Outliers in the 0Z Survey

TL;DR

This study analyzes over 100 candidate extremely metal-poor stars, identifying outliers in elemental abundances, and provides insights into the chemical diversity and rare abundance patterns in the early universe.

Contribution

It offers the first detailed abundance analysis of a large sample of extremely metal-poor stars, highlighting the frequency and nature of outliers in their elemental compositions.

Findings

Approximately 6% are strong outliers in one or more elements.

Around 15% show minor or strong outliers in key elements like Sr and Ba.

Identified stars with extremely low [Sr/Fe] and [Ba/Fe], including one with the lowest known ratios.

Abstract

We have now completed detailed abundance analyses of more than 100 stars selected as candidate extremely metal-poor stars with [Fe/H] < -3.0 dex. Of these 18 are below -3.3 dex on the scale of the First Stars VLT project led by Cayrel, and 57 are below -3.0 dex on that scale. Ignoring enhancement of carbon which ranges up to very large values, and two C-rich stars with very high N as well, there are 0 to 3 high or low strong outliers for each abundance ratio tested from Mg to Ni. The outliers have been checked and they are real. Ignoring the outliers, the dispersions are in most cases approximately consistent with the uncertainties, except those for [Sr/Fe] and [Ba/Fe], which are much larger. Approximately 6% of the sample are strong outliers in one or more elements between Mg and Ni. This rises to ~15% if minor outliers for these elements and strong outliers for Sr and Ba are included. There are 6 stars with extremely low [Sr/Fe and [Ba/Fe], including one which has lower [Ba/H] than Draco 119, the star found by Fulbright, Rich and Castro to have the lowest such ratio known previously. There is one extreme r-process star.