Magnetic Dipole Microwave Emission from Dust Grains
Magnetic Dipole Microwave Emission from Dust Grains
复制标题
尘埃颗粒的磁偶极子微波发射
DOI：
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发表时间：
1998
期刊：
影响因子：
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通讯作者：
A. Lazarian
中科院分区：
文献类型：
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作者：
B. Draine;A. Lazarian
星际颗粒磁化强度的热波动将产生ν > 100 GHz的磁偶极子发射。我们展示了如何计算由具有磁性和介电性质的材料组成的小颗粒的吸收和发射。推广了含尘介质的Kramers-Kronig关系，使其包含磁性颗粒的可能性。讨论了几种候选颗粒材料的磁导率随频率的变化规律。铁颗粒或含铁夹杂的颗粒在~50-100 GHz附近可能具有Fröhlich共振的磁模拟，这导致了大的磁偶极子吸收截面。我们计算了各种星际候选颗粒的发射光谱。虽然“普通”顺磁性颗粒甚至磁铁矿颗粒不能解释在14-90 GHz范围内观察到的尘埃“异常”发射，但如果颗粒材料的一小部分是铁磁性的，就会产生更强的磁偶极子发射，就像星际尘埃高铁含量的情况一样。从90 GHz附近的尘埃中观测到的发射表明，不超过5%的星际铁以金属铁颗粒或夹杂物的形式存在（例如，在“GEMS”中）。然而，我们表明，如果大多数星际铁存在于中等铁磁性材料中，并适当调整其磁性，那么它可以贡献观测到的14-90 GHz发射的很大一部分，也许可以与旋转的超小尘埃颗粒的贡献相媲美。这两种发射机制可以通过测量乌云的发射来区分。如果铁磁颗粒由单一磁畴组成并排列整齐，则磁偶极子发射将呈线性极化，极化程度与频率密切相关。
Thermal fluctuations in the magnetization of interstellar grains will produce magnetic dipole emission at ν≲100 GHz. We show how to calculate absorption and emission from small particles composed of material with magnetic, as well as dielectric, properties. The Kramers-Kronig relations for a dusty medium are generalized to include the possibility of magnetic grains. The magnetic permeability as a function of frequency is discussed for several candidate grain materials. Iron grains, or grains containing iron inclusions, are likely to have the magnetic analog of a Fröhlich resonance in the vicinity of ~50-100 GHz, which results in a large magnetic dipole absorption cross section. We calculate the emission spectra for various interstellar grain candidates. Although "ordinary" paramagnetic grains or even magnetite grains cannot account for the observed "anomalous" emission from dust in the 14-90 GHz range, stronger magnetic dipole emission will result if a fraction of the grain material is ferromagnetic, as could be the case given the high Fe content of interstellar dust. The observed emission from dust near 90 GHz implies that not more than ~5% of interstellar Fe is in the form of metallic iron grains or inclusions (e.g., in "GEMS"). However, we show that if most interstellar Fe is in a moderately ferromagnetic material, with the magnetic properties suitably adjusted, it could contribute a substantial fraction of the observed 14-90 GHz emission, perhaps comparable to the contribution from spinning ultrasmall dust grains. The two emission mechanisms can be distinguished by measuring the emission from dark clouds. If ferromagnetic grains consist of a single magnetic domain and are aligned, the magnetic dipole emission will be linearly polarized, with the polarization depending strongly on frequency.