Galactic r-process Abundance Feature Shaped by Radial Migration

Growing interest in the chemical feature of r-process elements among nearby disk stars represented by the [Eu/Fe] versus [Fe/H] diagram has sprouted because it can assess the origin of r-process elements through comparison with theoretical models, including a test of whether neutron star mergers can be major sites of r-process nucleosynthesis. On the other hand, recent studies have revealed that local chemistry is strongly coupled with the dynamics of the Galactic disk, which predicts that stars radially move on the disk where the observed elemental feature is different at various Galactocentric distances. Here, we show that radial migration of stars across the Galactic disk plays a crucial role in shaping the r-process abundance feature in the solar vicinity. In this proposed scenario, we highlight the importance of migration from the outer disk where [r-process/Fe] of some old stars is predicted to be enhanced to a level beyond the expectation of the observed Galactic Fe and Eu radial gradient, which results in a large span of [r-process/Fe] among nearby disk stars. The variation in the [r-process/Fe] ratio seen across the Galactic disk as well as in dwarf galaxies may be an outcome of different stellar initial mass functions, which change the occurrence frequency between supernovae, leaving behind neutron stars and ones ending with black holes. Here we propose that enhancement in [Eu/Fe] is attributed to the initial mass function lacking high-mass stars such as ≳25 M _⊙ in the scheme for which neutron star mergers are a major source of r-process elements.