The Pristine survey - XX. GTC follow-up observations of extremely metal-poor stars identified from Pristine and LAMOST

A B S T R A C T Ultra-metal-poor stars ( [Fe / H] < -4 . 0) are very rare, and finding them is a challenging task. Both narrow-band photometry and low-resolution spectroscopy have been useful tools for identifying candidates, and in this work, we combine both approaches. We cross-matched metallicity-sensitive photometry from the Pristine surv e y with the low-resolution spectroscopic Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) data base, and re-analysed all LAMOST spectra with [Fe / H]Pristine < -2 . 5. We find that similar to 1/3rd of this sample (selected without [Fe / H]Pristine quality cuts) also have spectroscopic [Fe / H] < -2 . 5. From this sample, containing many low signal-to-noise ratio (S/N) spectra, we selected 11 stars potentially having [Fe / H] < -4 . 0 or [Fe / H] < -3 . 0 with very high carbon abundances, and we performed higher S/N medium-resolution spectroscopic follow-up with the Optical System for Imaging and low Resolution Integrated Spectroscopy (OSIRIS) on the 10.4-m Gran Telescopio Canarias (GTC). We confirm their extremely low metallicities, with a mean of [Fe / H] = -3 . 4, and the most metal-poor star having [Fe / H] = -3 . 8. Three of these are clearly carbon-enhanced metal-poor (CEMP) stars with + 1 . 65 < [C / Fe] < + 2 . 45. The two most carbon-rich stars are either among the most metal-poor CEMP-s stars or the most carbon-rich CEMP-no stars known, the third is likely a CEMP-no star. We derived orbital properties for the OSIRIS sample and find that only one of our targets can be confidently associated with known substructures/accretion events, and that three out of four inner halo stars have prograde orbits. Large spectroscopic surveys may contain many hidden extremely and ultra-metal-poor stars, and adding additional information from e.g. photometry as in this work can unco v er them more efficiently and confidently.