First stars XIII. Two extremely metal-poor RR Lyrae stars?
C. J. Hansen, B. Nordstroem, P. Bonifacio, M. Spite, J. Andersen, T., C. Beers, R. Cayrel, F. Spite, P. Molaro, B. Barbuy, E. Depagne, P. Francois, V. Hill, B. Plez, and T. Sivarani

TL;DR
This study identifies two of the most metal-poor RR Lyrae stars, analyzes their detailed chemical compositions, and compares the results with supernova nucleosynthesis models to understand early Galactic chemical evolution.
Contribution
It is the first detailed abundance analysis of extremely metal-poor RR Lyrae stars, revealing their potential as tracers of early nucleosynthesis and Galactic evolution.
Findings
Stars are the most metal-poor RR Lyrae known.
Abundance patterns match energetic, possibly aspherical supernova yields.
EMP RR Lyrae stars retain original surface compositions, unaffected by internal mixing.
Abstract
The chemical composition of extremely metal-poor stars (EMP stars; [Fe/H]<~ -3) is a unique tracer of early nucleosynthesis in the Galaxy. As such stars are rare, we wish to find classes of luminous stars which can be studied at high resolution. We aim to determine the detailed chemical composition of the two EMP stars CS30317-056 and CS22881-039, originally thought to be red horizontal-branch (RHB) stars, and compare it to earlier results for EMP stars as well as to nucleosynthesis yields from various supernova (SN) models. In the analysis, we discovered that our targets are in fact the two most metal-poor RR Lyrae stars known. Our detailed abundance analysis, taking into account the variability of the stars, is based on VLT/UVES spectra (R~ 43000) and 1D LTE OSMARCS model atmospheres and synthetic spectra. For comparison with SN models we also estimate NLTE corrections for a number of…
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