Enrichment of r-Process Elements in Isolated Dwarf Galaxies

Abstract

The abundances of r-process elements in stars can be a signature for the astrophysical sites of the r-process elements and star formation histories of galaxies. Observations have confirmed that there are star-to-star scatters of the ratios r-process elements to iron at low metallicity in the Milky Way halo. However, the enrichment history of r-process elements has not yet been clarified. In this chapter, we have computed the enrichment of r-process elements using a series of N-body/hydrodynamic simulations of dwarf galaxies with various initial conditions. We find that neutron star mergers can enrich r-process elements at [Fe/H] \(\lesssim \) −2.5 if the star formation efficiency of the galaxy is less than 0.01 per billion years. In such galaxies, the mean metallicity is almost constant over several hundred million years. On the other hand, the r-process elements are produced at higher metallicity in galaxies with higher star formation efficiencies. The star-to-star scatters of the ratios of r-process elements to iron mainly come from the inhomogeneous spatial metallicity distribution in galaxies. The results demonstrate that it will be possible to estimate the early star formation histories of galaxies by using the abundances of r-process elements (Contents in this chapter have been in part published in Hirai et al. (Astrophys J 814:41, [19]), Hirai et al. (Mon Not Royal Astron Soc 466:2474-2487, [20]) reproduced permission of the AAS and Oxford University Press).