The Pristine survey XI: the FORS2 sample

Abstract : Extremely metal-poor (EMP) stars are old objects that mostly formed very early after the Big Bang. They are rare and, to select them, we have to rely on low-resolution spec-troscopic or photometric surveys; specifically the combination of narrow-and broad-band photometry provides a powerful and time efficient way to select metal-poor stars. The Pristine photometric Survey is using the CFHT MegaCam wide field imager to obtain narrow-band photometry by utilising a filter centred at 395.2 nm on the Caii-H and-K lines. Gaia DR 2 is providing us the wide band photometry as well as par-allaxes. Follow-up observations of metal-poor candidates allowed us to improve our photometric calibrations. In this paper of the series we analyse metal-poor stars observed with FORS2 at VLT. We demonstrate the Pristine calibration adopted in this work to be able to provide metallicities accurate to ±0.3 dex for metal-poor giant stars with good parallaxes, while it performs poorly for dwarf and turn off stars, whatever the accuracy on the parallaxes. We find some metal-poor and very metal-poor stars that are not enhanced in α elements. Such stars have already been found in several other searches, and a higher resolution follow-up of our sample would be useful to put our findings on a firmer ground. This sample of stars analysed has a low fraction of Carbon Enhanced Metal Poor (CEMP) stars, regardless of the definition adopted. This deficiency could indicate a small sensitivity of the Pristine filter to carbon abundance, issue to be addressed in the future.