Depolarization and polarization transfer rates for the C2 (X 1+g, a 3u) + H(2S1/2) collisions in the solar photosphere
Abstract
This paper is a continuation of a series of studies investigating collisional depolarization of solar molecular lines like those of MgH, CN and C2. It is focused on the case of the solar molecule C2 which exhibits striking scattering polarization profiles although its intensity profiles are inconspicuous and barely visible. In fact, interpretation of the C2 polarization in terms of magnetic fields is incomplete due to the almost complete lack of collisional data. This work aims at accurately computing the collisional depolarization and polarization transfer rates for the C2~(X 1+g, a 3u) by isotropic collisions with hydrogen atoms H~(2S1/2). We also investigate the solar implications of our findings. We utilize the MOLPRO package to obtain potential energy surfaces (PESs) for the electronic states X 1+g and a3u of C2, and the MOLSCAT code to study the quantum dynamics of the C2~(X 1+g, a 3u) + H(2S1/2) systems. We use the tensorial irreducible basis to express the resulting collisional cross-sections and rates. Furthermore, sophisticated genetic programming techniques are employed to determine analytical expressions for the temperature and total molecular angular momentum dependence of these collisional rates. We obtain quantum depolarization and polarization transfer rates for the C2 (X 1+g, a 3u) + H(2S1/2) collisions in the temperature range T=2,000--15,000~K. We also determine analytical expressions giving these rates as functions of the temperature and total molecular angular momentum. In addition, we show that isotropic collisions with neutral hydrogen can only partially depolarize the lower state of C2 lines, rather than completely. This highlights the limitations of the approximation of neglecting lower-level polarization while modeling the polarization of C2 lines.
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