Targeting Bright Metal-poor Stars in the Disk and Halo Systems of the Galaxy

Abstract

We present the results of spectroscopic follow-up for 1897 low-metallicity star candidates, selected from the Best & Brightest (B&B) Survey, carried out with the GMOS-N/S (Gemini North/South telescopes) and Goodman (SOAR Telescope) spectrographs. From these low-resolution ($R\sim 2000$) spectra, we estimate stellar atmospheric parameters, as well as carbon and magnesium (representative of $\alpha$ elements) abundance ratios. We confirm that $56\%$ of our program stars are metal-poor ([Fe/H] $<-1.0$), $30\%$ are very metal-poor (VMP; [Fe/H] $<-2.0$) and $2\%$ are extremely metal-poor (EMP; [Fe/H] $<-3.0$). There are 191 carbon-enhanced metal-poor (CEMP) stars, resulting in CEMP fractions of $19\%$ and $43\%$ for the VMP and EMP regimes, respectively. A total of 94 confirmed CEMP stars belong to Group I ($A(\mathrm{C})\gtrsim 7.25$) and 97 to Group II ($A(\mathrm{C})\lesssim 7.25$) in the Yoon-Beers $A$(C)$-$[Fe/H] diagram. Moreover, we combine these data with Gaia EDR3 astrometric information to delineate new target-selection criteria, which have been applied to the Goodman/SOAR candidates, to more than double the efficiency for identification of bona-fide VMP and EMP stars in comparison to random draws from the B&B catalog. We demonstrate that this target-selection approach can achieve success rates of $96\%$, $76\%$, $28\%$ and $4\%$ for [Fe/H] $\le -1.5$, $\le -2.0$, $\le -2.5$ and $\le -3.0$, respectively. Finally, we investigate the presence of dynamically interesting stars in our sample. We find that several VMP/EMP ([Fe/H] $\le -2.5$) stars can be associated with either the disk system or halo substructures like Gaia-Sausage/Enceladus and Sequoia.

Accepted for ApJ. Long tables at the end of the manuscript. Minor typos fixed and references updated to match the published version

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