Rotational (De-)excitation of Interstellar Silicon Dicarbide (c-SiC2) by He Collisions

Silicon dicarbide (c-SiC2) holds the distinction of being the first molecular ring identified in an astronomical source, marking a milestone in cosmic chemistry. This cyclic molecule, with its C2v symmetry, plays a pivotal role in the chemical dynamics of carbon-rich stellar environments, particularly in the cooler outer circumstellar envelopes (CSEs) of asymptotic giant branch (AGB) stars, where it contributes to the nucleation of silicon carbide dust. Its rotational transitions are critical diagnostics for probing the physical and chemical conditions of these astrophysical environments. In this study, we explore the rotational (de-)excitation of c-SiC2 induced by collisions with helium, a prevalent collisional partner in the interstellar medium. Using state-of-the-art quantum scattering calculations and a highly accurate three-dimensional potential energy surface derived from CCSD(T)-F12a/aug-cc-pVTZ computations, we determine collisional rate coefficients across a broad temperature range (1–70 K). Our analysis reveals pronounced energy-dependent trends in rotational excitation and de-excitation propensities, governed by the interplay of V10 and V20 terms in the interaction potential. These rate coefficients provide critical inputs for modeling the abundance and distribution of c-SiC2 in various astrophysical settings. Notably, our results identify specific rotational transitions that dominate at different temperature regimes, offering new opportunities for observational detection. These findings not only enhance our understanding of c-SiC2’s role in interstellar chemistry but also refine the broader models of dust formation and molecular evolution in carbon-rich stellar outflows.