Rotational excitation of methylidynium (CH) by a helium atom at high temperature

Context. The Herschel Space Observatory with its high-resolution instrument HIFI on board will observe the CH 1 → 0 and 2 → 1 rotational transitions in a wide range of gas temperatures up to 1000 K. Collisional parameters for such temperatures are thus welcome. Aims. We aim to obtain accurate rate coefficients for the collisional excitation of CH by He for high gas temperatures. Methods. The ab initio coupled-cluster [CCSD(T)] approximation was used to compute the interaction potential energy. Cross sections are then derived in the close coupling (CC) approach and rate coefficients inferred by averaging these cross sections over a Maxwell-Boltzmann distribution of kinetic energies. Results. Cross sections are calculated up to 10 000 cm for J ranging from 0 to 10. Rate coefficients are obtained at high temperatures up to 2000 K.