Hydrogen Atom Collision Processes in Cool Stellar Atmospheres: Effects on Spectral Line Strengths and Measured Chemical Abundances in Old Stars

• Published in: Journal of physics. Conference series Vol. 397; no. 1; pp. 1 - 7
• Main Author: Barklem, Paul S
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 01.01.2012
• Subjects: Abundance; Astronomical models; Astrophysics; Atmospheric models; Atomic collisions; Charge transfer; Chemical composition; Excitation; Galactic evolution; Hydrogen atoms; Inelastic collisions; Line spectra; Lithium; Local thermodynamic equilibrium; Luminosity; Mathematical models; Metallicity; Microphysics; Modelling; Physics; Quantum chemistry; Spectral lines; Stars; Stellar atmospheres; Stellar evolution; Three dimensional models
• ISSN: 1742-6588; 1742-6596
• DOI: 10.1088/1742-6596/397/1/012049

The precise measurement of the chemical composition of stars is a fundamental problem relevant to many areas of astrophysics. State-of-the-art approaches attempt to unite accurate descriptions of microphysics, non-local thermodynamic equilibrium (non-LTE) line formation and 3D hydrodynamical model atmospheres. In this paper I review progress in understanding inelastic collisions of hydrogen atoms with other species and their influence on spectral line formation and derived abundances in stellar atmospheres. These collisions are a major source of uncertainty in non-LTE modelling of spectral lines and abundance determinations, especially for old, metal-poor stars, which are unique tracers of the early evolution of our galaxy. Full quantum scattering calculations of direct excitation processes X(nl) + H [left right arrow] X(n'l')+ H and charge transfer processes X(nl) + H [left right arrow] X super(+) + H super(-) have been done for Li, Na and Mg [1,2,3] based on detailed quantum chemical data, e.g. [4]. Rate coefficients have been calculated and applied to non-LTE modelling of spectral lines in stellar atmospheres [5,6,7,8,9]. In all cases we find that charge transfer processes from the first excited S-state are very important, and the processes affect measured abundances for Li, Na and Mg in some stars by as much as 60%. Effects vary with stellar parameters (e.g. temperature, luminosity, metal content) and so these processes are important not only for accurate absolute abundances, but also for relative abundances among dissimilar stars.