SD 1313-0019: ANOTHER SECOND-GENERATION STAR WITH [Fe/H] =-5.0, OBSERVED WITH THE MAGELLAN TELESCOPE

We present a Magellan/MIKE high-resolution (R similar to 35,000) spectrum of the ancient star SD 1313-0019, which has an iron abundance of [Fe/H] = -5.0, paired with a carbon enhancement of [C/Fe] similar to 3.0. The star was initially identified by Allende Prieto et al. in the BOSS survey. Its medium-resolution spectrum suggested a higher metallicity of [Fe/H] = -4.3 due to the Ca II K line blending with a CH feature, which is a common issue related to the search for the most iron-poor stars. This star joins several other similar stars with [Fe/H]. -5.0 that all display a combination of low-iron and high-carbon abundances. Other elemental abundances of SD 1313-0019 follow that of more metal-rich halo stars. Fitting the abundance pattern with yields of Population III supernovae suggests that SD 1313-0019 had only one massive progenitor star with 20-30 M. that must have undergone a mixing and fallback episode. Overall, there are now five stars known with [Fe/H] less than or similar to -5.0 (1D local thermodynamic equilibrium abundances). This ever-increasing population of carbon-rich, iron-deficient stars can potentially constrain nucleosynthesis in Population III stars and their supernova explosions, the formation mechanisms of the first low-mass stars, and the nature of the first galaxies.