Abstract
Aims. Inelastic processes in low-energy Ca + H and Ca+ + H- collisions are treated for the states from the ground state up to the ionic state with the aim to provide rate coeffcients needed for non-LTE modeling of Ca in cool stellar atmospheres. Methods. The electronic molecular structure was determined using a recently proposed model approach that is based on an asymptotic method. Nonadiabatic nuclear dynamics were treated by means of multichannel formulas, based on the Landau-Zener model for nonadiabatic transition probabilities. Results. The cross sections and rate coeffcients for inelastic processes in Ca + H and Ca+ + H- collisions were calculated for all transitions between 17 low-lying covalent states plus the ionic state. It is shown that the highest rate coeffcient values correspond to the excitation, de-excitation, ion-pair formation, and mutual neutralization processes involving the Ca(4s5s 1;3S) and the ionic Ca+ + H- states. The next group with the second highest rate coeffcients includes the processes involving the Ca(4s5p 1;3P), Ca(4s4d 1;3D), and Ca(4s4p 1P) states. The processes from these two groups are likely to be important for non-LTE modeling.
Highlights
IntroductionNon-local thermodynamic equilibrium (non-LTE) modeling of stellar spectra is important for many fundamental problems in modern astrophysics (see, e.g., reviews Asplund 2005; Barklem 2012, and references therein)
Non-local thermodynamic equilibrium modeling of stellar spectra is important for many fundamental problems in modern astrophysics
Significant progress has recently been achieved in detailed quantum treatments of inelastic processes in collisions of hydrogen atoms and negative ions with atoms and positive ions of different chemical elements
Summary
Non-local thermodynamic equilibrium (non-LTE) modeling of stellar spectra is important for many fundamental problems in modern astrophysics (see, e.g., reviews Asplund 2005; Barklem 2012, and references therein). For many chemical elements of interest, accurate complete quantum cross sections are still not available It is known (Barklem et al 2011) that the so-called Drawin formula (Steenbock & Holweger 1984; Lambert 1993), which has been widely employed for estimates of inelastic collision rate coefficients, is not reliable. The Drawin formula is not applicable to charge transfer processes (ion-pair formation and mutual neutralization processes), which have been found to be the most important processes in the cases studied so far (e.g., Barklem et al 2003)1 For these reasons, approximate but physically reliable approaches to inelastic atomic collisions with hydrogen atoms are highly desirable. Since the treated inelastic processes are determined by nonadiabatic transitions, which are quantum by nature, the model approach is essentially quantum as well
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