原行星盘中尘埃环的几何：以LkCa 15为例
Geometry of dust rings in protoplanetary disks: the case of LkCa 15
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中文总结 AI 辅助
本文提出利用中等倾角尘埃环环颈的亮度与宽度几何特征约束其参数的方法，结合ALMA观测发现LkCa 15主环光学深度等参数随波长变化，证实其含多粒径尘埃组分，小粒径尘埃盈余或可解释蓬松尘埃环。
中文摘要 AI 辅助
原行星盘中的尘埃特性决定了行星形成的路径。本文提出一种测量中等倾角尘埃环尘埃特性的方法，该方法利用了一个简单的几何事实：对于这类尘埃环，其环颈（ansae）因视线穿过尘埃环物质的路径更长而显得更亮，且因短轴受投影缩短而显得更宽。由此得到的视亮度和宽度模式可用于约束三个参数：尘埃环的本征宽度、垂直厚度以及光学深度。我们将该方法应用于ALMA存档的LkCa 15盘在7、6、3波段的图像，发现其主环的光学深度从0.89毫米处的1.6降至3毫米处的0.4；同时，环宽度和环高度均从约2个气体标高降至1个气体标高。这种与波长相关的形态只能用多粒径尘埃组分的存在来解释。若采用简单的双粒径模型，我们推断该环包含大量小粒径尘埃（尺寸<20微米，总质量约100个地球质量），分布范围较广，以及数量较少的大粒径尘埃（尺寸>200微米），空间分布更集中。小粒径尘埃的这种大量盈余并非尘埃凝聚模型所预测，但它能自然解释LkCa 15的蓬松尘埃环，也可能解释其他原行星盘中的尘埃环。
英文摘要
Dust properties in proto-planetary disks shape the pathways for planet formation. Here, we present a method to measure these properties in moderately inclined dust rings. Our method exploits the simple geometric fact that, for such a ring, its ansae appear brighter because our line of sight traverses a longer path through the ring material, and appear broader because the minor axis are foreshortened by projection. The resultant patterns of apparent brightness and width can used to constrain three parameters: the intrinsic ring width, its vertical thickness and its optical depth. We apply this method to ALMA archival images of the LkCa 15 disk, in Bands 7, 6 and 3. We find that the optical depth of its main ring drops from 1.6 at 0.89mm to 0.4 at 3mm. Simultaneously, both the ring width and the ring height decrease from about two to one gas scale heights. Such wavelength-dependent morphology can only be explained by the presence of multiple grain populations. If we adopt a simple two-size model, we infer that the ring contains a massive population of small grains (size < 20 micron; total mass ~ 100 earth masses) that are broadly distributed, and a less massive population of large grains (size > 200 micron) that are more spatially concentrated. This large surplus of small grains is not predicted by models of dust coagulation, but it naturally explains the fluffy ring in LkCa 15, and possibly rings in other disks.
