Unraveling the Origins of Molybdenum and Ruthenium in the Milky Way Disk
Tatyana Mishenina and her collaborators investigate the cosmic origins of two relatively rare elements, molybdenum (Mo) and ruthenium (Ru), which are produced in stars through complex nuclear reactions. Despite decades of research, astronomers still debate which types of stars and stellar explosions made these elements in the first place. This study presents new measurements of Mo and Ru in 154 giant stars in our Galaxy’s disk and compares them with related elements, strontium (Sr) and zirconium (Zr), to better understand how these elements were forged and spread throughout the Milky Way.
Background and Motivation
Mishenina begins by reviewing the puzzle surrounding Mo and Ru. Each of these elements has several stable isotopes formed through different processes: the s-process (slow neutron capture in aging stars), the r-process (rapid neutron capture in extreme explosions like neutron star mergers), and possibly additional sources. Previous models struggled to reproduce the exact mix of Mo and Ru seen in the Sun and nearby stars. To explain the discrepancies, researchers proposed extra mechanisms, such as the intermediate neutron-capture process (i-process) or neutrino-driven winds from supernovae. These potential contributors, though theoretically plausible, have not yet been confirmed observationally.
Observations and Methods
To investigate further, the team observed stars using the 1.93-meter telescope at the Observatoire de Haute-Provence in France. They analyzed the starlight with a high-resolution instrument called ELODIE, which breaks light into its component wavelengths, revealing the chemical fingerprints of different elements. By fitting synthetic spectra to observed data, essentially matching model predictions to the actual absorption lines of Mo and Ru, the researchers derived precise element abundances. They used specific spectral lines for each element, such as Mo I at 5506 Å and Ru I at 4757 Å, and compared their measurements with known solar values to identify trends in element production.
Results: Abundance Trends in the Galactic Disk
The team found that [Mo/Fe] and [Ru/Fe] ratios decrease as stars become richer in metals ([Fe/H] increases), meaning that these elements were relatively more abundant in older, metal-poor stars. This trend mirrors the behavior of zirconium but differs from strontium, which remains nearly constant across the same metallicity range. The consistency between their results for giants and earlier studies of dwarf stars strengthens the reliability of these findings. These patterns suggest that Mo and Ru production is closely tied to the Galaxy’s chemical evolution, possibly reflecting varying contributions from multiple stellar processes over time.
Interpreting the Nucleosynthesis Origins
To dig deeper, Mishenina compared the ratios of [Zr/Mo] and [Ru/Mo] with predictions from models of the s-process, r-process, and LEPP (lighter element primary process), a proposed additional pathway to explain unexplained light-element abundances. The data show that none of the stars have [Zr/Mo] or [Ru/Mo] values inconsistent with these known processes, but the scatter in the results is too large to be fully explained by them alone. This variability might hint at extra contributions from the i-process or from material expelled by supernova-driven neutrino winds. Interestingly, the patterns in disk stars resemble those in r-II stars, ancient stars highly enriched in r-process material, suggesting shared nucleosynthesis origins across different environments.
Uncertainties and Future Directions
The paper also highlights the limits of current models. Part of the observed scatter might come from measurement uncertainties or from simplifications in the local thermodynamic equilibrium (LTE) assumption used in abundance calculations. The authors emphasize the need for future non-LTE (NLTE) corrections, which could refine the precision of Mo and Ru measurements. They also point to ongoing challenges in nuclear physics, such as uncertain reaction rates and decay probabilities, that affect theoretical predictions. Improved data from upcoming experiments on unstable isotopes and enhanced modeling will be essential to confirm whether exotic processes like the i-process are truly needed.
Conclusion
Mishenina and collaborators conclude that both the s-process and r-process can account for most of the Mo and Ru abundances seen in Galactic disk stars, but some stars may require additional contributions from other mechanisms. Their expanded dataset for giant stars fills an important observational gap and helps clarify how neutron-capture elements formed over the Milky Way’s history. Ultimately, the study underscores how measuring even trace elements in starlight can illuminate the long and intricate story of stellar alchemy that built the chemical richness of our Galaxy.
Source: Mishenina
