Molybdenum and ruthenium in the Galactic disk: A closer look at their nucleosynthesis components

TL;DR

This study provides new Mo and Ru abundances in a large sample of Galactic-disk giants (−1 < [Fe/H] < +0.3) using LTE spectral synthesis of Mo I and Ru I lines, complemented by Sr and Zr measurements. The authors find that [Mo/Fe] and [Ru/Fe] decline with metallicity, mirroring [Zr/Fe], while [Sr/Fe] remains roughly flat, and they show that the observed [Zr/Mo] and [Ru/Mo] ratios are generally lower and higher, respectively, than simple $s$-process expectations. The observed scatter in these ratios suggests that, in addition to the classical $s$- and $r$-process, other nucleosynthesis components (e.g., the $i$-process, LEPP, or neutrino-driven winds) may contribute to Mo and Ru in the Milky Way disk; however, nuclear-physics uncertainties and NLTE effects complicate definitive conclusions. The work highlights the need for NLTE corrections for Mo and Ru and improved nuclear data to disentangle the relative roles of multiple neutron-capture channels in Galactic chemical evolution, and it provides a valuable data resource via CDS for future modeling.

Abstract

The stellar origin of the elements molybdenum (Mo, Z=42) and ruthenium (Ru, Z=44) is still a matter of debate. Studying their abundances provides valuable insights into nucleosynthesis processes and the broader evolution of neutron-capture elements. We presented new observations of Mo and Ru, together with nearby neutron-capture elements strontium (Sr) and zirconium (Zr) for a new sample of 154 giant stars, located in the Galactic disk with metallicities -1 < [Fe/H] < +0.3. The abundances were determined under the assumption of the local thermodynamic equilibrium by fitting synthetic spectra. The abundances of Mo were derived from the Mo I lines at 5506 and 5533 A, the abundances of Ru were determined from Ru I lines at 4584, and 4757 A. For most of the giant stars observed in this work, Mo and Ru abundances were determined for the first time. We compare our observations with the signatures from different nucleosynthesis processes. Both the [Mo/Fe] and [Ru/Fe] in our stars show a decreasing trend with respect to increasing [Fe/H]. This pattern is similar to that of [Zr/Fe], whereas [Sr/Fe] exhibits a relatively flat trend with metallicity. Compared to the s-process ratios, all stars show a lower [Zr/Mo] and a higher [Ru/Mo], as expected from classical nucleosynthesis. Still, it is unclear if additional contributions from neutrino-wind components or the i-process is needed to explain the observed scatter of [Zr/Mo] and [Ru/Mo] in the Milky Way disk. Indeed, such a dispersion is consistent with the variations also seen in r-II stars at low metallicity and could therefore result from the combined contributions of r-process and s-process to galactic chemical evolution. The observed [Zr/Mo] and [Ru/Mo] scatter in r-II stars should be constrained by future investigations to define if any contributions of additional nucleosynthesis components are needed.

Figures (3)

• Figure 1: HD 221345: Synthetic (solid line) and observed (asterisks) spectra for Mo I lines. Spectra calculated with a Mo abundance differing by 0.1 dex and without Mo are shown as dashed and dotted lines, respectively.
• Figure 2: [Sr/Fe], [Zr/Fe], [Mo/Fe], and [Ru/Fe] with respect to [Fe/H] given for the giant stars in our sample (magenta points), the dwarf stars by mishenina:19amishenina:19bmishenina:13 (thin disk stars as black points, and thick disk stars as red points). The data from forsberg:19 (Zr) and forsberg:22 (Mo) are shown as blue points, while those from tautvai:21 -- Sr and Zr I -- are shown as open purple circles, and Zr II as open green triangles.
• Figure 3: [Ru/Mo] for the stellar sample presented in this work, shown with respect to [Zr/Mo]. For comparison, the metal-poor stars from honda:07hansen:12Hansen:14Aoki:17 and psaltis:24 are shown as white squares with green contours. The ratios from the $s$-process GCE simulations by bisterzo:14 (green line and error range) and from the r-II star CS 22892-052 sneden:03 (green line and error range) are included as references. The production range derived from a collection of r-II stars is indicated by the blue horizontal and vertical error bars for [Zr/Mo] and [Ru/Mo], respectively (see the text for details). We also report the LEPP prediction by travaglio:04 (large cyan square), the ratios from different neutrino-driven ejecta conditions in psaltis:24 with $Y_{\rm e}$$>$ 0.5 (orange triangles, $\nu p$-process) and with $Y_{\rm e}$$<$ 0.5 (red squares, weak r-process), along with $i$-process predictions for mild neutron exposures, characterized by production peaking in the Zr-Ru region, and no relevant production at the Ba peak (magenta-lined squares; see text for more details). The typical observation error for our stars is given in the lower left corner of the plot (black error bar).