The Pristine survey - XXI. Exploring the metal-poor boundary with ESPaDoNS

In this paper, we study high-resolution spectra of 19 stars that have metallicity estimates below -3.5 using at least two metallicity-sensitive photometric indices based on Pristine photometry. The purpose is to understand what kind of stars populate this parameter space, together with extremely metal-poor stars. This because we plan to extensively use the Pristine photometry to provide extremely metal-poor targets to the WEAVE spectroscopic survey and wish to understand the nature of possible contaminants. We find that this extreme sample of stars is heavily contaminated by variable stars, in particular short period eclipsing binaries. We thus found, serendipitously, eight double-lined spectroscopic eclipsing binaries that could be followed-up in future studies to provide reliable masses and distances for these systems. We also found two stars that have metallicity below -3.0, one of which may belong to the Gaia-Sausage-Enceladus structure. The lesson to be learned from this investigation is that to select truly metal-poor stars one should be able to remove all photometrically variable stars, which requires complementary information beyond the Pristine photometry. We show how the Gaia photometry can be used to remove about 85 per cent of the photometrically variable stars. Our investigation also shows that there is a clear potential for Pristine photometry to find double-lined spectroscopic binaries among short period eclipsing binaries.

Automated summary

In this paper, the authors studied high-resolution spectra of 19 stars with metallicity estimates below -3.5 and used metallicity-sensitive photometric indices based on Pristine photometry. They aimed to understand the characteristics of stars in this parameter space, especially extremely metal-poor stars, and investigate possible contaminants for future studies. The results showed that the sample of extreme stars was heavily contaminated by variable stars, particularly short period eclipsing binaries. Serendipitously, they discovered eight double-lined spectroscopic eclipsing binaries, which could be further studied to determine their masses and distances. They also identified two stars with metallicity below -3.0, one of which may belong to the Gaia-Sausage-Enceladus structure. The investigation highlighted the importance of removing photometrically variable stars when selecting truly metal-poor stars, and suggested that Gaia photometry can effectively remove about 85% of such stars. Additionally, the study demonstrated the potential of Pristine photometry in discovering double-lined spectroscopic binaries among short period eclipsing binaries.