Collisional excitation of SiC$_6$ by helium: cross sections, rate coefficients, and astrophysical implications
Year: 2026
Type: preprint
Abstract: Rotational transitions of SiC$_6$ with $j=26$--$40$ have been detected in IRC+10216, but collisional rate coefficients for SiC$_6$ with He or H$_2$ were previously unavailable. We present rate coefficients for SiC$_6$ rotational excitation by He, calculated using a two-dimensional potential energy surface (PES) at the RCCSD(T)-F12 level under the rigid-rotor approximation. Close-coupling (CC) calculations were performed for low rotational levels, while the coupled-states (CS) method was used for higher levels. Convergence tests were conducted, and CC and CS scattering cross sections were compared. The CS rate coefficients cover rotational levels up to $j=40$ and temperatures of 3--60\,K. Although CC and CS rate coefficients differ by up to a factor of $\sim 2$ for some transitions, particularly those affected by weak coupling and resonances, they yield nearly identical excitation and brightness temperatures for the high-$j$ transitions examined, including $19\rightarrow18$. These results support the reliability of the CS method for non-LTE modelling of the high-$j$ transitions considered, for which CC calculations are computationally impractical. We use reduced-mass scaling of the SiC$_6$--He rate coefficients to estimate those for SiC$_6$--H$_2$ and model the observed transitions. A $χ^2$ analysis of the observed line fluxes, excluding heavily blended features, yields a best-fitting of $n(\mathrm{H_2})=4.33\times10^5$\,cm$^{-3}$ and $N(\mathrm{SiC_6})=8.11\times10^{11}$\,cm$^{-2}$. The latter agrees excellently with the previous observational estimate of $(7.9\pm1.5)\times10^{11}$\,cm$^{-2}$, supporting the consistency of our non-LTE modelling with observational constraints.
Source: arXiv (Cornell University)
Institutions: Zhejiang A & F University, Tsinghua University
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7220739824