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Neutron star mergers (NSM) are likely to be the main production sites for therapid (r-) neutron capture process elements. We study the r-process enrichmentof the stellar halo of the Milky Way through NSM, by tracing the typicalr-process element Eu in the Munich-Groningen semi-analytic galaxy formationmodel, applied to three high resolution Aquarius dark matter simulations. Inparticular, we investigate the effect of the kick velocities that neutron starbinaries receive upon their formation, in the building block galaxies (BBs)that partly formed the stellar halo by merging with our Galaxy. When this kickis large enough to overcome the escape velocity of the BB, the NSM takes placeoutside the BB with the consequence that there is no r-process enrichment. Wefind that a standard distribution of NS kick velocities decreases [Eu/Mg]abundances of halo stars by $\sim 0.5$~dex compared to models where NS do notreceive a kick. With low NS kick velocities, our simulations match observed[Eu/Mg] abundances of halo stars reasonably well, for stars with metallicities[Mg/H]$\geq -1.5$. Only in Aquarius halo B-2 also the lower metallicity starshave [Eu/Mg] values similar to observations. We conclude that our assumption ofinstantaneous mixing is most likely inaccurate for modelling the r-processenrichment of the Galactic halo, or an additional production site for r-processelements is necessary to explain the presence of low-metallicity halo starswith high Eu abundances.

Semi-analytic modelling of the europium production by neutron star mergers in the halo of the Milky Way