Depolarizing collisions with hydrogen: neutral and singly ionized alkaline earths

TL;DR

This paper calculates depolarizing collision rates between hydrogen and alkali-earth atoms relevant for solar magnetic field diagnostics, using quantum mechanical methods and ab initio potential curves.

Contribution

It provides new ab initio calculations of depolarizing rates for multiple alkali-earth atoms due to hydrogen collisions, improving previous estimates.

Findings

Depolarizing rates vary with temperature and atomic species.

Results impact interpretation of solar polarization and magnetic field measurements.

Comparison with previous literature shows improved accuracy.

Abstract

Depolarizing collisions are elastic or quasielastic collisions that equalize the populations and destroy the coherence between the magnetic sublevels of atomic levels. In astrophysical plasmas, the main depolarizing collider is neutral hydrogen. We consider depolarizing rates on the lowest levels of neutral and singly ionized alkaly-earths Mg I, Sr I, Ba I, Mg II, Ca II, and Ba II, due to collisions with H. We compute ab initio potential curves of the atom-H system and solve the quantum mechanical dynamics. From the scattering amplitudes we calculate the depolarizing rates for Maxwellian distributions of colliders at temperatures T <10000 K. A comparative analysis of our results and previous calculations in the literature is done. We discuss the effect of these rates on the formation of scattering polarization patterns of resonant lines of alkali-earths in the solar atmosphere, and their effect on Hanle effect diagnostics of solar magnetic fields.