sample

The authors utilized a well-defined sample of 42 Milky Way stars showing heavy-element enhancement via the r-process, carefully excluding contamination from s-process or other nucleosynthesis pathways. The selection hinges on homogeneous data acquired across several documented studies, standardized using consistent atomic data. The stars were further categorized by their metallicity, specifically within the range of −3.57 to −0.99 in [Fe/H], and r-process enhancement identified by −0.52 to +1.69 in [Eu/Fe].

Findings and Correlations

A pivotal aspect of this study lies in the correlation (or lack thereof) of element abundances with the [Eu/Fe] ratio. Two distinct groups emerged showing significant correlations: the lighter elements Ru, Rh, Pd, and Ag (44 ≤ Z ≤ 47) and heavier elements Gd, Tb, Dy, Ho, Er, Tm, Yb, Hf, Os, and Pt (64 ≤ Z ≤ 78). These contrasts the lack of correlation for elements like Se, Sr, Y, Zr, Nb, Mo, Cd, Sn, Te, Ba, La, Ce, Pr, Nd, and Sm, which underscores a possible fission origin for the former groups as fission fragments of transuranic elements.

The study also challenges current two-component models of r-process explanations, which traditionally view element production as isolated to specific mass ranges. Instead, it suggests that fission of heavy, neutron-rich transuranic nuclei, initially formed during the r-process, contributes to the abundance of these lighter and heavier elements.

Implications and Theoretical Integration

The implications of these findings are significant. They propose a fission-mediated mechanism where transuranic nuclei, having mass numbers greater than 260, disintegrate, thereby altering the stellar abundance patterns observed. This could have broader implications for nucleosynthesis modeling, urging reconsideration of the roles that supernovae and neutron star mergers play in producing the heaviest chemical elements in the universe.

Furthermore, the study posits that such fission processes could harmonize relative abundances of lighter and heavy elements even under varied nucleosynthesis conditions, reflecting a potential universality within specific r-process events that has been previously unaccounted for.

Speculation on Future Research Directions

For future research, this study suggests that a more nuanced understanding of the r-process could emerge via refinement of nucleosynthesis models that incorporate comprehensive fission yield data. Future work could explore isotopic analysis of fission products, leveraging spectroscopic methods capable of resolving fine isotopic fractions. Additionally, expanding observational datasets to include more metal-poor stars with precise individual elemental abundances could yield further insights into the nature and sites of r-process nucleosynthesis beyond those currently recognized.

Conclusion

The paper by Roederer et al. opens avenues for reconsidering traditional narratives around the synthesis of heavy elements, postulating a role for fission from transuranic nuclei within r-process events. These findings call for more extensive examinations into the cosmic events leading to heavy element formation, potentially reshaping our understanding of element genesis in our universe. The exploration of such element abundance patterns provides a formidable framework to challenge and enhance current astrophysical nucleosynthesis theories.