GeminiFocus April 2019 - Page 9

The distribution of the carbon abundances as a function of the metallicity for these stars is shown in Figure 4. The lower and side panels show marginal distributions for each quantity. The behavior is simi- lar to that expected from high-resolution spectroscopic samples, which makes this subset important for two reasons: 1) as a tool for target selection, and 2) to have an independent estimate of quantities, such as the fraction of carbon-enhanced metal- poor stars as a function of [Fe/H], which is a crucial observational constraint to Galactic chemical evolution models. What Have We Learned and What's Next? The objectives of such follow-up studies, which can include Gemini Poor Weather ob- servations, are two-fold: 1) build statistics of metallicities and carbon abundances deter- mined from medium-resolution spectros- copy, which are crucial for studies of stellar populations and formation of the Milky Way, and 2) select interesting stars for further, more targeted, high-resolution spectrosco- py efforts. One effort that is feeding directly from the Gemini data is called the "R-Process Alliance" (RPA) — a multi-stage, multi-year effort to provide observational, theoretical, and experimental constraints on the nature and origin of the astrophysical r-process (rapid neutron-capture). The parameters determined using the Gem- ini spectra are extremely useful to tailor target lists for the type of (high-resolution) follow-up conducted by the RPA, and there is already a study published based on an extremely metal-poor star first identified at Gemini (Cain et al., 2018). This star, J2005- 3057, shows enhancements in elements formed by the r-process, such as europium, iridium and thorium, among others. An- other effort currently underway is gathering April 2019 high-resolution data for the most carbon- enhanced stars identified by Gemini and the results are also promising. Collectively, these discoveries help us paint a more cohesive picture of how the Universe evolved chemi- cally and how we can reshape our current understanding of stellar evolution and gal- axy formation. In the near future, such bright stars will be perfect targets for high-resolu- tion spectroscopic follow-up with GHOST, which will be a great asset in pushing these efforts forward. Figure 4. Carbon abundances, A(C), as a function of the metallicity, [Fe/H], for the program stars observed with Gemini. The side and lower panels show the marginal distributions for each quantity. References Cain, M., Frebel, A., Gull, M., et al., The Astro- physical Journal, 864: 43, 2018 Placco, V. M., Beers, T. C., Santucci, R. M., et al., The Astrophysical Journal, 155: 256, 2018 Placco, V. M., Santucci, R. M., Beers, T. C., et al., The Astrophysical Journal, 870: 122, 2019 Schlaufman, K. C., and Casey, A. R., The Astro- physical Journal, 797: 13, 2014 Steinmetz, M., Zwitter, T., Siebert, A., et al., The Astrophysical Journal, 132: 1645, 2006 Vinicius Placco is Research Assistant Professor at the Department of Physics at the University of Notre Dame and is located at Notre Dame, Indi- ana. He can be reached at [email protected] GeminiFocus 7