Mapping the Milky Way with Gaia BP/RP Spectra. III. The First Medium-resolution Follow-up Spectroscopy of Metal-poor Stars^*

Abstract The Gaia BP/RP spectra provide an excellent opportunity to search for extremely metal-poor (EMP; [M/H] ≤ –3.0) stars. In this study, we assess the potential of our Gaia BP/RP-based candidate catalog, vetting newly identified EMP candidates with higher-resolution follow-up observations. The candidates are selected based on the metallicity derived from BP/RP spectra and the goodness of fit. Fifteen candidates were observed with the Intermediate Dispersion Spectrograph (IDS) on the Isaac Newton Telescope to validate the sample. We analyzed the data with 1D local thermodynamic equilibrium models and assessed the reliability of the [C/Fe] estimates. Three well-studied metal-poor stars are included as a sanity check. For most targets, the metallicities estimated from IDS data agree well with those derived from BP/RP spectra. Five of these stars are EMP stars (four of them are newly identified), including two with [M/H] ≤ –3.5, one of which is a carbon-enhanced metal-poor star. Reliable [C/Fe] estimates are obtained for eight stars. The efficiency in detecting EMP candidates (as eight candidates with [M/H] ≤ –3.0 yielding five EMP stars) is estimated to be <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>6</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>24</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>19</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>%</mml:mo> </mml:math> at a 68% confidence level, which is comparable to or higher than that of the Pristine-Gaia synthetic metallicity catalogue (38%), highlighting the potential of our method.