Vol. 579
In section 8. Stellar atmospheres

An equatorial ultra iron-poor star identified in BOSS

by C. Allende Prieto, E. Fernandez-Alvar, D. S. Aguado, et al. A&A 579, A98


The authors present a new member of the rare class of ultra iron-poor red giant stars with [Fe/H]=-4.3 and a very high carbon abundance, [C/Fe]=+2.5. This object is only the fifth star known of this kind. It was found in observations of the Baryonic Oscillations Spectroscopic Survey (BOSS) obtained with the Sloan 2.5m telescope, and released in DR12, the final data release of the Sloan Digital Sky Survey III. It was picked out by a code (FERRE) that compares observed spectra with a library of models to find likely candidates for ultra metal-poor stars. The combination of a fairly low effective temperature (Teff ≃ 5300 K), which enhances line absorption, with its brightness (g = 16.9) made it possible to measure the abundances of calcium, carbon, and iron using the low-resolution spectrum. The authors compared the measured carbon and iron abundance ratios in this star to other similar objects in the light of predicted nucleosynthetic yields from metal-free massive stars, and concluded 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.