Depolarization and polarization transfer rates for the C2 (X1 Σ+g, a 3Πu) + H(2S1/2) collisions in the solar photosphere

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Context. This paper is a continuation of a series of studies that investigated the collisional depolarization of solar molecular lines such as those of MgH, CN and C2. It is focused on the solar molecule C2, which exhibits striking scattering polarization profiles, although its intensity profiles are inconspicuous and barely visible. The current interpretation of the C2 polarization in terms of magnetic fields is incomplete because collisional data are almost completely lacking. Aims. We accurately compute the collisional depolarization and polarization transfer rates for the C2(X1Σg+,α3Πu) by isotropic collisions with hydrogen atoms H (2S l/2). We also investigate the solar implications of our findings. Methods. We used the package MOLPRO to obtain potential energy surfaces for the electronic states X1Σg+ and a3Πu of C2, and the code MOLSCAT to study the quantum dynamics of the C2(X1Σg+,α3Πu) + H(2S1/2) systems. We used the tensorial irreducible basis to express the resulting collisional cross sections and rates. Furthermore, sophisticated genetic programming techniques were employed to determine analytical expressions for the temperature and total molecular angular momentum dependence of these collisional rates. Results. We obtained quantum depolarization and polarization transfer rates for the C2(X1Σg+,α3Πu) + H(2S1/2) collisions in the temperature range T = 2000−15 000 K. We also determined analytical expressions that write 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 the C2 lines. This highlights that the approximation of neglecting lower-level polarization is limited in modeling the polarization of C2 lines. Conclusions. Isotropic collisions with neutral hydrogen atoms are a fundamental ingredient for understanding C2 polarization.