[论文解读] Molecules and dust in Cas A: I - Synthesis in the supernova phase and processing by the reverse shock in the clumpy remnant

Chiara Biscaro, Isabelle Cherchneff|arXiv (Cornell University)|Jan 22, 2014

Gamma-ray bursts and supernovae参考文献 43被引用 24

一句话总结

本研究模拟了大犬座A（Cassiopeia A）这一IIb型超新星遗迹中的尘埃与分子形成过程，表明低密度喷射物无法形成显著的尘埃团簇；相反，必须依赖致密团块。反向激波会破坏原有的尘埃，但允许CO和SiO等分子重新形成——尽管由于激波后气体密度不足，尘埃团簇无法重新形成，这表明团块状喷射物对观测到的尘埃多样性和质量至关重要。

ABSTRACT

Aims: We study the chemistry of the Type IIb supernova ejecta that led to the Cas A supernova remnant to assess the chemical type and quantity of dust that forms and evolves in the remnant phase. We later model a dense oxygen-rich ejecta knot that is crossed by the reverse shock in Cas A to study the evolution of the clump gas phase and the possibility to reform dust clusters in the post-reverse shock gas. Methods: A chemical network including all processes efficient at high gas temperatures and densities is considered. The formation of key bimolecular species (CO, SiO) and dust clusters is described. Stiff, coupled, ordinary, differential equations are solved for the conditions pertaining to both the SN ejecta and the post-reverse shock gas. Results: We find that the ejecta of Type IIb SNe are unable to form large amounts of molecules and dust clusters as opposed to their Type II-P counterparts because of their diffuse ejecta. The gas density needs to be increased by several orders of magnitude to allow the formation of dust clusters. We show that the chemical composition of the dust clusters changes drastically and gains in chemical complexity with increasing gas density. Hence, the ejecta of the Cas A supernova progenitor must have been in the form of dense clumps to account for the dust chemical composition and masses inferred from infrared observations of Cas A. We show that the ejecta molecules in a clump that is processed by the reverse shock reform in the post-reverse shock gas with lower abundances than those of the initial ejecta clump, except SiO. These molecules include CO, SiS and O2. Dust clusters are destroyed by the reverse shock and do not reform in the post-reverse shock gas, even for the highest gas density. These results indicate that the synthesis of dust grains from the gas phase in the dense knots of Cas A and in other supernova remnants is unlikely.

研究动机与目标

• 评估IIb型超新星（特别是Cas A）喷射物中形成的尘埃的化学成分与质量。
• 确定Cas A中观测到的尘埃是否可能在均匀喷射物中形成，或必须依赖致密团块。
• 模拟富含氧的致密喷射物团块在反向激波穿越过程中的化学演化。
• 评估分子与尘埃团簇在激波后气体相中的重新形成潜力。
• 将Cas A中观测到的尘埃化学多样性与形成环境的物理条件联系起来。

提出的方法

• 通过详细化学网络模拟与超新星喷射物及激波后条件相关的高温、高密度气相反应。
• 使用刚性耦合常微分方程组，模拟在不同气体密度与温度下随时间演化的化学过程。
• 包括热分解、双分子反应、辐射结合、碰撞离解以及离子-分子反应等过程。
• 对橄榄石、刚玉、二氧化硅、金属碳化物、硫化物、纯金属及非晶碳等物质的尘埃团簇形成进行建模。
• 通过施加140–200 km/s速度的反向激波于致密团块上，模拟激波穿越过程，随后模拟激波后演化。
• 将结果与斯皮兹勒（Spitzer）、赫歇尔（Herschel）及阿塔卡马大型亚毫米波阵列（ALMA）的观测数据对比，以验证模型预测。

实验结果

研究问题

• RQ1 在像Cas A这样的IIb型超新星低密度喷射物中，能否发生显著的尘埃团簇形成？
• RQ2 气体密度在超新星喷射物气相中尘埃团簇成核过程中起何种作用？
• RQ3 在激波诱导破坏后，CO、SiO和SiS等分子在激波后气体中能多大程度上重新形成？
• RQ4 尘埃团簇能否在激波后气体中重新形成？若不能，原因是什么？
• RQ5 尘埃颗粒的化学复杂性如何依赖于喷射物初始气体密度？

主要发现

• IIb型超新星的喷射物过于稀薄，无法形成显著的尘埃团簇；气体密度必须提高200至2000倍才能实现成核。
• 尘埃团簇的化学复杂性随气体密度增加而提升，其中非晶碳仅在最高密度下形成。
• CO、SiO、SiS和O2等分子可在激波后气体中重新形成，且在更强激波下其丰度更高。
• 包括硅酸盐、碳化物和非晶碳在内的尘埃团簇无法在激波后气体中重新形成，原因在于密度不足。
• Cas A中观测到的尘埃多样性和质量要求前身星喷射物以致密团块形式存在，其密度对比至少为200倍于均匀喷射物。
• 激波后区域中存在温暖致密气体（n_gas ~ 10^6 cm⁻³），支持模型预测的分子在约100天 timescale 内重新形成。