Neutron Star Mergers as the Main Source ofr-process: Natal Kicks and Inside-out Evolution to the Rescue

Binary neutron star mergers (BNSMs) are currently the most promising source ofr-process thanks to the detection of GW170817. The estimated occurring frequency and the amount of mass ejected per merger indicate that BNSMs by themselves can account for all ther-process enrichment in the Galaxy. However, the decreasing trend of [Eu/Fe] versus [Fe/H] of disk stars for [Fe/H] greater than or similar to -1 in the solar neighborhood is inconsistent with the flat trend expected from BNSMs with a standard delay time distribution (DTD) proportional to t(-1). This has led to the suggestion that either additional sources or modification to the DTD of BNSMs is required to match the observations. We investigate the effects of natal kicks received during the birth of neutron star binaries on the chemical evolution ofr-process element Eu in the Milky Way by combining the results from the galactic dynamics codegalpywith a one-zone Galactic chemical evolution modelomega. We show that when key inputs from simulations of the inside-out disk evolution are combined with natal kicks, BNSMs can naturally reproduce the observed decreasing trend of [Eu/Fe] with [Fe/H] in the solar neighborhood without the need for modification to the DTD or additionalr-process sources.