标题： 氦气对氢氰化物（HCN）和异氢氰酸酯（HNC）的激发：分子云中HCN/HNC丰度比的新见解 显示英文标题

标题： The rotational excitation of HCN and HNC by He: New insights on the HCN/HNC abundance ratio in molecular clouds

摘要： 星际云分子发射的建模需要计算与最丰富的物种碰撞激发的速率。本文的重点是计算基态振动条件下 HCN 和 HNC 分子与 He 碰撞时的转动激发速率系数。这些计算基于从高度相关的\textit{从头开始}计算得到的新二维势能面。通过 He 对 HCN 和 HNC 的纯转动（去）激发截面进行了计算，采用了本质上精确的紧密耦合方法。对于 HCN 和 HNC 的前 8 个转动能级之间的跃迁，计算了高达 1000 cm$^{-1}$动能下的截面。这些截面用于确定温度范围从 5 K 到 100 K 的碰撞速率常数。在 HCN-He 碰撞中观察到偶数$\Delta j$转动跃迁的倾向，在 HNC-He 碰撞中则观察到奇数$\Delta j$转动跃迁的倾向。评估了这些结果对天体物理模型的影响，并表明使用 HCN 的速率系数来解释 HNC 观测可能会导致确定 HNC 丰度的重大误差，特别是在冷暗云中，新得出的 HNC 碰撞速率显示该物种的$j=1-0$能级比 HCN 更容易被激发。新的 HNC-He 碰撞速率的一个重要结果是，冷云中观测到的 HNC/HCN 丰度比需要从$>$1 修正为$\simeq$1，这与文献中现有的详细化学模型非常一致。 显示英文摘要

摘要： Modeling of molecular emission from interstellar clouds requires the calculation of rates for excitation by collisions with the most abundant species. The present paper focuses on the calculation of rate coefficients for rotational excitation of the HCN and HNC molecules in their ground vibrational state in collision with He. The calculations are based on new two-dimensional potential energy surfaces obtained from highly correlated \textit{ab initio} calculations. Calculations of pure rotational (de)excitation cross sections of HCN and HNC by He were performed using the essentially exact close-coupling method. Cross sections for transitions among the 8 first rotational levels of HCN and HNC were calculated for kinetic energies up to 1000 cm$^{-1}$. These cross sections were used to determine collisional rate constants for temperatures ranging from 5 K to 100 K. A propensity for even $\Delta j$ transitions is observed in the case of HCN--He collisions whereas a propensity for odd $\Delta j$ transitions is observed in the case of HNC--He collisions. The consequences for astrophysical models are evaluated and it is shown that the use of HCN rate coefficients to interpret HNC observations can lead to significant inaccuracies in the determination of the HNC abundance, in particular in cold dark clouds for which the new HNC rates show that the $j=1-0$ line of this species will be more easily excited by collisions than HCN. An important result of the new HNC-He rates is that the HNC/HCN abundance ratio derived from observations in cold clouds has to be revised from $>$1 to $\simeq$1, in good agreement with detailed chemical models available in the literature.