Non-LTE Analyses of High-resolution H-band Spectra. III. Neutral and Singly Ionized Calcium

A calcium atomic model is updated using collisional rates from new quantum-mechanical computations. We investigated the reliability of the model atom and the non-local thermodynamic equilibrium (NLTE or non-LTE) effects on the formation of Ca I lines in both optical and H bands. NLTE and local thermodynamical equilibrium calcium abundances of 13 sample stars are derived from high-resolution and high signal-to-noise ratio optical spectra and the Apache Point Observatory Galactic Evolution Experiment (APOGEE) data. The results suggest that the updated calcium atomic model is appropriate for studying the formation of H-band calcium lines because our NLTE analyses not only reduce the line-to-line spread of derived abundances, but also lead to consistent optical-line-based and H-band-line-based abundances with a difference of -0.009 ± 0.035 dex and a calcium ionization balance between Ca I and Ca II. In the stellar fundamental parameter space that our sample stars cover, NLTE corrections for H-band Ca I lines are small, within 0.03 dex. Even in the extreme cases of the APOGEE data, the NLTE corrections on selected H-band Ca I lines are within 0.1 dex.