Nordlander, ThomasBessell, MichaelDa Costa, GaryMackey, DougalAsplund, MartinCasey, A. R.Chiti, AnirudhEzzeddine, RanaFrebel, AnnaLind, K.Marino, Anna FabiolaMurphy, SimonNorris, JohnSchmidt, BrianYong, David2021-05-072021-05-070035-8711http://hdl.handle.net/1885/232540We report the discovery of SMSS J160540.18−144323.1, a new ultra metal-poor halo star discovered with the SkyMapper telescope. We measure [Fe/H] = −6.2 ± 0.2 (1D LTE), the lowest ever detected abundance of iron in a star. The star is strongly carbonenhanced, [C/Fe] = 3.9 ± 0.2, while other abundances are compatible with an α-enhanced solar-like pattern with [Ca/Fe] = 0.4 ± 0.2, Mg/Fe = 0.6 ± 0.2, [Ti/Fe] = 0.8 ± 0.2, and no significant s- or r-process enrichment, [Sr/Fe] < 0.2 and [Ba/Fe] < 1.0 (3σ limits). Population III stars exploding as fallback supernovae may explain both the strong carbon enhancement and the apparent lack of enhancement of odd-Z and neutron-capture element abundances. Grids of supernova models computed for metal-free progenitor stars yield good matches for stars of about 10 M imparting a low kinetic energy on the supernova ejecta, while models for stars more massive than roughly 20 M are incompatible with the observed abundance pattern.Parts of this research were conducted by the Australian Research Council Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), through project number CE170100013. Research on extremely metal-poor stars has been supported in part through Australian Research Council Discovery Grant Program DP150103294 (GSDC, MSB, and BPS). ARC is supported in part by Australian Research Council Discovery Project DP160100637. ADM is supported by an Australian Research Council Future Fellowship (FT160100206). The national facility capability for SkyMapper has been funded through ARC LIEF grant LE130100104 from the Australian Research Council, awarded to the University of Sydney, the Australian National University, Swinburne University of Technology, the University of Queensland, the University of Western Australia, the University of Melbourne, Curtin University of Technology, Monash University and the Australian Astronomical Observatory.application/pdfen-AU© 2019 The Author(s)stars: abundancesstars: individual: SMSS J160540.18−144323.1stars: Population III2019-06-2710.1093/mnrasl/slz1092020-12-27