The R-process Alliance: First Release from the Southern Search for R-process-enhanced Stars in the Galactic Halo*

Abstract

The recent detection of a binary neutron star merger and the clear evidence for the decay of radioactive material observed in this event have, after sixty years of effort, provided an astrophysical site for the rapid neutron-capture (r-) process which is responsible for the production of the heaviest elements in our Universe. However, observations of metal-poor stars with highly-enhanced r-process elements have revealed abundance patterns suggesting that multiple sites may be involved. To address this issue, and to advance our understanding of the r-process, we have initiated an extensive search for bright (V < 13.5), very metal-poor ([Fe/H] < −2) stars in the Milky Way halo exhibiting stronglyenhanced r-process signatures. This paper presents the first sample collected in the Southern Hemisphere, using the echelle spectrograph on du Pont 2.5m telescope at Las Campanas Observatory. We have observed and analyzed 107 stars with −3.13 < [Fe/H] < −0.79. Of those, 12 stars are strongly enhanced in heavy r-process elements (r-II), 42 stars show moderate enhancements of heavy r-process material (r-I), and 20 stars exhibit low abundances of the heavy r-process elements and higher abundances of the light r-process elements relative to the heavy ones (limited-r).This search is more successful at finding r-process-enhanced stars compared to previous searches, primarily due to a refined target selection procedure that focuses on red giants.

Enrichment of the Galactic disc with neutron capture elements: Sr

“…Again, we are quite close to the error range limit. The same uncommon signature is observed in few metal poor stars (e.g., Roederer et al 2010;Frebel 2010;Hansen et al 2018), indicating the contribution from different r-process components or some additional nucleosynthesis component which is not taken into account in this analysis.…”

“…A number of nucleosynthesis processes needed at low metallicity have been also discussed by Hansen, Montes & Arcones (2014), utilizing the approach by Qian & Wasserburg (2001). These authors considered neutrino-driven winds in CCSNe as a source of Sr. More recent analyses of production of elements at the Sr peak in metal-poor stars are provided by Hansen et al (2018) and Spite et al (2018).…”

Gamma Rays from Kilonova: A Potential Probe of r-process Nucleosynthesis

“…Over four decades ago, Lattimer & Schramm (1974) proposed that the mergers of a neutron star with either another neutron star (NS/NS) or a black hole (NS/BH) could be primary sites of the r-process production in the Universe. Proving this point requires demonstrating that the rate of these mergers is sufficiently high and the cumulative nucleosynthetic yield is plentiful, given the merger rate, and further, produces the solar-like distribution in proper agreement with r-process enriched metal-poor stars (Sneden et al 1996;Beers & Christlieb 2005;Ji & Frebel 2018;Hansen et al 2018). Rates of these mergers from theoretical (e.g.…”

Evidence for the accretion origin of halo stars with an extreme r-process enhancement

“…Such large excesses of r-process elements are not expected in stars with higher metallicity (moderately metal-poor stars), because the chemical composition of these stars are usually determined by contributions of a large number of nucleosynthesis events until the stars were formed, and any single event could not significantly change the abundance ratios. Only a few of such moderately metal-poor r-II stars have been found in the Galactic halo (HD 222925 and J1802-4404) 17,18 and bulge 19 . J1124+4535 is unique among them for being deficient in α-elements.…”