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

Abstract

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 10M[subscript⨀] imparting a low kinetic energy on the supernova ejecta, while models for stars more massive than roughly 20M[subscript⨀] are incompatible with the observed abundance pattern.