Galactic evolution of rapid neutron capture process abundances: the inhomogeneous approach

Abstract: For the origin of heavy r-process elements, different sources have been proposed, e.g., core-collapse supernovae or neutron star mergers. Old metal-poor stars carry the signature of the astrophysical source(s). Among the elements dominantly made by the r-process, europium (Eu) is relatively easy to observe. In this work we simulate the evolution of europium in our galaxy with the inhomogeneous chemical evolution model 'ICE', and compare our results with spectroscopic observations. We test the most important parameters affecting the chemical evolution of Eu: (a) for neutron star mergers the coalescence time scale of the merger ($t_{\mathrm{coal}}$) and the probability to experience a neutron star merger event after two supernova explosions occurred and formed a double neutron star system ($P_{\mathrm{NSM}}$) and (b) for the sub-class of magneto-rotationally driven supernovae ("Jet-SNe"), their occurrence rate compared to standard supernovae ($P_{\mathrm{Jet-SN}}$). We find that the observed [Eu/Fe] pattern in the galaxy can be reproduced by a combination of neutron star mergers and magneto-rotationally driven supernovae as r-process sources. While neutron star mergers alone seem to set in at too high metallicities, Jet-SNe provide a cure for this deficiency at low metallicities. Furthermore, we confirm that local inhomogeneities can explain the observed large spread in the europium abundances at low metallicities. We also predict the evolution of [O/Fe] to test whether the spread in $\alpha$-elements for inhomogeneous models agrees with observations and whether this provides constraints on supernova explosion models and their nucleosynthesis.