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] ≈ −6.0 and [Sr, Ba/Fe] ≈ −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 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.

Chemical Abundances in the Ultra-faint Dwarf Galaxies Grus I and Triangulum II: Neutron-capture Elements as a Defining Feature of the Faintest Dwarfs*

“…Roederer (2017) found three halo stars with low Sr and Ba as well as Eu detections consistent with the r-process (−4 < [Eu/H] < −3.5). Casey & Schlaufman (2017) found a halo star with [Sr,Ba/H] ≈ −6, with [Sr/Ba] ∼ 0 consistent with the full r-process. Assuming that these halo stars originated in now-tidally-disrupted UFDs, that might imply that a low-yield but robust r-process does occur.…”

Keck HIRES spectroscopy of SkyMapper commissioning survey candidate extremely metal-poor stars

“…The solar system rprocess pattern has [Sr/Ba]=−0.5 dex, though lower values are seen in some ultra-faint dwarf galaxy member stars (see Frebel & Norris 2015, and references therein) and do occur in both the Yong et al (2013) and Jacobson et al (2015) datasets. Casey & Schlaufman (2017) found a star with [Ba/Fe]∼ −3, with typical main r-process abundance patterns, a signature of the universality of the r-process. Six stars with very low [Ba/Fe] ([Ba/Fe]< −1.5) but which show a range of [Sr/Ba] of ∼2 dex, were identified by Jacobson et al (2015).…”

The R-Process Alliance: Spectroscopic Follow-up of Low-metallicity Star Candidates from the Best & Brightest Survey

“…The Best & Brightest Survey (B&B; Schlaufman & Casey 2014) made use of the contrast in the mid-IR photometric bands from the WISE (Wide-field Infrared Survey Explorer; Wright et al 2010) satellite mission to ground-based optical and near-IR photometry to select over 11,000 candidate VMP and EMP stars, with an overall success rate of 30% (VMP) and ∼5% (EMP), which is competitive with previous surveys (see, e.g., Schörck et al 2009;Youakim et al 2017). High-resolution spectroscopic follow-up of selected candidates successfully identified some of the first metal-poor stars in the inner Galaxy (Casey & Schlaufman 2015) and also the most neutron-capture poor star ever observed (Casey & Schlaufman 2017). The B&B survey has the advantage that all of their candidates are brighter than V=14.0, where many other surveys saturate.…”