Activity in Very Cool Stars: Magnetic Dissipation in Late M and L Dwarf Atmospheres
Activity in Very Cool Stars: Magnetic Dissipation in Late M and L Dwarf Atmospheres
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DOI：
10.1086/339911
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发表时间：
2002-01
期刊：
影响因子：
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通讯作者：
S. Mohanty;G. Basri;F. Shu;F. Allard;G. Chabrier
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文献类型：
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作者：
S. Mohanty;G. Basri;F. Shu;F. Allard;G. Chabrier
最近的观测表明，M矮星和L矮星晚期的色球H-α活动比早期M矮星要低得多。考虑到晚期M矮星和L矮星的自转速度相对较快，这一点尤其令人惊讶：在早期的M矮星中，快速自转与高活动水平有关。一种可能性是，M矮星和L矮星活动的减弱是由于其致密、凉爽和以中性为主的大气中的高电阻率所致。我们使用Allard和Hausschilt的大气结构模型计算了3000-1500K(M中期到L晚期)具有Tef的天体在大气中的磁场扩散率。我们发现，在这些大气中，非常低的电离分数和高密度的结合导致了非常大的电阻率，从而产生了有效的场扩散。由于离子-电子碰撞引起的电流的双极扩散和欧姆衰变都会发生，而主要的扩散效应是由于带电粒子与中性粒子的碰撞而产生的电流衰变。此外，后者是有效温度和光学厚度的强函数，并且随着有效温度的减小或光学厚度的减小而迅速增大。这有两个含义：(1)当一个人从M中期移动到L时，存在的任何磁场都与大气流体运动日益解耦。在M晚期和L矮星中，大气运动不能导致与势场有很大不同的平衡场构型。也就是说，在这些天体中，大气运动产生的磁应力的大小非常小。我们通过一个简单的雷诺数计算来量化这种影响。(2)即使磁应力很容易由热的内部(场与物质的耦合良好)中的流体运动产生，但它在大气层中的传播将受到大气电阻率增加的越来越大的阻碍。因此，在这些寒冷的矮星中，磁应力的产生和传播都随着TJeff的减小而受到越来越大的抑制。结果，可用于支撑色球的磁自由能变得越来越小，随着色球类型的增加，活度变得越来越小。这可以解释从M中期到L期间观测到的Hα活动下降的原因，假设这些矮星的活动是由磁驱动的。为了验证后一种假设，我们通过Lighthill-Proudman计算来估计这些天体中的出射声通量。虽然声学通量也随着Tef的减小而减小，但它们似乎不足以解释在M中期到L矮星观测到的Hα通量。在没有声加热的情况下，磁加热似乎确实是产生活动的最可行的方式。我们强调，我们的计算是平衡的，不涉及依赖时间的现象。我们也不检查最高的大气层，那里不需要修正，但我们展示的是足够稀薄的，以允许带电粒子保持与场的耦合。最后，虽然我们的计算没有处理M矮星和L矮星晚期的耀斑，但我们推测后者可能是由浮力通量管产生的，这些浮力通量管在大气层中迅速上升，在高层大气中耗散了相关的电流。
Recent observations show that chromospheric Hα activity in late M and L dwarfs is much lower than in the earlier M types. This is particularly surprising given that the late M and L dwarfs are comparatively very rapid rotators: in the early M dwarfs, rapid rotation is associated with high activity levels. One possibility is that the drop-off in activity in the late M and L dwarfs is a result of very high electrical resistivities in their dense, cool, and predominantly neutral atmospheres.We calculate the magnetic field diffusivity in the atmospheres of objects with Teff in the range 3000-1500 K (mid M to late L) using the atmospheric structure models of Allard and Hauschildt. We find that the combination of very low ionization fraction and high density in these atmospheres results in very large resistivities and thus efficient field diffusion. While both ambipolar diffusion and Ohmic decay of currents due to ion-electron collisions occur, the primary diffusion effects are due to current decay through collisions of charged particles with neutrals. Moreover, the latter resistivity is a strong function of both effective temperature and optical depth, increasing rapidly as either Teff or optical depth decreases. This has two implications: (1) Any magnetic field present is increasingly decoupled from atmospheric fluid motions as one moves from mid M to L. In the late M and L dwarfs, atmospheric motions cannot lead to equilibrium field configurations very different from potential ones. That is, the magnitude of magnetic stresses generated by atmospheric motions is very small in these objects. We quantify this effect by a simple Reynolds number calculation. (2) Even if magnetic stresses are easily produced by fluid motions in the hot interior (where the coupling between field and matter is good), their propagation up through the atmosphere will be increasingly hampered by the growing atmospheric resistivity as one moves from mid M to late L. Thus both the generation and propagation of magnetic stresses are increasingly damped with decreasing Teff in these cool dwarfs. As a result, the magnetic free energy available for the support of a chromosphere, and activity becomes smaller and smaller with later type. This can account for the observed drop in Hα activity from mid M to L, assuming that activity in these dwarfs is magnetically driven. To check the latter assumption, we estimate the emergent acoustic fluxes in these objects through a Lighthill-Proudman calculation. While the acoustic fluxes also decrease with decreasing Teff, they appear inadequate to explain the observed Hα fluxes in mid M to L dwarfs. In the absence of acoustic heating, magnetic heating indeed seems the most viable way of generating activity. We emphasize that our calculations are equilibrium ones and do not address time-dependent phenomena. We also do not examine the highest atmospheric layers, where correction is not expected, but which we show are rarefied enough to permit charged particles to remain coupled to the field. Finally, while our calculations do not address flares in late M and L dwarfs, we speculate that the latter could be created by buoyant flux tubes that are generated in the interior and rise rapidly through the atmosphere, dissipating their associated currents in the upper atmospheric layers.