The most metal-poor stars in the Galactic halo carry important information about the formation and early evolution of the chemistry in the early Universe, as well as in the assembly of the Milky Way. Two subclasses are of great interest: (i) the ultra-metal-poor (UMP; [Fe/H] < ( 4.0, e.g., Beers & Christlieb, 2005) stars are believed to be formed by gas clouds polluted by the chemical yields of the very first (Population III) stars formed after the Big Bang (Iwamoto et al., 2005). It has been shown that more than 80% of the observed UMP stars in the Galaxy present enhancements in carbon (e.g., Placco et al., 2014a), the so-called Carbon-Enhanced Metal-Poor (CEMP) stars; (ii) the highly r-process-element enhanced stars (r-II; [Fe/H] < ( 2.0 and [Eu/Fe] > +1:0), provide crucial information about the astrophysical site(s) of the rapid neutron-capture process, which has remained elusive since the seminal work of Burbidge et al. (1957). Recent observations of the electromagnetic counterpart of the first neutron star merger detected by LIGO can possibly provide the final piece of this cosmic chemical puzzle (Abbott et al., 2017; Shappee et al., 2017).
The identification of UMP, CEMP, and r-II stars is a challenging endeavor. They are intrinsically rare and can only be properly classified by spectroscopic studies. In addition, most of these stars found to date are faint, which limits the amount of spectroscopic information that can be obtained, even with large telescopes. Thus, there is a need to find additional bright examples of the type of phenomena (low-metallicity, carbon-enhancement, and neutron-capture element enhancement) believed to probe the early stages of the chemical evolution of the Galaxy and the Universe. This has already been started for a representative sample of UMP and CEMP stars (e.g., Placco et al., 2014b, 2015b), but additional work is needed to address questions raised by recent theoretical studies (Nomoto et al., 2013).
As mentioned before, it has been recently confirmed that neutron star mergers (NSMs) are a (perhaps the only) astrophysical site for the production of heavy elements by the rapid neutron-capture process (r-process). We plan to complete a survey that will identify and study a large number of highly r-process enhanced stars in the halo of the Milky Way, and thereby address numerous fundamental questions that remain about the nature and the origin of the r-process. Open questions include – Do NSMs produce all of the r-process elements, or are other astrophysical sites required? If NSMs can produce both light and heavy r-process elements, in what ratios are they produced and what underlying physics determines this ratio? Are r-process elements produced exclusively at early times, or can they be synthesized over the full history of our Galaxy?
Answers to these and other questions require more detailed constraints than can be obtained solely from estimates of the frequencies of NSMs based on gravitational wave detections in other galaxies, or observations of their associated electromagnetic signatures (kilonovae). Previous samples of highly r-process-enhanced stars have provided valuable clues to the nature of the r-process and its possible astrophysical production site(s). However, once sliced into subsamples that highlight possibly contrasting behaviors among these sites, the opportunity to statistically quantify the results with any precision is lost. Much larger samples of these rare stars are required. The R-Project Alliance, an international collaboration, aims to provide these constraints, based on a quadrupling of the numbers of known highly r-process-enhanced stars in the Milky Way, and detailed study of their observed elemental abundance patterns.
Understanding our origin is a central theme for much of contemporary research, covering a wide range of scientific endeavors, and a subject of enormous interest to society as a whole. With the recent confirmation that NSMs provide at least a portion of the heavy elements associated with the r-process, which are eventually incorporated into stars such as the Sun, and planets such as Earth, researchers and the public alike are anxious to learn more about the origin of the elements, and where and when they were created.