Collisional excitation of SiC6 by helium: cross sections, rate coefficients, and astrophysical implications
Abstract
Rotational transitions of SiC6 with j=26--$40$ have been detected in IRC+10216, but collisional rate coefficients for SiC6 with He or H2 were previously unavailable. We present rate coefficients for SiC6 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 ∼ 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→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 SiC6--He rate coefficients to estimate those for SiC6--H2 and model the observed transitions. A χ2 analysis of the observed line fluxes, excluding heavily blended features, yields a best-fitting of n(H2)=4.33×105\,cm−3 and N(SiC6)=8.11×1011\,cm−2. The latter agrees excellently with the previous observational estimate of (7.9±1.5)×1011\,cm−2, supporting the consistency of our non-LTE modelling with observational constraints.
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BibTeX
@misc{tian2026collisional,
title = {{Collisional excitation of SiC$_6$ by helium: cross sections, rate coefficients, and astrophysical implications}},
author = {Liman Tian and Ziyu Xiang and Chuangang Ning and Xiaolin Chen},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.06525},
eprint = {2610.06525},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.06525},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Collisional excitation of SiC_6 by helium: cross sections, rate coefficients, and astrophysical implications
AU - Liman Tian
AU - Ziyu Xiang
AU - Chuangang Ning
AU - Xiaolin Chen
PY - 2026
DA - 2026/10/21
PB - arXiv
DO - 10.48550/arxiv.2610.06525
UR - https://arxiv.org/abs/2610.06525
AB - 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 ∼ 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→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 χ^2 analysis of the observed line fluxes, excluding heavily blended features, yields a best-fitting of n(H_2)=4.33×10^5\,cm^−3 and N(SiC_6)=8.11×10^11\,cm^−2. The latter agrees excellently with the previous observational estimate of (7.9±1.5)×10^11\,cm^−2, supporting the consistency of our non-LTE modelling with observational constraints.
ER -