Rotational excitation and de-excitation of interstellar calcium dicarbide (<i>c-</i>CaC2) by He collisions

ABSTRACT Calcium dicarbide (c-CaC2) has been recently detected in the IRC+10216 molecular envelope, confirming predictions of metal dicarbides in circumstellar environments. Its identification, alongside MgC2, provides key insights into metal-carbon circumstellar chemistry in evolved stars. Understanding its distribution, excitation, and abundance could provide valuable insights into processes such as dust formation and the incorporation of metals into gas-phase molecules through ion–molecule reactions in carbon-rich circumstellar environments. In this study, we present state-to-state collisional cross-sections and rate coefficients for the rotational excitation and de-excitation of c-CaC2 in collisions with He for a wide range of rotational transitions and temperatures (T ≤ 80 K) relevant to the interstellar medium. We used ab initio high-level CCSD(T)-F12/aug-cc-pVTZ calculations to compute a three-dimensional potential energy surface for the c-CaC2–He van der Waals complex. Close-coupling calculations were performed using the analytical potential to determine cross-sections for rotational transitions of c-CaC2 by He collisions, up to 400 cm−1. These cross-sections were then thermally averaged to obtain inelastic collision rate coefficients. Our results show that rotational de-excitation with Δj = Δkc = −1 has the highest rate coefficients at temperatures up to 80 K. However, for rotational excitation, transitions with Δj = Δkc = 1 are dominant only at very low temperatures (T ≤ 10 K). The newly computed rate coefficients are expected to play a crucial role in the determination of the abundances of metal carbides within circumstellar and interstellar environments.