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arXiv 1106.3265astro-ph.SRastro-ph.EP
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分子云中的凝聚和碎裂。II。尘埃聚集体粒度分布的不透明度

Coagulation and Fragmentation in molecular clouds. II. The opacity of the dust aggregate size distribution

C. W. Ormel, M. Min, A. G. G. M. Tielens, C. Dominik, D. Paszun

备注：
10 pages, accepted for publication in A&A
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摘要：

分子云中的尘埃粒度分布受冰幔形成和（随后的）颗粒凝聚的强烈影响。根据以前的工作，其中粉尘粒度分布已计算出从国家的最先进的碰撞模型的粉尘聚集体，涉及凝聚和破碎（纸I），相应的不透明度在这项研究中。应用有效介质理论，假定尘埃聚集体是0.1μm的硅酸盐和石墨颗粒的混合物，真空条件下，计算了尘埃的不透明度。特别是，我们探讨了凝聚如何影响近红外不透明度和9.7μm硅酸盐特征的不透明度。我们发现，随着尘埃聚集体生长到μ m尺寸，9.7 μm特征中的近红外颜色过剩和不透明度都增加。尽管它们凝结，多孔聚集体有助于延长9.7μm特征的存在。我们发现，在硅酸盐功能和近红外颜色过剩的不透明度之间的比例变得较低的ISM，根据许多观测的乌云。然而，这一趋势主要是由于冰幔的形成和混合材料组成的聚集体，而不是由凝固驱动。随着更强的增长，当大多数尘埃质量存在于10μm或更大的颗粒中时，近红外颜色过剩和9.7μm硅酸盐特征都显着减少。在其他波长，特别是在亚毫米范围内的观测，是必不可少的，以提供密集的核心内的尘埃粒度分布的定量约束。我们的结果表明，亚毫米指数β将略有增加，如果聚集体生长到100μm的大小。

英文摘要：

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.

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