横向磁场作用下丝状等温云的磁流体静力平衡结构和质量
Magnetohydrostatic Equilibrium Structure and Mass of Filamentary Isothermal Cloud Threaded by Lateral Magnetic Field
摘要：
赫歇尔观测最近揭示了星际分子云由许多细丝组成。光学和红外波段的偏振观测表明，磁场通常垂直于灯丝。本文研究了热压力和洛伦兹力与自引力平衡，磁场整体垂直于丝轴的等温气体的磁流体静力学位形。该模型由三个参数控制：中心-表面密度比（ρ c/ρ s）、周围星际气体的等离子体β（β_0）和由标度高度归一化的假设母云半径（R '_0），尽管质量相对于磁通量（质量负载）的分布仍有一定的自由度。在R '_0足够小的情况下，磁场起着约束气体的作用。然而，磁场通常具有支撑云的作用。有一个最大的线质量（单位长度的质量），超过这个线质量，云就不能抵抗重力。与非磁化云的最大线质量（2cs ^2/G，其中cs和G分别代表等温声速和引力常量）相比，磁化云的最大线质量大于非磁化云的最大线质量。最大线质量的数值计算公式为λ_max \simeq 0.24 Φ_cl/G^1/2 + 1.66 c_s^2/G，其中Φ_cl表示单位长度穿过灯丝的磁通量的一半。当单位长度的磁通量超过Φ_cl \gtrsim 3 pc μG（c_s/190 m s^-1）^[2]时，对流云的最大质量会受到磁场的显著影响。
英文摘要：
Herschel observation has recently revealed that interstellar molecular clouds consist of many filaments. Polarization observations in optical and infrared wavelengths indicate that the magnetic field often runs perpendicular to the filament. In this paper, the magnetohydrostatic configuration of isothermal gas is studied, in which the thermal pressure and the Lorentz force are balanced against the self-gravity and the magnetic field is globally perpendicular to the axis of the filament. The model is controlled by three parameters: center-to-surface density ratio (ρ_c/ρ_s), plasma βof surrounding interstellar gas (β_0) and the radius of the hypothetical parent cloud normalized by the scale-height (R'_0), although there remains a freedom how the mass is distributed against the magnetic flux (mass loading). In the case that R'_0 is small enough, the magnetic field plays a role in confining the gas. However, the magnetic field generally has an effect in supporting the cloud. There is a maximum line-mass (mass per unit length) above which the cloud is not supported against the gravity. Compared with the maximum line-mass of non-magnetized cloud (2c_s^2/G, where c_s and G represent respectively the isothermal sound speed and the gravitational constant), that of the magnetized filament is larger than the non-magnetized one. The maximum line-mass is numerically obtained as λ_max \simeq 0.24 Φ_cl/G^1/2 + 1.66 c_s^2/G, where Φ_cl represents one half of the magnetic flux threading the filament per unit length. The maximum mass of the filamentary cloud is shown to be significantly affected by the magnetic field when the magnetic flux per unit length exceeds Φ_cl \gtrsim 3 pc μG (c_s/190 m s^-1)^2.
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