Astronomy and Astrophysics, volume 605A, 53-53 (2017/9-1)
Influence of inelastic collisions with hydrogen atoms on the non-LTE modelling of Ca I and Ca II lines in late-type stars.
MASHONKINA L., SITNOVA T. and BELYAEV A.K.
Abstract (from CDS):
We performed the non-local thermodynamic equilibrium (non-LTE, NLTE) calculations for CaI-II with the updated model atom that includes new quantum-mechanical rate coefficients for CaI+HI collisions from two recent studies and investigated the accuracy of calcium abundance determinations using the Sun, Procyon, and five metal-poor (MP, -2.6≤[Fe/H]≤-1.3) stars with well-determined stellar parameters.Including HI collisions substantially reduces over-ionisation of CaI in the line formation layers compared with the case of pure electronic collisions and thus the NLTE effects on abundances derived from CaI lines. We show that both collisional recipes lead to very similar NLTE results. As for CaII, the classical Drawinian rates scaled by SH=0.1 are still applied. When using the subordinate lines of CaI and the high-excitation lines of CaII, 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 CaI lines seen in LTE and leads to consistent [Ca/H] abundances from the two ionisation stages.In the MP stars, the NLTE abundance from CaII 8498Å agrees well with the abundance from the CaI subordinate lines, in contrast to LTE, where the abundance difference grows towards lower metallicity and reaches 0.46 dex in BD -13°3442 ([Fe/H]=-2.62). NLTE largely removes abundance discrepancies between the high-excitation lines of CaII and CaII 8498Å obtained for our four [Fe/H]< -2 stars under the LTE assumption. We investigated the formation of the CaI resonance line in the [Fe/H]< -2 stars. When the calcium abundance varies between [Ca/H]~=-1.8 and -2.3, photon loss in the resonance line itself in the uppermost atmospheric layers drives the strengthening of the line core compared with the LTE case, and this effect prevails over the weakening of the line wings, resulting in negative NLTE abundance correction and underestimation of the abundance derived from CaI 4226Å compared with that from the subordinate lines, by 0.08 to 0.32dex. This problem may be related to the use of classical homogeneous (1D) model atmospheres. The situation is improved when the calcium abundance decreases and the CaI 4226Å line formation depths are shifted into deep atmospheric layers that are dominated by over-ionisation of CaI. However, the departures from LTE are still underestimated for CaI 4226Å at [Ca/H]~=-4.4 (HE 0557-4840). Consistent NLTE abundances from the CaI resonance line and the CaII lines are found for HE 0107-5240 and HE 1327-2326 with [Ca/H]≤-5. Thus, the CaI/CaII ionisation equilibrium method can successfully be applied to determine surface gravities of [Ca/H]≤-5 stars. We provide the NLTE abundance corrections for 28 lines of CaI in a grid of model atmospheres with 5000K≤Teff≤6500K, 2.5≤logg≤4.5, -4≤[Fe/H]≤0, which is suitable for abundance analysis of FGK-type dwarfs and subgiants.