发表机构
Zhejiang A & F University; Tsinghua University（浙江农林大学; 清华大学）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
本文通过量子散射计算提供 SiC$_6$--He 碰撞的态到态速率系数，验证 CS 近似适用性，并模拟 IRC+10216 中 SiC$_6$ 的激发，发现高 $J$ 跃迁存在非局部热动平衡效应。
AI 中文摘要
背景。SiC$_6$ 最近通过 $J=26$--$40$ 的转动跃迁在富碳星周包层 IRC+10216 中被识别。为了模拟其激发，需要可靠的碰撞速率系数，但目前尚未有 SiC$_6$ 与 He 或 H$_2$ 碰撞的态到态转动非弹性速率的报道。目标。我们提供 SiC$_6$--He 碰撞的态到态热速率系数，并评估耦合态（CS）近似对观测相关的高转动能级的适用性。方法。在刚性转子近似下，利用 RCCSD(T)-F12 方法获得的二维 SiC$_6$--He 势能面上进行了量子散射计算。对较低转动能级进行了紧耦合（CC）计算，对较高能级进行了 CS 计算。获得了 $j \leq 40$ 和 3--100 K 范围内的速率系数，并用于 RADEX 以研究 IRC+10216 中 SiC$_6$ 的激发。结果。CC 和 CS 计算显示出相似的能量依赖性，尽管某些弱跃迁的速率系数差异可达 2--3 倍，特别是在低能共振区域。然而，RADEX 计算对所考虑的高 $J$ 跃迁给出了几乎相同的激发温度和亮温度，表明在所探索的条件下 CS 适用于非局部热动平衡模拟。所得数据集提供了高达 $j=40$ 的态到态速率，能够模拟观测到的 $J=26$--$40$ 跃迁。在 IRC+10216 的典型低密度（$n(\mathrm{H}_2) \sim 10^4$ cm$^{-3}$）下，$40 \rightarrow 39$ 跃迁是亚热激发的，突出了非局部热动平衡效应对 SiC$_6$ 高 $J$ 发射的重要性。
英文摘要
Context. SiC$_6$ was recently identified toward the carbon-rich circumstellar envelope IRC+10216 through rotational transitions with $J=26$--$40$. Reliable collisional rate coefficients are required to model its excitation, but state-to-state rotationally inelastic rates for SiC$_6$ with He or H$_2$ collisions have not been reported. Aims. We provide state-to-state thermal rate coefficients for SiC$_6$--He collisions and assess the applicability of the coupled-states (CS) approximation to the high rotational levels relevant to the observations. Methods. Quantum scattering calculations were performed on a two-dimensional SiC$_6$--He potential energy surface obtained using the RCCSD(T)-F12 method within the rigid-rotor approximation. Close-coupling (CC) calculations were performed for lower rotational levels and CS calculations for higher levels. Rate coefficients were obtained for $j \leq 40$ and 3--100 K and used in RADEX to investigate SiC$_6$ excitation in IRC+10216. Results. CC and CS calculations show similar energy dependence, although rate coefficients for some weak transitions differ by up to a factor of 2--3, particularly in the low-energy resonance region. Nevertheless, RADEX calculations give nearly identical excitation and brightness temperatures for the high-$J$ transitions considered, indicating that CS is adequate for non-LTE modelling under the conditions explored. The resulting dataset provides state-to-state rates up to $j=40$, enabling modelling of the observed $J=26$--$40$ transitions. At the typical low density of IRC+10216, ($n(\mathrm{H}_2) \sim 10^4$ cm$^{-3}$), the $40 \rightarrow 39$ transition is subthermally excited, highlighting the importance of non-LTE effects for the high-$J$ emission of SiC$_6$.
Comments8 pagee, 9 figures
