The metal-poor tail of APOGEE I

Once identified within APOGEE, we compute the orbits of our metal-poor candidates using AGAMA. The calibrated APOGEE stellar parameters provide a clear signature of the most metal-poor stars in the survey, indicated by null values for their metallicities while having effective temperatures and surface gravities determined by the pipeline. From comparison with the literature, we find that, within a temperature range of 3700 - 6700 K and above a threshold of SNR>30, the vast majority of APOGEE stars without calibrated metallicities are very metal-poor.

Additional cleaning by visual inspection of the spectra improves the purity of the sample further. The radial velocities provided by APOGEE, Gaia DR3 positions and astrometry along with spectrophotometric distances derived in this work, allow for the computation of their orbits. In this work, we carefully select 289 very metal-poor red giant stars (likely below [Fe/H]=-2.5) from the APOGEE results.

Our sample contains 16 very metal-poor member candidates of the Magellanic Clouds as well as 14 very metal-poor stars with orbits confined to the inner Galaxy, and 13 inner Galaxy halo interlopers. These samples significantly add to the very metal-poor stars known in these regions and allow for a more detailed picture of early chemical evolution across different environments.