We perform the non-local thermodynamic equilibrium (NLTE) calculations for Ca
I-II with the updated model atom that includes new quantum-mechanical rate
coefficients for Ca I + H I collisions from two recent studies, that is, by
Barklem and by Mitrushchenkov, Guitou, Belyaev, Yakovleva, Spielfiedel, and
Feautrier, and investigate the accuracy of calcium abundance determinations
using the Sun, Procyon, and five metal-poor (MP) stars with well-determined
stellar parameters. We show that both collisional recipes lead to very similar
NLTE results. When using the subordinate lines of Ca I and the high-excitation
lines of Ca II, NLTE provides the smaller line-to-line scatter compared with
the LTE case for each star. For Procyon, NLTE removes a steep trend with line
strength among strong Ca I lines seen in LTE and leads to consistent [Ca/H]
abundances from the two ionisation stages. In the MP stars, the NLTE abundance
from Ca II 8498 A agrees well with that from the Ca I subordinate lines. NLTE
largely removes abundance discrepancies between the high-excitation lines of Ca
I and Ca II 8498 A obtained for our four [Fe/H] < -2 stars under the LTE
assumption. We investigate the formation of the Ca I resonance line in the
[Fe/H] < -2 stars. Consistent NLTE abundances from the Ca I resonance line and
the Ca II lines are found for two hyper metal-poor stars HE0107-5240 and
HE1327-2326. We provide the NLTE abundance corrections for 28 lines of Ca I in
a grid of model atmospheres suitable for abundance analysis of FGK-type dwarfs
and subgiants.Comment: 12 pages, 3 tables, 10 figures; accepted for publication in A&
