Molybdenum and Ruthenium in the Galactic Disc

We derive Sr, Zr, and Mo abundances for all stars in our sample, and Ru for 110 stars. For most of the giant stars observed in this work, the Mo and Ru abundances were determined for the first time. We compare our observations with the signature from different nucleosynthesis processes.

Both the [Mo/Fe] and [Ru/Fe] in our stars show a decreasing trend with respect to the increasing [Fe/H]. This pattern is similar to [Zr/Fe], whereas [Sr/Fe] shows a rather at trend with respect to metallicity. These results are compatible with our previous measurements obtained for dwarf stars over the same metallicity range.

Compared to the s-process ratios, all stars show a lower [Zr/Mo] and a higher [Ru/Mo], as expected from classical nucleosynthesis. Instead, it is unclear if additional contributions from neutrino-wind components or the i-process are needed to explain the observed scatter of [Zr/Mo] and [Ru/Mo] in the Milky Way disk. Indeed, such a dispersion is compatible with the variation also seen in r-II stars at low metallicity, and therefore it could just be a product of the combination of r-process and s-process contributions 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.