New H-band stellar spectral libraries for the SDSS-III/APOGEE survey

The Sloan Digital Sky Survey-III (SDSS-III) Apache Point Observatory Galactic Evolution Experiment (APOGEE) has obtained high-resolution (R ∼ 22,500), high signal-to-noise ratio (>100) spectra in the H-band (∼1.5-1.7 μm) for about 146,000 stars in the Milky Way galaxy. We have computed spectral libraries with effective temperature (T_eff) ranging from 3500 to 8000 K for the automated chemical analysis of the survey data. The libraries, used to derive stellar parameters and abundances from the APOGEE spectra in the SDSS-III data release 12 (DR12), are based on ATLAS9 model atmospheres and the ASST spectral synthesis code. We present a second set of libraries based on MARCS model atmospheres and the spectral synthesis code Turbospectrum. The ATLAS9/ASS∈T (T_eff= 3500-8000 K) and MARCS/Turbospectrum (T_eff= 3500-5500 K) grids cover a wide range of metallicity (-2.5 ≤[M/H] ≤ +0.5 dex), surface gravity (0 ≤ log g ≤5 dex), microturbulence (0.5 ≤ χ ≤ 8 km s^-1), carbon (-1 ≤ [C/M] ≤ +1 dex), nitrogen (-1 ≤ [N/M] ≤+1 dex), and α-element (-1 ≤ [α/M] ≤ +1 dex) variations, having thus seven dimensions. We compare the ATLAS9/ASS∈T and MARCS/Turbospectrum libraries and apply both of them to the analysis of the observed H-band spectra of the Sun and the K2 giant Arcturus, as well as to a selected sample of well-known giant stars observed at very high resolution. The new APOGEE libraries are publicly available and can be employed for chemical studies in the H-band using other high-resolution spectrographs.