# High-resolution spectroscopy of extremely metal-poor stars from SDSS/SEGUE. III. Unevolved Stars with $[\mathrm{Fe/H}]\lesssim -3.5$

## TL;DR

This study analyzes elemental abundances in eight extremely metal-poor unevolved stars from SDSS/SEGUE, confirming low lithium levels at low metallicity with minimal star-to-star scatter, and exploring element abundance variations and stellar evolution constraints.

## Contribution

It provides detailed abundance measurements for very metal-poor unevolved stars, comparing methods, and discusses implications for lithium depletion and stellar mixing processes.

## Key findings

- Average Li abundance at [Fe/H] < -3.5 is 1.90 dex with small scatter.
- Significant scatter and bimodality observed in some element ratios like [Na/Fe].
- Detection of CEMP-s star at very low metallicity constrains stellar mixing models.

## Abstract

We present elemental abundances for eight unevolved extremely metal-poor stars with $T_{\rm eff}>5500\,\mathrm{K}$, among which seven have $[\mathrm{Fe/H}]<-3.5$. The sample is selected from the Sloan Digital Sky Survey / Sloan Extension for Galactic Understanding and Exploration (SDSS/SEGUE), and our previous high-resolution spectroscopic follow-up with the Subaru Telescope (Aoki et al.). Several methods to derive stellar parameters are compared, and no significant offset in the derived parameters is found in most cases. From an abundance analysis relative to the standard extremely metal-poor star G 64-12, an average Li abundance for stars with $[\mathrm{Fe/H}]<-3.5$ is $A(\mathrm{Li}) =1.90$, with a standard deviation of $\sigma =0.10$ dex. This result confirms that lower Li abundances are found at lower metallicity, as suggested by previous studies, and demonstrates that the star-to-star scatter is small. The small observed scatter could be a strong constraint on Li-depletion mechanisms proposed for explaining the low Li abundance at lower metallicity. Our analysis for other elements obtained the following results: i) A statistically significant scatter in $[\mathrm{X/Fe}]$ for Na, Mg, Cr, Ti, Sr, and Ba, and an apparent bimodality in $[\mathrm{Na/Fe}]$ with a separation of $\sim 0.8\, \mathrm{dex}$, ii) an absence of a sharp drop in the metallicity distribution, and iii) the existence of a CEMP-$s$ star at $[\mathrm{Fe/H}]\simeq -3.6$ and possibly at $[\mathrm{Fe/H}]\simeq-4.0$, which may provide a constraint on the mixing efficiency of unevolved stars during their main-sequence phase.