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arXiv 1003.2722astro-ph.SR
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磁星形成的非等温阶段。II。结果

The nonisothermal stage of magnetic star formation. II. Results

M. W. Kunz, T. Ch. Mouschovias

备注：
20 pages, 18 figures, submitted to MNRAS
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摘要：

在以前的论文中，我们制定的问题的形成和演变的碎片（或核心）的磁支持，自引力分子云的轴对称几何形状，占双极扩散和欧姆耗散，晶粒化学和动力学，和辐射转移的影响。在这里，我们展示了恒星形成模拟的结果，这些模拟准确地跟踪了密度超过11个数量级（从300 cm^-3到\approx 10 ^14 cm^-3）的原恒星碎片的演化，即，从早期的双极扩散-引发的碎裂阶段，通过磁超临界，动态收缩阶段和随后的磁去耦阶段，形成一个接近流体静力平衡的原恒星核心。正如Fiedler和Mouschovias（1993）所发现的，引力驱动的双极扩散导致了磁超临界核心的形成和随后的动态收缩。此外，我们还发现双极扩散（而非欧姆耗散）导致了除电子之外的所有物质与磁场的去耦，其密度为\approx 3 x 10^12 cm^-3。磁退耦先于中心恒星物体的形成，并最终在流体静力学核心之外产生磁通量的集中（“磁墙”）-Tennis和Mouschovias（2005 a，b）通过不同的方法也发现了这一点。在欧姆耗散变得比双极扩散更重要的密度（\gtrsim 7 x 10^12 cm^-3）大致相同的情况下，颗粒携带大部分电荷以及电流。星前核在半径约10天文单位的范围内仍保持盘状，其中热压力变得重要。恒星形成的磁通量问题至少在强磁性新生恒星的演化阶段得到解决，即，中心密度为\approx 10 ^14 cm^-3。流体静力核的半径为\approx 2 AU，密度为\approx 10 ^14 cm^-3，温度\approx 300 K，磁场强度\approx 0.2 G，磁通量\approx 5 x 10^18 Wb，光度~ 10^-3 L_ \odot，质量约为10^-2 M_ \odot。

英文摘要：

In a previous paper we formulated the problem of the formation and evolution of fragments (or cores) in magnetically-supported, self-gravitating molecular clouds in axisymmetric geometry, accounting for the effects of ambipolar diffusion and Ohmic dissipation, grain chemistry and dynamics, and radiative transfer. Here we present results of star formation simulations that accurately track the evolution of a protostellar fragment over eleven orders of magnitude in density (from 300 cm^-3 to \approx 10^14 cm^-3), i.e., from the early ambipolar-diffusion--initiated fragmentation phase, through the magnetically supercritical, dynamical-contraction phase and the subsequent magnetic decoupling stage, to the formation of a protostellar core in near hydrostatic equilibrium. As found by Fiedler & Mouschovias (1993), gravitationally-driven ambipolar diffusion leads to the formation and subsequent dynamic contraction of a magnetically supercritical core. Moreover, we find that ambipolar diffusion, not Ohmic dissipation, is responsible for decoupling all the species except the electrons from the magnetic field, by a density \approx 3 x 10^12 cm^-3. Magnetic decoupling precedes the formation of a central stellar object and ultimately gives rise to a concentration of magnetic flux (a `magnetic wall') outside the hydrostatic core --- as also found by Tassis & Mouschovias (2005a,b) through a different approach. At approximately the same density at which Ohmic dissipation becomes more important than ambipolar diffusion (\gtrsim 7 x 10^12 cm^-3), the grains carry most of the electric charge as well as the electric current. The prestellar core remains disclike down to radii ~ 10 AU, inside which thermal pressure becomes important. The magnetic flux problem of star formation is resolved for at least strongly magnetic newborn stars by this stage of the evolution, i.e., by a central density \approx 10^14 cm^-3. The hydrostatic core has radius \approx 2 AU, density \approx 10^14 cm^-3, temperature \approx 300 K, magnetic field strength \approx 0.2 G, magnetic flux \approx 5 x 10^18 Wb, luminosity ~ 10^-3 L_\odot, and mass ~ 10^-2 M_\odot.

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