The universality of the rapid neutron-capture process revealed by a possible disrupted dwarf galaxy star

Abstract

The rapid neutron-capture process or r-process is thought to produce the majority of the heavy elements (Z > 30) in extremely metal-poor stars. The same process is also responsible for a significant fraction of the heavy elements in the Sun. This universality of the r-process is one of its characteristic features as well as one of the most important clues to its astrophysical origin. We report the discovery of an extremely metal-poor field giant with [Sr,Ba/H] $\approx$ -6.0 and [Sr, Ba/Fe] $\approx$ -3.0, the lowest abundances of strontium and barium relative to iron ever observed. Despite its low abundances, the star 2MASS J151113.24-213003.0 has [Sr/Ba] = -0.11 +/- 0.14 and therefore its neutron-capture abundances are consistent with the main solar r-process pattern that has [Sr/Ba] = -0.25. It has been suggested that extremely low neutron-capture abundances are a characteristic of dwarf galaxies, and we find that this star is on a highly-eccentric orbit with apocenter $\gtrsim$100 kpc that lies in the disk of satellites in the halo of the Milky Way. We show that other extremely metal-poor stars with low [Sr,Ba/H] and [Sr,Ba/Fe] plus solar [Sr/Ba] tend to have orbits with large apocenters, consistent with a dwarf galaxy origin for this class of object. The nucleosynthesis event that produced the neutron-capture elements in 2MASS J151113.24-213003.0 must produce both strontium and barium together in the solar ratio. We exclude contributions from the s-process in intermediate-mass AGB or fast-rotating massive metal-poor stars, pair-instability supernovae, the weak r-process, and neutron-star mergers. We argue that the event was a Pop III or extreme Pop II core-collapse supernova explosion.

arXiv commentAccepted to ApJ

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