Enrichment of the Galactic disc with neutron-capture elements: Mo and Ru
  We present new observational data for the heavy elements molybdenum (Mo, Z =
42) and ruthenium (Ru, Z = 44) in F-, G-, and K-stars belonging to different
substructures of the Milky Way. The range of metallicity covered is --1.0 $<$
[Fe/H] $<$ +0.3. The spectra of Galactic disc stars have a high resolution of
42,000 and 75,000 and signal-to-noise ratio better than 100. Mo and Ru
abundances were derived by comparing the observed and synthetic spectra in the
region of Mo I lines at 5506, 5533 \AA~ for 209 stars and Ru I lines at 4080,
4584, 4757 \AA~ for 162 stars using the LTE approach. For all the stars, the Mo
and Ru abundance determinations are obtained for the first time with an average
error of 0.14 dex. This is the first extended sample of stellar observations
for Mo and Ru in the Milky Way disc, and together with earlier observations in
halo stars it is pivotal in providing a complete picture of the evolution of Mo
and Ru across cosmic timescales.
  The Mo and Ru abundances were compared with those of the neutron-capture
elements (Sr, Y, Zr, Ba, Sm, Eu). The complex nucleosynthesis history of Mo and
Ru is compared with different Galactic Chemical Evolution (GCE) simulations. In
general, present theoretical GCE simulations show underproduction of Mo and Ru
at all metallicities compared to observations. This highlights a significant
contribution of nucleosynthesis processes not yet considered in our
simulations. A number of possible scenarios are discussed.
