Depolarization of MgH Solar Lines by Collisions with Hydrogen Atoms

Interpretations of the very rich second solar spectrum of the MgH molecule face serious problems owing to the complete lack of any information about rates of collisions between the MgH and hydrogen atoms. This work seeks to begin the process of filling this lacuna by providing, for the first time, quantum excitation, depolarization, and polarization transfer collisional rates of the MgH ground state $X^2\Sigma$. To achieve the goals of this work, potential energy surfaces are calculated and then are included in the Schr\"odinger equation to obtain the probabilities of collisions and, thus, all collisional rates. Our rates are obtained for temperatures ranging from $T \!\!=$2000 K to $T \!\!=$15,000 K. Sophisticated genetic programming methods are adopted in order to fit all depolarization rates with useful analytical functions of two variables: the total molecular angular momentum and temperatures. We study the solar implications of our results, and we find that the $X^2\Sigma$ state of MgH is partially depolarized by isotropic collisions with neutral hydrogen in its ground state $^2S$. Our findings show the limits of applicability of the widely used approximation in which the lower-level polarization is neglected.