An equatorial ultra iron-poor star identified in BOSS

We report the discovery of SDSS J131326.89 - 001941.4, an ultra iron-poor red giant star ([Fe/H] similar or equal to -4.3) with a very high carbon abundance ([C/Fe] similar or equal to +2.5). This object is the fifth star in this rare class, and the combination of a fairly low effective temperature (T-eff similar or equal to 5300 K), which enhances line absorption, with its brightness (g = 16.9), makes it possible to measure the abundances of calcium, carbon and iron using a low-resolution spectrum from the Sloan Digital Sky Survey. We examine the carbon and iron abundance ratios in this star and other similar objects in the light of predicted yields from metal-free massive stars, and conclude that they are consistent. By way of comparison, stars with similarly low iron abundances but lower carbon-to-iron ratios deviate from the theoretical predictions.