M-dwarf stellar winds: the effects of realistic magnetic geometry on rotational evolution and planets

• Published in: Monthly notices of the Royal Astronomical Society Vol. 438; no. 2; pp. 1162 - 1175
• Main Authors: Vidotto, A. A., Jardine, M., Morin, J., Donati, J. F., Opher, M., Gombosi, T. I.
• Format: Journal Article
• Language: English
• Published: London Oxford University Press 21.02.2014; Oxford University Press (OUP): Policy P - Oxford Open Option A
• Subjects: Astronomy; Astrophysics; Cosmic rays; Dwarf stars; Earth and Planetary Astrophysics; Effects; Evolution; Magnetic fields; Physics; Planets; Sciences of the Universe; Solar and Stellar Astrophysics; Wind; MHD; stars: magnetic field; planetary systems; stars: low-mass; methods: numerical; stars: winds, outflows; MHD methods: numerical stars: low-mass stars: magnetic field planetary systems stars: winds; outflows
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stt2265

We perform three-dimensional numerical simulations of stellar winds of early-M-dwarf stars. Our simulations incorporate observationally reconstructed large-scale surface magnetic maps, suggesting that the complexity of the magnetic field can play an important role in the angular momentum evolution of the star, possibly explaining the large distribution of periods in field dM stars, as reported in recent works. In spite of the diversity of the magnetic field topologies among the stars in our sample, we find that stellar wind flowing near the (rotational) equatorial plane carries most of the stellar angular momentum, but there is no preferred colatitude contributing to mass-loss, as the mass flux is maximum at different colatitudes for different stars. We find that more non-axisymmetric magnetic fields result in more asymmetric mass fluxes and wind total pressures p tot (defined as the sum of thermal, magnetic and ram pressures). Because planetary magnetospheric sizes are set by pressure equilibrium between the planet's magnetic field and p tot, variations of up to a factor of 3 in p tot (as found in the case of a planet orbiting at several stellar radii away from the star) lead to variations in magnetospheric radii of about 20 per cent along the planetary orbital path. In analogy to the flux of cosmic rays that impact the Earth, which is inversely modulated with the non-axisymmetric component of the total open solar magnetic flux, we conclude that planets orbiting M-dwarf stars like DT Vir, DS Leo and GJ 182, which have significant non-axisymmetric field components, should be the more efficiently shielded from galactic cosmic rays, even if the planets lack a protective thick atmosphere/large magnetosphere of their own.