  Recently Lian et al. (2023), thanks to Gaia-ESO data, studied the chemical
evolution of neutron-capture elements in the regime [Fe/H]>-1. We aim here to
complement this study down to [Fe/H]=-3, and focus on Ba, Y, Sr, and abundance
ratios of [Ba/Y] and [Sr/Y], which give comprehensive views on s-process
nucleosynthesis channels. We measured LTE and NLTE abundances of Ba, Y, and Sr
in 323 Galactic metal-poor stars using high-resolution optical spectra with
high S/N. We used the spectral fitting code TSFitPy, together with 1D model
atmospheres using previously determined LTE and NLTE atmospheric parameters.
The NLTE effects are on the order of -0.1 to ~0.2dex depending on the element.
T he ratio between heavy and light s-process elements [Ba/Y] varies weakly with
[Fe/H] even in the metal-poor regime, consistently with the behavior in the
metal-rich regime. The [Ba/Y] scatter at a given metallicity is larger than the
abundance measurement uncertainties. Homogeneous chemical evolution models with
different yields prescriptions are unable to accurately reproduce the [Ba/Y]
scatter at low-[Fe/H]. Adopting the stochastic chemical evolution model by
Cescutti & Chaippini (2014) allows to reproduce the observed scatter in the
abundance pattern of [Ba/Y] and [Ba/Sr]. With our observations, we rule out the
need for an arbitrary scaling of the r-process contribution as previously
suggested by the model authors. We have showed how important it is to properly
include NLTE effects when measuring chemical abundances, especially in the
metal-poor regime. This work shows that the choice of the Galactic chemical
evolution model (stochastic vs. 1-zone) is key when comparing models to
observations. The upcoming surveys such as 4MOST and WEAVE will deliver high
quality spectra of many thousands of metal-poor stars, and this work gives a
typical case study of what could be achieved with such surveys.
