横向磁场作用下多变丝状云的磁流体静力平衡结构和质量
Magnetohydrostatic Equilibrium Structure and Mass of Polytropic Filamentary Cloud Threaded by Lateral Magnetic Field
摘要：
在赫歇尔卫星的观测中，丝状结构被认为是星际分子云的基本组成部分。这些暗条，特别是质量很大的暗条，通常沿着垂直于星际磁场的方向延伸。此外，细丝有时具有明显的负温度梯度，即它们的温度朝向中心降低。本文研究了负指数多变气体在磁场垂直于丝轴作用下的磁流体静力平衡态。该模型由四个参数控制：中心-表面密度比（$ρ_c/ρ_s$），周围气体的等离子体$β$，由尺度高度归一化的母云半径$R'_0$和多变指数$N$。温度梯度的陡度由$N$表示。我们发现，当温度梯度较大时，柱密度分布的包络变浅。这调和了观察到的配置文件和等温模型的预期之间的不一致。我们比较了最大线质量（每单位长度的质量），在此之上没有平衡，与等温非磁化灯丝。本文得到了一个经验公式，用$λ_{max}\simeq\left[{\left(λ_{0,max}(N)/M_\odot{\rm pc^{-1}}\right)^2+\left[5.9(1.0+1.2/N)^{1/2}({Φ_{cl}}/{1μ{\rm G\,pc}})\right]^2}\right]^{1/2}M_\odot {\rm pc^{-1}}$来表示磁化的多变细丝的最大线质量，其中$λ_{0,max}(N)$表示非磁化细丝的最大线质量，$Φ_{cl}$表示单位长度穿过细丝的磁通量的一半。虽然负折射率多面体使最大线质量比等温模型减小，但穿过灯丝的磁场使线质量增大。
英文摘要：
Filamentary structures are recognized as a fundamental component of interstellar molecular clouds in observations by the Herschel satellite. These filaments, especially massive filaments, often extend in a direction perpendicular to the interstellar magnetic field. Furthermore, the filaments sometimes have an apparently negative temperature gradient, that is, their temperature decreases towards the center. In this paper, we study the magnetohydrostatic equilibrium state of negative-indexed polytropic gas with the magnetic field running perpendicular to the axis of the filament. The model is controlled by four parameters: center-to-surface density ratio ($ρ_c/ρ_s$), plasma $β$ of the surrounding gas, radius of the parent cloud $R'_0$ normalized by the scale height, and the polytropic index $N$. The steepness of the temperature gradient is represented by $N$. We found that the envelope of the column density profile becomes shallow when the temperature gradient is large. This reconciles the inconsistency between the observed profiles and those expected from the isothermal models. We compared the maximum line-mass (mass per unit length), above which there is no equilibrium, with that of the isothermal non-magnetized filament. We obtained an empirical formula to express the maximum line-mass of a magnetized polytropic filament as $λ_{max}\simeq\left[{\left(λ_{0,max}(N)/M_\odot{\rm pc^{-1}}\right)^2+\left[5.9(1.0+1.2/N)^{1/2}({Φ_{cl}}/{1μ{\rm G\,pc}})\right]^2}\right]^{1/2}M_\odot {\rm pc^{-1}}$, where $λ_{0,max}(N)$ represents the maximum line-mass of the non-magnetized filament and $Φ_{cl}$ indicates one-half of the magnetic flux threading the filament per unit length. Although the negative-indexed polytrope makes the maximum line-mass decrease compared with that of the isothermal model, a magnetic field threading the filament increases the line-mass.
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