Helium-induced rotational (de-)excitation of interstellar rhomboidal silicon tricarbide (c-SiC _3)

Silicon carbide molecules are key tracers of dust formation and molecular evolution in carbon-rich circumstellar environments, yet their collisional excitation remains largely unexplored. We report the first quantum scattering study of the collisional (de-)excitation of interstellar rhomboidal silicon tricarbide (c-mathrm SiC _3) by helium atoms under low-temperature conditions relevant to astrophysical environments. We computed a high-accuracy 3D potential energy surface (3D-PES) for the c-mathrm SiC _3- interaction at the operatorname He CCSD ( )-mathrm T F 12 / - ^ a aug level of theory and analytically represented in terms of spherical harmonics. The global minimum of the PES lies at a T-shaped configuration with a well depth of about 42 mathrm cm -1 , revealing a highly anisotropic interaction. State-to-state inelastic cross-sections were computed using the close-coupling formalism for total energies up to 400 ^ cm -1 . The corresponding thermal rate coefficients were evaluated for kinetic temperatures between 5 and 50 K and found to display a strong propensity for even Δ j transitions, dominated by Δ j= ± 2 transitions induced by the V_20 anisotropic term. These data represent the first set of collisional rate coefficients for c-mathrm SiC _3 and provide essential input for nonlocal thermodynamic equilibrium (NLTE) radiative transfer models. They are crucial for interpreting observed line emission transitions in carbon-rich circumstellar envelopes such as IRC+10216 and offer valuable guidance for future searches for silicon-carbon ring species in the interstellar medium.