A statistical investigation of the rate constants for the H+ + HD reaction at temperatures of astrophysical interest

The H+ + HD(v, j) reaction has been investigated in detail by means of a statistical quantum method. State-to-state cross sections and rate constants for transitions between reactants and rovibrational states HD(v′, j′) of the product arrangement with energies below 0.9 eV collision energy [that is, HD(v = 0, j = 0–11) and HD(v = 1, j = 0–6)] have been calculated. For the other product channel, D+ + H2(v′, j′), rovibrational states up to (v′ = 0, j′ = 9) have been considered for the calculation of the corresponding thermal rate. Present predictions are compared with previously reported theoretical and experimental rates. Finally, cooling functions for HD due to proton and atomic hydrogen collisions are computed in the low-density regime. We find that the much larger HD–H+ cooling function, as compared with that of HD–H, does not compensate for the low H+/H abundance ratio in astrophysical media so that HD cooling is dominated by HD–H (or HD–H2) collisions.