Collisional excitation of SiC${}_6$ by helium: cross sections, rate coefficients, and astrophysical implications

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 $19\to 18$. 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 ${\chi }^2$ analysis of the observed line fluxes, excluding heavily blended features, yields a best-fitting of $n({\mathrm{H}}_2)=4.33\times 10^5$\,cm${}^{-3}$ and $N(\mathrm{Si}{\mathrm{C}}_6)=8.11\times 10^{11}$\,cm${}^{-2}$. The latter agrees excellently with the previous observational estimate of $(7.9\pm 1.5)\times 10^{11}$\,cm${}^{-2}$, supporting the consistency of our non-LTE modelling with observational constraints.

arXiv comment13 pagee, 7 figures