GEPI

Extremely metal-poor stars (EMPs, [Fe/H]<-3) are extremely rare objects located mainly in the Milky Way halo. Because they are extremely metal-poor, also relative to their neighbourhood, it is assumed that they formed in the relative pristine Galaxy short after the Big Bang. As such, they belong to the earliest generations of stars formed in the Universe. Because of their age (> 11 Gyr), observable EMPs must be low-mass stars, but the minimum metallicity at which low-mass stars can form is still an open question. The search for, and study of, stars at the lowest metallicities are therefore important tools to answer questions on the masses of the first generation of stars and the universality of the IMF, as well as on the early formation stages of galaxies and the first supernovae.
In the first part of the talk I will speak about the Pristine survey, a spectrophotometric survey based on a narrow-band Ca H&K filter aims to detect and analyse the EMPs. In the second part of the talk I will focus on the results of a Bayesian framework to derive stellar parameters, distances and orbits for all the ultra metal-poor stars (UMPs, [Fe/H] < -4) available in the literature. The inferred distances and orbital parameters are directly linked to the formation stages and building blocks of our Galaxy. I will show that, even though most UMP stars have properties that link them to the inner halo or the accreted halo, a strikingly large fraction of those stars follow disc-like orbits. I will discuss how this discovery affects the different scenarios of the formation of the proto-MW.