2019MNRAS.488L.109N

We 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 carbon-enhanced, [ 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.