GeminiFocus April 2019 - Page 6

Extremely Metal-poor Stars: Windows into the Early Universe The only way to understand and character- ize the first generation of stars is to look for their direct descendants that would still be alive today: second-generation low-mass, low-metallicity stars. A subset of these, the Extremely Metal-Poor (EMP; [Fe/H] < -3.0) stars, with iron abundances of 1/1,000 of the solar value, are believed to carry in their atmospheres the chemical fingerprints of the evolution of as few as one Pop. III mas- sive star. Apart from the very low iron abun- dance, the majority (more than 60%) of the observed EMP stars show a very strong molecular carbon signature in their optical spectrum. Such high carbon abundances are one of the expected yields of the final stages of evolution of zero-metallicity Pop. III stars and can help trace back the nature of the first stars in the Universe. Finding the Needle in the Haystack Figure 1. Equatorial and Galactic coordinate distribution of the stars observed with Gemini North and Gemini South in poor weather conditions. Identifying such pristine objects is a chal- lenging endeavor. EMP stars are intrinsically rare (less than 30 stars identified to date with [Fe/H] < -4.0) and can only be properly char- acterized as such via spectroscopic studies. In addition, metal-poor stars are generally found in higher fractions in the halo popu- lations of the Galaxy, making most of them faint and "expensive" in terms of telescope time. Thus, it is important to have reliable se- lection criteria in the search for the brightest metal-poor star candidates for high-resolu- tion spectroscopic follow-up. Since changes in metallicity affect the col- ors in optical wavelengths in predictable ways, we pre-selected a number of such candidates from broadband or narrowband photometry. Even though these methods can successfully identify metal-poor star candidates, they become more and more uncertain as metallicities decrease. As a re- sult, medium-resolution (R = λ/Δλ ≈ 1,500) spectroscopy becomes a valuable tool not only for pre-selection of targets to be fol- lowed-up in high-resolution (R ≈ 30,000) but also for parameter determination and stellar population studies. Recently, our team published two studies in The Astronomical Journal (Placco et al., 2018; Placco et al., 2019), aiming to increase the inventory of EMP star candidates observed with medium-resolution spectroscopy. We observed these stars over the course of sev- en semesters (from 2014A to 2017A) with a variety of telescopes, including the Gemini North and South telescopes, the Southern Astrophysical Research telescope, Kitt Peak National Observatory’s Mayall telescope, and the European Southern Observatory’s New Technology Telescope. In total, 2,551 stars were observed. We selected the (bright) candi- dates from two sources — the RAdial Velocity Experiment (RAVE) and the Best & Brightest Survey (B&B) — and used the Gemini North and South tele- scopes to observe 666 stars out of the 2,551. Figure 1 shows the distribution of equatorial and 4 GeminiFocus April 2019