VMP stars [Fe/H] estimates in GSP-Spec catalog

The recovered equivalent widths agree with the directly measured values with a scatter of 0.05dex for the stars that pass the GSP-Spec quality cuts. Among stars that were recommended to filter out, we observed a similar scatter for FGK-type stars initially misidentified as hot stars. Contrarily, we found a poorer agreement in general for stars that the GSP-Spec identified as cool K and M-type giants, although we can still define subsets that show reasonable agreement.

At the low metallicity end ([Fe/H]<-1.5), our metallicity estimates have a typical uncertainty of 0.18dex, which is about half of the quoted GSP-Spec metallicity uncertainty at the same metallicity. Our metallicities also show better agreement with the high-resolution literature values than the original GSP-Spec metallicities at low metallicity; the scatter in the comparison decreases from 0.36-0.46dex to 0.17-0.29dex for stars that satisfy the GSP-Spec quality cuts.

While the GSP-Spec metallicities show increasing scatter when misidentified "hot" stars and the subsets of the "cool K and M-type giants" are included (up to 1.06dex), we can now identify them as FGK-type stars and provide metallicities that show a small scatter in the comparisons (up to 0.34dex), which helps us to increase the number of VMP stars with reliable and precise metallicity. The inclusion of photometric information greatly contributes to breaking parameter degeneracy, enabling precise metallicity estimates for VMP stars from Gaia RVS spectra.

We produce a publicly-available catalog of bright metal-poor stars suitable for high-resolution follow-up. The sample contains about 2345 VMP stars with an estimated contamination rate of 5%.