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arXiv 1306.4789astro-ph.EPastro-ph.SR
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M矮星磁场对潜在宜居行星的影响

Effects of M dwarf magnetic fields on potentially habitable planets

A. A. Vidotto, M. Jardine, J. Morin, J. -F. Donati, P. Lang, A. J. B. Russell

摘要:

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手段来评估哪些行星最受恒星磁场的影响。我们发现,假设类地行星与类似的地球磁化(1G)轨道的内部(外部)边缘的宜居带的这些恒星将目前的磁层延伸到最多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)需要在比宜居带的传统界限远得多的轨道上运行;要么(2)如果它在宜居带内运行,它至少需要一个磁场,从几个G到几千个G不等。通过假设更适合年轻地球(3.4光年前)的磁层尺寸,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。当恒星变老,自转速率和磁场降低,行星际磁压降低,行星的磁层可能扩大。使用经验得出的旋转活动/磁性关系,.(续)

英文摘要:

We investigate the effect 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 affected by the stellar magnetic field. We show that hypothetical Earth-like planets with similar terrestrial magnetisation (1G) 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 (3.4Gyr 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. 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,... (continues)

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