Evolution of PAHs in photodissociation regions: Hydrogenation and charge states
Evolution of PAHs in photodissociation regions: Hydrogenation and charge states
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光解区域 PAH 的演变：氢化和电荷状态
DOI：
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发表时间：
2013
期刊：
影响因子：
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通讯作者：
D. Toublanc
中科院分区：
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作者：
J. Montillaud;J. Montillaud;J. Montillaud;C. Joblin;C. Joblin;D. Toublanc;D. Toublanc
语境。各种研究都强调了光解离区（PDR）中星际多环芳烃（PAH）群体的电荷状态和组成的变化。这些变化预计将影响这些区域的能量学和化学，需要进行定量描述。目标。我们的目标是模拟 PDR 中 PAH 的电荷和氢化状态的空间演化。我们重点关注 NGC 7023 西北 (NW) PDR 的具体情况，该情况有许多观测约束。我们还讨论了使用星际介质（ISM）的情况。方法。 NGC 7023 NW 的物理条件是使用最先进的 PDR 代码进行建模的。然后，我们使用新的多环芳烃化学演化模型，其中包括最新的多环芳烃实验数据并描述多光子事件。我们考虑了一系列含有最多 96 个碳原子的紧凑型 PAH。结果。计算出的电离比与 NGC 7023 NW 中观测到的电离比非常一致。在 PDR 内，PAH 演变为三个主要种群。我们发现中等尺寸的多环芳烃 (50 . NC . 90) 通常会氢化，而较大的多环芳烃 (NC & 90) 可以超氢化，而较小的多环芳烃 (NC . 50) 则完全脱氢。在最近检测到富勒烯 C60 的腔体中使用较多的气体中，发现所有研究的 PAH 都很快完全脱氢。由于多光子事件，PAH 化学演化表现出复杂的非线性行为，作为紫外线辐射场的函数。当 AV=1 时，达到氢化稳态的时间尺度范围从小 PAH 不到一年到大 PAH 长达 10 4 年。需要更多研究的关键反应是阳离子与电子的重组、阳离子与 H2 的反应性以及中性 PAH 与 H 的反应性。结论。我们根据最新的分子数据开发了一种新的多环芳烃化学演化模型。该模型使我们能够在没有任何拟合参数的情况下合理化观测约束。小于 50 个碳原子的 PAH 预计不会在 NGC 7023 NW PDR 中存活。对于di use ISM 也得到了类似的结论。碳簇是 PAH 光解的最终产物，需要进一步研究这些簇的演化，以评估它们对 PDR 化学和物理演化的影响。
Context. Various studies have emphasized variations of the charge state and composition of the interstellar polycyclic aromatic hydrocarbon (PAH) population in photodissociation regions (PDRs). These changes are expected to impact the energetics and chemistry in these regions calling for a quantitative description. Aims. We aim to model the spatial evolution of the charge and hydrogenation states of PAHs in PDRs. We focus on the specific case of the north-west (NW) PDR of NGC 7023, for which many observational constraints are available. We also discuss the case of the di use interstellar medium (ISM). Methods. The physical conditions in NGC 7023 NW are modelled using a state-of-the-art PDR code. We then use a new PAH chemical evolution model that includes recent experimental data on PAHs and describes multiphoton events. We consider a family of compact PAHs bearing up to 96 carbon atoms. Results. The calculated ionization ratio is in good agreement with the observed ratio in NGC 7023 NW. Within the PDR, PAHs evolve into three major populations. We find medium-sized PAHs (50 . NC . 90) to be normally hydrogenated, while larger PAHs (NC & 90) can be superhydrogenated, and smaller species (NC . 50) are fully dehydrogenated. In the more di use gas of the cavity, where the fullerene C60 was recently detected, all the studied PAHs are found to be quickly fully dehydrogenated. PAH chemical evolution exhibits a complex non-linear behaviour as a function of the UV radiation field because of multiphoton events. Steady state for hydrogenation is reached on timescales ranging from less than a year for small PAHs, up to 10 4 years for large PAHs at AV=1. Critical reactions that would need more studies are the recombination of cations with electrons, the reactivity of cations with H2 and the reactivity of neutral PAHs with H. Conclusions. We developed a new model of PAH chemical evolution based on the most recent available molecular data. This model allows us to rationalize the observational constraints without any fitting parameter. PAHs smaller than 50 carbon atoms are not expected to survive in the NGC 7023 NW PDR. A similar conclusion is obtained for the di use ISM. Carbon clusters turn out to be end products of PAH photodissociation, and the evolution of these clusters needs to be investigated further to evaluate their impact on the chemical and physical evolution of PDRs.