Hyperfine-resolved transition rates of the DCO+ ion by collisions with helium


E-mail: buffa@df.unipi.it


The importance of rotational transition rates in the analysis of cold interstellar clouds is well known. We present results, for temperatures ranging from 5 to 80 K, for the hyperfine-resolved rotational transitions of DCO+ induced by collision with helium. Since the isotopic substitution is not expected to introduce significant changes, close-coupling calculations are based on a potential energy surface obtained for He–HCO+ and checked by accurate pressure broadening and shift measurements. The well-grounded assumption that deuterium nuclear spin is not affected by the collisions allowed us to obtain the hyperfine-resolved transition matrix elements as a sum of spin-free transition matrix elements, which account for collision dynamics, multiplied by purely geometrical factors, which account for the hyperfine dependence. The temperature dependence of the rates is weak for downward transitions jj′, j′ < j, while for upward transitions (j′ > j) it can be large due to the need of energy to be transferred from translation to rotation. The dependence of the rates on j and j′ and hyperfine propensities is discussed. The rates for quasi-elastic purely hyperfine transitions j, FjF′ are also obtained.