Effects of M dwarf magnetic fields on potentially habitable planets
Effects of M dwarf magnetic fields on potentially habitable planets
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M矮星磁场对潜在宜居行星的影响
DOI：
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发表时间：
2013
期刊：
影响因子：
--
通讯作者：
A. Russell
中科院分区：
文献类型：
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作者：
A. Vidotto;M. Jardine;J. Morin;J. Donati;P. Lang;A. Russell
我们研究了M矮星的磁场对类地行星的影响。这些磁场可以减少行星磁层的大小，以至于行星大气的很大一部分可能会受到恒星风的侵蚀。我们使用了15颗活跃的dM恒星的样本，重建了这些恒星的表面磁场图，以确定行星轨道上的磁压，从而确定其磁层的最大尺寸，这只会通过考虑恒星风而减少。我们的方法提供了一种快速的方法来评估哪些行星最受恒星磁场的影响，这可以作为第一个研究，然后是更复杂的模型。我们发现，假设类地行星与类似的地球磁化（1 G）轨道在这些恒星的可居住区的内（外）边缘将提出磁层，延伸到最多6（11：7）行星半径。为了维持地球大小的磁层，除了少数情况外，类地行星要么（1）需要在比可居住带的传统界限更远的地方运行;要么（2）如果它在可居住带内运行，它至少需要一个从几G到几千G的磁场。通过假设磁层的大小更适合于行星-地球（3：4 Gyr ago），所需的行星磁场要弱一个数量级。然而，在这种情况下，行星的极冠面积是两倍大，这是不受行星间空间粒子运输的保护。目前，我们不知道行星表面的最小面积有多小，可以暴露出来，并且仍然不会影响行星上生命形成和发展的潜力。当星星变老时，它的自转速率和磁场会减小，行星际磁压会减小，行星的磁层可能会膨胀。使用经验推导的旋转活动/磁性关系，我们提供了一个分析表达式，估计最短的恒星旋转周期的地球模拟在可居住区可以维持地球大小的磁层。我们发现，dM早期和中期恒星（周期为37 - 202天）所需的旋转速率比太阳慢，而dM晚期恒星（&63 - 263天）甚至更慢。在dM恒星的可居住带中运行的行星，其旋转速度比这更快，其磁层尺寸比地球磁层小。由于许多dM晚期的恒星都是快速旋转的恒星，类地行星拥有地球大小的磁层的条件，对于缓慢旋转的dM早期和中期恒星的行星来说更容易实现。
We investigate the e ect of the magnetic fields of M dwarf (dM) stars on potentially habitable Earth-like planets. These fields can reduce the size of planetary magnetospheres to such an extent that a significant fraction of the planet’s atmosphere may be exposed to erosion by the stellar wind. We used a sample of 15 active dM stars, for which surface magnetic-field maps were reconstructed, to determine the magnetic pressure at the planet orbit and hence the largest size of its magnetosphere, which would only be decreased by considering the stellar wind. Our method provides a fast means to assess which planets are most a ected by the stellar magnetic field, which can be used as a first study to be followed by more sophisticated models. We show that hypothetical Earth-like planets with similar terrestrial magnetisation ( 1 G) orbiting at the inner (outer) edge of the habitable zone of these stars would present magnetospheres that extend at most up to 6 (11:7) planetary radii. To be able to sustain an Earth-sized magnetosphere, with the exception of only a few cases, the terrestrial planet would either (1) need to orbit significantly farther out than the traditional limits of the habitable zone; or else, (2) if it were orbiting within the habitable zone, it would require at least a magnetic field ranging from a few G to up to a few thousand G. By assuming a magnetospheric size that is more appropriate for the young-Earth (3:4 Gyr ago), the required planetary magnetic fields are one order of magnitude weaker. However, in this case, the polar-cap area of the planet, which is unprotected from transport of particles to/from interplanetary space, is twice as large. At present, we do not know how small the smallest area of the planetary surface is that could be exposed and would still not a ect the potential for formation and development of life in a planet. As the star becomes older and, therefore, its rotation rate and magnetic field reduce, the interplanetary magnetic pressure decreases and the magnetosphere of planets probably expands. Using an empirically derived rotation-activity/magnetism relation, we provide an analytical expression for estimating the shortest stellar rotation period for which an Earth-analogue in the habitable zone could sustain an Earth-sized magnetosphere. We find that the required rotation rate of the early- and mid-dM stars (with periods &37‐202 days) is slower than the solar one, and even slower for the late-dM stars (&63‐263 days). Planets orbiting in the habitable zone of dM stars that rotate faster than this have smaller magnetospheric sizes than that of the Earth magnetosphere. Because many late-dM stars are fast rotators, conditions for terrestrial planets to harbour Earth-sized magnetospheres are more easily achieved for planets orbiting slowly rotating early- and mid-dM stars.
亚阿尔弗尼克行星恒星和月球行星相互作用中的磁能通量
影响因子：
6.5
作者：
Grambusch;Duling;Neubauer
通讯作者：
Neubauer