The lowest detected stellar Fe abundance: The halo star SMSS J160540.18-144323.1

We report the discovery of SMSS J160540.18-144323.1, a new ultra metal-poor halo star discovered with the SkyMapper telescope. We measure left[rm Fe/rm Hright]= -6.2 pm 0.2 (1D LTE), the lowest ever detected abundance of iron in a star. The star is strongly carbon-enhanced, left[rm C/rm Feright] = 3.9 pm 0.2, while other abundances are compatible with an α-enhanced solar-like pattern with left[rm Ca/rm Feright] = 0.4 pm 0.2, left[rm Mg/rm Feright] = 0.6 pm 0.2, left[rm Ti/rm Feright] = 0.8 pm 0.2, and no significant s- or r-process enrichment, left[rm Sr/rm Feright] lt 0.2 and left[rm Ba/rm Feright] lt 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, rm M_odot imparting a low kinetic energy on the supernova ejecta, while models for stars more massive than roughly 20, rm M_odot are incompatible with the observed abundance pattern.