This work presents a large consistent study of molybdenum ( Mo ) and ruthenium ( Ru ) abundances in the Milky Way .
These two elements are important nucleosynthetic diagnostics .
In our sample of 71 Galactic metal-poor field stars , we detect Ru and/or Mo in 51 of these ( 59 including upper limits ) .
The sample consists of high-resolution , high signal-to-noise spectra covering both dwarfs and giants from [ Fe/H ] = -0.63 down to -3.16 .
Thus we provide information on the behaviour of Mo I and Ru I at higher and lower metallicity than is currently known .
In this sample we find a wide spread in the Mo and Ru abundances , which is typical of heavy elements .
We confirm earlier findings of Mo enhanced stars around [ Fe/H ] = -1.5 and add \sim 15 stars both dwarfs and giants with normal ( < 0.3 dex ) Mo and Ru abundances , as well as more than 15 Mo and Ru enhanced ( > 0.5 dex ) stars to the currently known stellar sample .
This indicates that several formation processes , in addition to high entropy winds , can be responsible for the formation of elements like Mo and Ru .
We trace the formation processes by comparing Mo and Ru to elements ( Sr , Zr , Pd , Ag , Ba , and Eu ) with known formation processes .
Based on how tight the two elements correlate with each other , we are able to distinguish if they share a common formation process and how important this contribution is to the element abundance .
We find clear indications of contributions from several different formation processes , namely the p-process , and the slow ( s- ) , and rapid ( r- ) neutron-capture processes .
From these correlations we find that Mo is a highly convolved element that receives contributions from both the s-process and the p-process and less from the main and weak r-processes , whereas Ru is mainly formed by the weak r-process as is silver .
We also compare our absolute elemental stellar abundances to relative isotopic abundances of presolar grains extracted from meteorites .
Their isotopic abundances can be directly linked to the formation process ( e.g .
r-only isotopes ) providing a unique comparison between observationally derived abundances and the nuclear formation process .
The comparison to abundances in presolar grains shows that the r-/s-process ratios from the presolar grains match the total elemental chemical composition derived from metal-poor halo stars with [ Fe/H ] around -1.5 to -1.1 dex .
This indicates that both grains and stars around and above [ Fe/H ] = -1.5 are equally ( well ) mixed and therefore do not support a heterogeneous presolar nebula .
An inhomogeneous interstellar medium ( ISM ) should only be expected at lower metallicities .
Our data , combined with the abundance ratios of presolar grains , could indicate that the AGB yields are less efficiently mixed into stars than into presolar grains .
Finally , we detect traces of s-process material at [ Fe/H ] = -1.5 , indicating that this process is at work at this and probably at even lower metallicity .