Spectropolarimetric constraints on the nature of interstellar grains
Spectropolarimetric constraints on the nature of interstellar grains
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星际颗粒性质的分光偏振约束
DOI：
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发表时间：
2014
期刊：
影响因子：
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通讯作者：
Aigen Li
中科院分区：
文献类型：
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作者：
Qi Li;S. Liang;Aigen Li
虽然星际颗粒是由无定形硅酸盐和某种形式的碳质材料组成的，但关于碳质成分的确切化学和物理形式仍然存在争议。现代的颗粒模型假设硅酸盐和碳组分要么是物理分离的，要么形成核幔结构，要么聚集形成多孔复合物。核幔模型假设地幔是由某种脂肪烃物质组成的，并负责在银河系和外部星系的弥漫星际介质（ISM）中普遍可见的3.4微米吸收特征。由于观察到9.7微米硅酸盐特征被偏振，因此该模型受到3.4微米吸收特征中未检测到偏振的挑战。
为了减轻这一挑战，我们计算了3.4微米的球形硅酸盐粉尘涂层的球形脂肪烃的功能的偏振度。发现3.4微米特征偏振仍然超过了观测上限，即使预期球形脂肪烃地幔引起的偏振比非球形少得多（例如，球状）地幔。
我们还表明，由无定形硅酸盐，脂肪烃，和真空组成的复合颗粒模型也预测3.4微米的功能极化远远超过所观察到的。这些结果支持了先前的论点，即脂肪烃组分与硅酸盐组分物理分离，除非3.4微米的吸收特征只是ISM中的次要碳汇。
While it is well recognized that interstellar grains are made of amorphous silicates and some form of carbonaceous materials, it remains debated regarding what exact chemical and physical form the carbonaceous component takes. Contemporary grain models assume that the silicate and carbon components are either physically separated, or they form a core-mantle structure, or they agglomerate to form porous composites. The core-mantle model posits that the mantle is made of some sort of aliphatic hydrocarbon materials and is responsible for the 3.4 micrometer absorption feature ubiquitously seen in the diffuse interstellar medium (ISM) of the Milky Way and external galaxies. This model is challenged by the nondetection of polarization in the 3.4 micrometer absorption feature as the 9.7 micrometer silicate feature is observed to be polarized.
To alleviate this challenge, we calculate the degree of polarization of the 3.4 micrometer feature for spheroidal silicate dust coated by a layer of spherical aliphatic hydrocarbon. It is found that the 3.4 micrometer feature polarization still exceeds the observational upper limit, even though spherical aliphatic hydrocarbon mantles are expected to cause much less polarization than nonspherical (e.g., spheroidal) mantles.
We have also shown that the composite grain model which consists of amorphous silicate, aliphatic hydrocarbon, and vacuum also predicts the 3.4 micrometer feature polarization to well exceed what is observed. These results support the earlier arguments that the aliphatic hydrocarbon component is physically separated from the silicate component unless the 3.4 micrometer absorption feature is just a minor carbon sink in the ISM.