An Update on the 0Z Project

TL;DR

This paper reports on the discovery of extremely metal-poor stars from the 0Z Survey, highlighting a star with unique abundance ratios and analyzing the metallicity distribution function, revealing contamination in previous estimates.

Contribution

It provides new extremely metal-poor star discoveries, including a star with anomalous abundance ratios, and critically assesses the metallicity distribution function from the HES survey.

Findings

Discovery of a star with highly anomalous abundance ratios.

Identification of contamination in the MDF for [Fe/H] < -3 dex.

Evidence that many candidate EMP stars are of higher metallicity or spurious.

Abstract

We give an update on our 0Z Survey to find more extremely metal poor (EMP) stars with [Fe/H] < -3 dex through mining the database of the Hamburg/ESO Survey. We present the most extreme such stars we have found from ~1550 moderate resolution follow up spectra. One of these, HE1424-0241, has highly anomalous abundance ratios not seen in any previously known halo giant, with very deficient Si, moderately deficient Ca and Ti, highly enhanced Mn and Co, and low C, all with respect to Fe. We suggest a SNII where the nucleosynthetic yield for explosive alpha-burning nuclei was very low compared to that for the hydrostatic alpha-burning element Mg, which is normal in this star relative to Fe. A second, less extreme, outlier star with high [Sc/Fe] has also been found. We examine the extremely metal-poor tail of the HES metallicity distribution function (MDF). We suggest on the basis of comparison of our high resolution detailed abundance analyses with [Fe/H](HES) for stars in our sample that the MDF inferred from follow up spectra of the HES sample of candidate EMP stars is heavily contaminated for [Fe/H](HES) < -3 dex; many of the supposed EMP stars below that metallicity are of substantially higher Fe-metallicity, including most of the very C-rich stars, or are spurious objects.