Heating of dense interstellar clouds by magnetic ion slip: a constraint on cloud field strengths.
Heating of dense interstellar clouds by magnetic ion slip: a constraint on cloud field strengths.
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磁离子滑移加热致密星际云：对云场强度的限制。
DOI：
10.1086/155200
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发表时间：
1977
期刊：
影响因子：
--
通讯作者：
J. Scalo
中科院分区：
文献类型：
--
作者：
J. Scalo
观测结果表明，在密度与大质量星际云相当的情况下，碎片会变成恒星。然而，如果磁场强度和密度由B = B/sub 0/n/sup y/y = 2/3相关，则不会发生碎裂，如具有冻结场的各向同性收缩。一个半经验上限的参数y是通过考虑加热磁离子滑移（双极扩散）的云的热平衡，并要求理论温度不大大超过观测温度。冷却CO，H/sub 2/，和HD的旋转过渡和非弹性气体-颗粒碰撞包括在平衡计算。发现当密度大于10/sup 4/cm/sup-3/时，y值大到2/3将导致气体温度超过100 K，因此可以被观测排除。如果磁场梯度的尺度与云半径的量级相同，则需要一个近似小于0.55的y值。该上限的计算值对B/sub 0/的选择敏感。y的值可以通过气体沿沿着场线的优先流动或通过磁离子滑移来减小。重新研究了磁离子滑移的时间尺度。«少
Observations suggest that fragmentation into stars occurs at densities comparable to those of massive interstellar clouds. However, fragmentation cannot occur if the magnetic field strength and density are related by B=B/sub 0/n/sup y/ with y=2/3, as for isotropic contraction with a frozen-in field. A semiempirical upper limit to the parameter y is derived by considering the thermal equilibrium of a cloud heated by magnetic ion slip (ambipolar diffusion) and requiring that the theoretical temperatures do not greatly exceed observed temperatures. Cooling by rotational transitions of CO, H/sub 2/, and HD and by inelastic gas-grain collisions is included in the equilibrium calculations. It is found that a value of y as large as 2/3 would result in gas temperatures in excess of 100 K at densities greater than 10/sup 4/ cm/sup -3/ and can therefore be excluded by the observations. A value of yapproximately-less-than0.55 is required if the scale of the magnetic field gradient is of the same order as the cloud radius. The calculated value of this upper limit is sensitive to the choice of B/sub 0/. The value of y can be decreased by preferential flow of gas along field lines or by magnetic ion slip. The time scalemore » for magnetic ion slip is reexamined.« less