Rotational excitation of CaC2 by He: a comprehensive collisional dataset and non-LTE modelling

Metal dicarbides are predicted to be important chemical constituents of evolved stars. The identification of CaC2 in the envelope of the evolved carbon star IRC+10216 confirms this long-standing hypothesis. We present quantum-mechanical calculations of the rotational excitation and de-excitation of interstellar calcium dicarbide, CaC2, induced by collisions with helium. A new, highly accurate three-dimensional potential energy surface for the CaC2–He complex is developed at the RCCSD(T)-F12/aug-cc-pVTZ level of theory. The potential exhibits a global minimum with a well depth of approximately 33.8 cm−1. State-to-state cross sections are computed for transitions involving CaC2 rotational levels below 130 cm−1, using both the close-coupling and coupled states methods. The corresponding rate coefficients are obtained up to 60 K by thermal averaging over a Maxwell–Boltzmann velocity distribution. Comparison with previous collisional data shows that the existing datasets are incomplete and miss several transitions. Using the He-induced rates as a template for para-H2 in non-LTE radiative-transfer calculations, we show that the available literature data cannot reliably model CaC2 excitation because of their incomplete transition coverage. We therefore recommend the use of the present collisional dataset for future modelling of CaC2 line emission.