Tracing the formation of the Milky Way through ultra metal-poor stars.

Ultra metal-poor stars (UMPs, [Fe/H] 11 Gyr), observable UMPs 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. Thanks to the exquisite photometric and astrometric information from Gaia DR2, we discovered that, even though most UMP stars have properties that link them to the inner halo or the accreted halo, a strikingly large fraction (26%) of these objects follow disc-like orbits and, in particular, the most metal poor star known (the Caffau's star) has a quasi-circular orbit. One intriguing interpretation is that these stars with disc-like orbits belonged to the massive building block(s) of the proto-Milky Way that formed the backbone of the MW disc. Alternatively, they might have formed in the early disc and have been dynamically heated, or have been brought into the Milky Way by one or more accretion events whose orbit was dragged into the plane by dynamical friction before disruption.For a deeper reading, please have a look at the article BIBCODE: 2019MNRAS.484.2166S and arXiv: arxiv.org/pdf/1811.0309