# WHT follow-up observations of extremely metal-poor stars identified from SDSS and LAMOST

## TL;DR

This study identifies and confirms extremely metal-poor stars from SDSS and LAMOST data using follow-up observations with the WHT telescope, employing a robust two-step spectral analysis to accurately determine their metallicity.

## Contribution

The paper introduces a reliable two-step methodology combining SDSS/LAMOST spectra analysis with WHT follow-up to accurately measure metallicity in extremely metal-poor stars.

## Key findings

- Confirmed several stars with [Fe/H] < -4
- Discovered 12 new extremely metal-poor candidates
- Validated methodology accuracy down to [Fe/H] < -5

## Abstract

We have identified several tens of extremely metal-poor star candidates from SDSS and LAMOST, which we follow-up with the 4.2m WHT telescope to confirm their metallicity.We follow a robust two-step methodology. We first analyze the SDSS and LAMOST spectra. A first set of stellar parameters is derived from these spectra with the FERRE code, taking advantage of the continuum shape to determine the atmospheric parameters, in particular, the effective temperature. Second, we select interesting targets for follow-up observations, some of them with very low-quality SDSS or LAMOST data. We then obtain and analyze higher-quality medium-resolution spectra obtained with ISIS on the WHT telescope to arrive at a second, more reliable, set of atmospheric parameters. This allows us to derive, with accuracy, the metallicity, and confirm the extremely metal-poor nature in most cases. In this second step we also employ FERRE, but we take a running mean to normalize both the observed and the synthetic spectra, and therefore the final parameters do not rely on having an accurate flux calibration or continuum placement. In order to verify our results we have analyzed with the same tools, and following the same procedure, six well-known metal-poor stars, five of them at [Fe/H]<-4, showing that our methodology is able to get accurate metallicity determinations down to [Fe/H]<-5.0. The results for these six reference stars give us confidence on the metallicity scale for the rest of the sample. In addition, we present 12 new extremely metal-poor candidates: two stars at [Fe/H]=-4, six more in the range -4<[Fe/H]<-3.5, and four more at -3.5<[Fe/H]<-3.0.