  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 alpha-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 (3{\sigma} 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 solar mass imparting
a low kinetic energy on the supernova ejecta, while models for stars more
massive than roughly 20 solar mass are incompatible with the observed abundance
pattern.
