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 spectroscopic or photometric surveys; specifically the combination of narrow- and broad-band photometry provides a powerful and time efficient way to select MP stars. The Pristine photometric survey is using the Canada–France–Hawaii Telescope MegaCam wide-field imager to obtain narrow-band photometry by utilizing a filter centred at 395.2 nm on the Ca ii-H and -K lines. Gaia DR 2 is providing us the wide-band photometry as well as parallaxes. Follow-up observations of MP candidates allowed us to improve our photometric calibrations. In this paper of the series we analyse MP 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 MP giant stars with good parallaxes, while it performs poorly for dwarf and turn-off stars, whatever the accuracy on the parallaxes. We find some MP and very MP 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 MP 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.

TOPoS

“…The TOPoS sample provides an analysis of the same stars based on higher resolution spectra obtained by our group. We have a total of 70 stars (we excluded stars with upper limits in the high-resolution analysis, as well as stars for which MyGIsFOS did not converge), and they are summarised in Caffau et al (2018), with resolving power R ∼ 5000 and high S/N (typically above 100, 30 stars in common), and by Caffau et al (2020), with FORS at VLT, a resolving power of R ∼ 2000, and a high S/N (again above 100, three stars in common). In the Caffau et al (2018) sample, one star is analysed independently using two spectra.…”

“…The green dots are data fromCaffau et al (2018), and open symbols are stars with uncertain abundance according toCaffau et al (2018). The blue dots are data fromCaffau et al (2020). The stars circled in black are removed from the average (see text).…”

High-speed stars: Galactic hitchhikers

“…We computed a synthetic spectrum with SYNTHE (see Kurucz 2005;Sbordone et al 2004) and the radial velocities were derived by templatematching (see e.g. Caffau et al 2020). Using the star's coordinates, radial velocities, Gaia Dr2 distances, and proper motions we characterised the stellar orbital parameters.…”

“…and for the 70 stars for which we have A(Ca) determination from Ca i lines [Ca/Fe] = +0.31 ± 0.10. To make this statement stronger, we compared our sample to the FORS-Pristine sample (Caffau et al 2020), which is a sample of similar metallicity stars, observed with the same spectrograph. These stars (open blue squares in the figure) generally show a larger scatter in [Mg/Fe] and [Ca/Fe], overall showing several stars with no evidence for the typical α-enhancement expected for the Galactic metal-poor stars.…”

“…In Fig. 11, the most metal-poor stars in our sample ([Fe/H] < −2) are compared to the stars with [Fe/H] < −2 in Caffau et al (2020) and to literature values for extremely metal-poor stars.…”