Starkenburg, ElseAguado, David S.Bonifacio, PiercarloCaffau, ElisabettaJablonka, PascaleLardo, CarmelaMartin, NicolasSanchez-Janssen, RubenSestito, FedericoVenn, Kim A.Youakim, KrisAllende Prieto, CarlosArentsen, AnkeGentile, MarcGonzalez Hernandez, Jonay I.Kielty, CollinKoppelman, Helmer H.Longeard, NicolasTolstoy, ElineCarlberg, Raymond G.Cote, PatrickFouesneau, MorganHill, VanessaMcConnachie, Alan W.Navarro, Julio F.2018-12-132018-12-132018-12-132018-12-0110.1093/mnras/sty2276https://infoscience.epfl.ch/handle/20.500.14299/152929WOS:000450984100072The early Universe presented a star formation environment that was almost devoid of heavy elements. The lowest metallicity stars thus provide a unique window into the earliest Galactic stages, but are exceedingly rare and difficult to find. Here, we present the discovery of an ultra-metal-poor star, Pristine(-)221.8781+9.7844, using narrow-band Ca H&K photometry from the Pristine survey. Follow-up medium- and high-resolution spectroscopy confirms the ultra-metal-poor nature of Pristine(-)221.8781+9.7844 ([Fe/H] = -4.66 +/- 0.13 in 1D LTE) with an enhancement of 0.3-0.4 dex in alpha-elements relative to Fe, and an unusually low carbon abundance. We derive an upper limit of A(C) = 5.6, well below typical A(C) values for such ultra-metal-poor stars. This makes Pristine (-)221.8781+9.7844 one of the most metal-poor stars; in fact, it is very similar to the most metal-poor star known (SDSS J102915+172927). The existence of a class of ultra-metal-poor stars with low(er) carbon abundances suggest that there must have been several formation channels in the early Universe through which long-lived, low-mass stars were formed.Astronomy & Astrophysicsstars: abundancesgalaxy: abundancesgalaxy: evolutiongalaxy: formationgalaxy: halosky surveychemical-compositionshamburg/eso surveymodel atmosphereshalo-stars1st starsabundancelithiumphotometrytelescopetext::journal::journal article::research article