Grid of red giant synthetic spectra with 0<[M/H]<=-6

We investigate the dependence of the complete system of 22 Lick indices on overall metallicity scaled from solar abundances, [M/H], from the solar value, 0.0, down to the extremely metal-poor (XMP) value of -6.0, for late-type giant stars (MK luminosity class III, logg=2.0) of MK spectral class late-K to late-F (3750<Teff<6500K) of the type that are detected as "fossils" of early galaxy formation in the Galactic halo and in extra-galactic structures.

Our investigation is based on synthetic index values, I, derived from atmospheric models and synthetic spectra computed with PHOENIX in Local Thermodynamic Equilibrium (LTE) and Non-LTE (NLTE), where the synthetic spectra have been convolved to the spectral resolution, R, of both IDS and SDSS (and LAMOST) spectroscopy. We identify nine indices, that we designate "Lick-XMP", that remain both detectable and significantly [M/H]-dependent down to [M/H] values of at least ~-5.0, and down to [M/H]~-6.0 in five cases, while also remaining well-behaved (single-valued as a function of [M/H] and positive in linear units).