恒星形成中的三流体等离子体I。磁流体动力学方程
Three-fluid plasmas in star formation I. Magneto-hydrodynamic equations
摘要：
星际磁场影响恒星形成过程的各个阶段，从分子云核心的坍塌到原恒星喷流的形成。这就要求我们对不同电离度的磁化等离子体在不同密度和温度下的物理性质有一个全面的了解。我们推导了任意电离的三流体介质的磁流体动力学演化的一般方程，其中也包括带电尘埃颗粒作为主要载流子的可能性。在一篇配套论文（Pinto & Galli 2007）中，我们补充了这一分析，计算了碰撞耦合系数的精确表达式。在空间和时间尺度大于所谓的大尺度等离子体的限制和碰撞为主的等离子体的限制，并为非相对论流体的速度，我们得到一个对流扩散的磁场。本文导出了三流体介质中的扩散率、扩散时间尺度和加热率的一般表达式，并利用它们来估计分子云和原恒星喷流中磁场的演化。带电粒子间的碰撞使云坍缩过程中的欧姆电阻率值显著增大，特别是影响了物质与磁场的退耦，提高了能量耗散速率。当负电荷主要由尘埃颗粒携带时，霍尔电阻率可以取比以前发现的更大的值。在弱或轻度电离的原恒星喷流，双极扩散被发现发生在一个时间尺度可比的动力学时间尺度，限制了稳态和非耗散模型的有效性，研究射流的结构。
英文摘要：
Interstellar magnetic fields influence all stages of the process of star formation, from the collapse of molecular cloud cores to the formation of protostellar jets. This requires us to have a full understanding of the physical properties of magnetized plasmas of different degrees of ionization for a wide range of densities and temperatures. We derive general equations governing the magneto-hydrodynamic evolution of a three-fluid medium of arbitrary ionization, also including the possibility of charged dust grains as the main charge carriers. In a companion paper (Pinto & Galli 2007), we complement this analysis computing accurate expressions of the collisional coupling coefficients. Over spatial and temporal scales larger than the so-called large-scale plasma limit and the collision-dominated plasma limit, and for non-relativistic fluid speeds, we obtain an advection-diffusion for the magnetic field. We derive the general expressions for the resistivities, the diffusion time scales and the heating rates in a three-fluid medium and we use them to estimate the evolution of the magnetic field in molecular clouds and protostellar jets. Collisions between charged particles significantly increase the value of the Ohmic resistivity during the process of cloud collapse, affecting in particular the decoupling of matter and magnetic field and enhancing the rate of energy dissipation. The Hall resistivity can take larger values than previously found when the negative charge is mostly carried by dust grains. In weakly-or mildy-ionized protostellar jets, ambipolar diffusion is found to occur on a time scale comparable to the dynamical time scale, limiting the validity of steady-state and nondissipative models to study the jet's structure.
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