论文标题
星际云中水的矫正比
The ortho-to-para ratio of water in interstellar clouds
论文作者
论文摘要
使用格伦布尔大学的阿尔普斯天文学网络（UGAN）研究了星际水的核旋转化学。该网络包括涉及碳，氮，氧气和硫的氢化物的不同核旋转状态及其剥离形式。在涉及多达七个质子的反应的争夺假设中实施了核旋转选择规则。气相水和水离子（H $ _2 $ o $ $^+$和H $ _3 $ o $^+$）的丰度和正para比（OPR）是在代表深色分子云的稳态条件下计算的。该模型结合了分子的冷冻到晶粒，简单的晶粒表面化学和宇宙射线诱导并直接吸收IC。发现预测的OPR与热和统计值显着偏离，并且独立于$ \ sim $ 30〜K的温度。根据H $ _2 $ _2 $ o的h $ _2 $ O的OPR，取决于H $ _2 $的自旋状态，这与在灭绝相对较高的半透明云中得出的值非常吻合。在PRESTELLAR CORE倒塌计算中，显示H $ _2 $ O的OPR显示出严重耗竭区域的统计值3（$ n _ {\ rm H}> 10^7 $ 〜cm $^{ - 3} $）。我们得出的结论是，低水OPR（$ \ lyssim 2.5 $）与气相离子中性化学一致，并反映了使用OPR（H $ _2）\ Lessim 1 $的气体。最终讨论了原月球磁盘和彗星中可用的OPR测量。
The nuclear-spin chemistry of interstellar water is investigated using the University of Grenoble Alpes Astrochemical Network (UGAN). This network includes reactions involving the different nuclear-spin states of the hydrides of carbon, nitrogen, oxygen and sulphur, as well as their deuterated forms. Nuclear-spin selection rules are implemented within the scrambling hypothesis for reactions involving up to seven protons. The abundances and ortho-to-para ratios (OPRs) of gas-phase water and water ions (H$_2$O$^+$ and H$_3$O$^+$) are computed under the steady-state conditions representative of a dark molecular cloud and during the early phase of gravitational collapse of a prestellar core. The model incorporates the freezing of the molecules on to grains, simple grain surface chemistry and cosmic-ray induced and direct desorption of ices. The predicted OPRs are found to deviate significantly from both thermal and statistical values and to be independent of temperature below $\sim $30~K. The OPR of H$_2$O is shown to lie between 1.5 and 2.6, depending on the spin-state of H$_2$, in good agreement with values derived in translucent clouds with relatively high extinction. In the prestellar core collapse calculations, the OPR of H$_2$O is shown to reach the statistical value of 3 in regions with severe depletion ($n_{\rm H}> 10^7$~cm$^{-3}$). We conclude that a low water OPR ($\lesssim 2.5$) is consistent with gas-phase ion-neutral chemistry and reflects a gas with OPR(H$_2)\lesssim 1$. Available OPR measurements in protoplanetary disks and comets are finally discussed.
