Retainment of r-process material in dwarf galaxies

Abstract

The synthesis of $r$-process elements is known to involve extremely energetic\nexplosions. At the same time, recent observations find significant $r$-process\nenrichment even in extremely small ultra-faint dwarf (UFD) galaxies. This\nraises the question of retainment of those elements within their hosts. We\nestimate the retainment fraction and find that it is large $\\sim 0.9$, unless\nthe $r$-process event is very energetic ($\\gtrsim 10^{52}$erg) and / or the\nhost has lost a large fraction of its gas prior to the event. We estimate the\n$r$-process mass per event and rate as implied by abundances in UFDs, taking\ninto account imperfect retainment and different models of UFD evolution. The\nresults are consistent with previous estimates (Beniamini et al. 2016) and with\nthe constraints from the recently detected macronova accompanying a neutron\nstar merger (GW170817). We also estimate the distribution of abundances\npredicted by these models. We find that $\\sim 0.07$ of UFDs should have\n$r$-process enrichment. The results are consistent with both the mean values\nand the fluctuations of [Eu/Fe] in galactic metal poor stars, supporting the\npossibility that UFDs are the main 'building blocks' of the galactic halo\npopulation.\n