HAC演化对Orion Bar和Horsehead PDR小烃形成的影响
Impact of HAC evolution on the formation of small hydrocarbons in the Orion Bar and the Horsehead PDRs
摘要：
我们研究了年轻大质量恒星附近光解离区（PDR）中氢化非晶碳（HAC）颗粒在强紫外辐射下的演化。我们的目的是评价HAC颗粒对PDR中观察到的小烃形成的影响：C $_2$ H、C $_2$ H $_2$、C $_3$ H $^+$、C $_3$ H、C $_3$ H $_2$、C $_4$ H。我们开发了一个微观模型的HAC颗粒的基础上，现有的实验结果。该模型包括尘埃表面的光解吸、热解吸、氢原子和碳原子的吸积以及随后的碳质地幔的形成过程。H $_2$，CH $_4$，C $_2$ H $_2$，C $_2$ H $_4$，C $_2$ H $_6$，C $_3$ H $_4$，C $_3$ H $_6$，C $_3$ H $_8$被认为是HAC光破坏的主要碎片。我们在猎户座星云和马头星云两个PDR的物理条件下模拟了HAC颗粒的演化。我们估计了气相化学反应中HAC碎片的产生速率，并将其与由于HAC破坏而产生的碎片的产生速率进行比较。在某些条件下，即在两种PDR中A $_V$ =0.1时，后一种速率可能占主导地位。我们把我们的模型与气体-颗粒化学模型MONACO相结合，并计算了观测到的小碳氢化合物的丰度。我们的结论是，HAC破坏碎片的化学贡献是不足以匹配所观察到的丰度，虽然它增加了几个数量级的丰度在猎户座酒吧在A $_V$ =0.1。此外，我们发现，尘埃表面上的碳质地幔的形成过程可以是一个抑制剂，在PDR中观察到的小碳氢化合物的形成。
英文摘要：
We study evolution of hydrogenated amorphous carbon (HAC) grains under harsh UV radiation in photo-dissociation regions (PDRs) near young massive stars. Our aim is to evaluate the impact of the HAC grains on formation of observed small hydrocarbons: C$_2$H, C$_2$H$_2$, C$_3$H$^+$, C$_3$H, C$_3$H$_2$, C$_4$H, in PDRs. We developed a microscopic model of the HAC grains based on available experimental results. The model includes processes of photo- and thermodesorption, accretion of hydrogen and carbon atoms and subsequent formation of carbonaceous mantle on dust surface. H$_2$, CH$_4$, C$_2$H$_2$, C$_2$H$_4$, C$_2$H$_6$, C$_3$H$_4$, C$_3$H$_6$, C$_3$H$_8$ are considered as the main fragments of the HAC photo-destruction. We simulated evolution of the HAC grains under the physical conditions of two PDRs, the Orion Bar and the Horsehead nebula. We estimated the production rates of the HAC fragments in gas phase chemical reactions and compared them with the production rates of fragments due to the HAC destruction. The latter rates may dominate under some conditions, namely, at A$_V$=0.1 in both PDRs. We coupled our model with the gas-grain chemical model MONACO and calculated abundances of observed small hydrocarbons. We conclude that the contribution of the HAC destruction fragments to chemistry is not enough to match the observed abundances, although it increases the abundances by several orders of magnitude in the Orion Bar at A$_V$=0.1. Additionally, we found that the process of carbonaceous mantle formation on dust surface can be an inhibitor for the formation of observed small hydrocarbons in PDRs.
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