[论文解读] Coagulation and Fragmentation in molecular clouds. II. The opacity of the dust aggregate size distribution

Chris W. Ormel, M. Min+3 more|arXiv (Cornell University)|Jun 16, 2011

Astrophysics and Star Formation Studies参考文献 59被引用 71

一句话总结

本研究通过有效介质理论，模拟了分子云中尘埃聚集体的消光特性，考虑了聚集、破碎以及冰层包覆的影响。结果表明，多孔且覆冰的聚集体可延长9.7 μm的硅酸盐特征，并降低硅酸盐特征与近红外色指数之比——与致密核心的观测结果一致；当聚集体尺寸超过约10 μm时，两项特征均减弱；若聚集体尺寸达到约100 μm，则亚毫米波段消光显著增强。

ABSTRACT

The dust size distribution in molecular clouds can be strongly affected by ice-mantle formation and (subsequent) grain coagulation. Following previous work where the dust size distribution has been calculated from a state-of-the art collision model for dust aggregates that involves both coagulation and fragmentation (Paper I), the corresponding opacities are presented in this study. The opacities are calculated by applying the effective medium theory assuming that the dust aggregates are a mix of 0.1μm silicate and graphite grains and vacuum. In particular, we explore how the coagulation affects the near-IR opacities and the opacity in the 9.7μm silicate feature. We find that as dust aggregates grow to μm-sizes both the near-IR color excess and the opacity in the 9.7 μm feature increases. Despite their coagulation, porous aggregates help to prolong the presence of the 9.7μm feature. We find that the ratio between the opacity in the silicate feature and the near-IR color excess becomes lower with respect to the ISM, in accordance with many observations of dark clouds. However, this trend is primarily a result of ice mantle formation and the mixed material composition of the aggregates, rather than being driven by coagulation. With stronger growth, when most of the dust mass resides in particles of size 10μm or larger, both the near-IR color excess and the 9.7μm silicate feature significantly diminish. Observations at additional wavelengths, in particular in the sub-mm range, are essential to provide quantitative constraints on the dust size distribution within dense cores. Our results indicate that the sub-mm index β will increase appreciably, if aggregates grow to ~100μm in size.

研究动机与目标

• 量化尘埃聚集与冰层包覆如何改变分子云中尘埃聚集体的消光特性。
• 解释致密核心中近红外消光曲线变平以及9.7 μm硅酸盐特征减弱的观测现象。
• 评估聚集体形态（孔隙率、成分）对分子云光谱能量分布的影响。
• 评估尘埃增长对亚毫米波段消光及尘埃光谱指数β的影响。
• 将理论消光趋势与斯皮itzer、2MASS及亚毫米波段巡天的观测约束相联系。

提出的方法

• 利用包含聚集与破碎过程的碰撞模型（来自论文I）模拟尘埃聚集体的尺寸分布。
• 应用有效介质理论，计算由0.1 μm硅酸盐、石墨与真空组成的混合物的消光特性，考虑冰层包覆。
• 固定气体密度为10⁵ cm⁻³，并假设球形均匀颗粒用于碰撞建模。
• 通过改变硅酸盐与石墨组分的混合方式（空间尺度与聚集体尺度混合），测试消光特性的变化。
• 计算波长依赖的消光特性，重点关注近红外波段（J、K波段）、9.7 μm硅酸盐特征及亚毫米波段。
• 分析9.7 μm光学厚度与近红外色指数之比（q = τ_sil / E(J−K)）作为尘埃演化程度的诊断指标。

实验结果

研究问题

• RQ1 尘埃聚集如何影响分子云中近红外色指数与9.7 μm硅酸盐特征的强度？
• RQ2 冰层包覆与混合材料组成在致密核心中对观测到的消光比有何影响？
• RQ3 聚集体尺寸增长对亚毫米波段消光及尘埃光谱指数β有何影响？
• RQ4 为何致密核心中硅酸盐特征与近红外消光之比（q）低于弥散星际介质中的值？
• RQ5 仅靠聚集过程能否解释观测到的消光趋势，还是需要成分解耦？

主要发现

• 随着尘埃聚集体增长至微米量级，近红外色指数因散射与消光增强而增加。
• 多孔且覆冰的聚集体可使9.7 μm硅酸盐特征持续更久，其减弱过程相比致密颗粒被延迟。
• 9.7 μm消光与近红外色指数之比（q）低于弥散星际介质中的约0.34，主要归因于冰层包覆与混合材料组成，而非仅由聚集引起。
• 当大部分尘埃质量集中于大于约10 μm的聚集体时，近红外色指数与9.7 μm特征均显著减弱。
• 若聚集体尺寸增长至约100 μm，亚毫米波段消光显著增强，导致尘埃光谱指数β明显上升。
• 当q值远小于1时，需假设碳质与硅酸盐颗粒发生解耦——即仅硅酸盐颗粒聚集且冰层仅包覆硅酸盐颗粒——表明尘埃演化路径并非普适。