Helium-induced rotational (de-)excitation of interstellar rhomboidal silicon tricarbide (c-SiC₃)

Institute of Physics, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University in Toruń, Grudzidzka 5, 87-100 Toruń, Poland

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 12 November 2025
Accepted: 7 April 2026

Abstract

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-SiC₃) 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-SiC₃ − He interaction at the CCSD(T)-F12a/aug-cc-pVTZ 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 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 Δj transitions, dominated by Δj = ±2 transitions induced by the V₂₀ anisotropic term. These data represent the first set of collisional rate coefficients for c-SiC₃ 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.