Shock formation around planets orbiting M-dwarf stars

Bow shocks can be formed around planets due to their interaction with the coronal medium of the host stars. The net velocity of the particles impacting on the planet determines the orientation of the shock. At the Earth's orbit, the (mainly radial) solar wind is primarily responsible for the formation of a shock facing towards the Sun. However, for close-in planets that possess high Keplerian velocities and are frequently located at regions where the host star's wind is still accelerating, a shock may develop ahead of the planet. If the compressed material is able to absorb stellar radiation, then the signature of bow shocks may be observed during transits. Bow-shock models have been investigated in a series of papers (Llama et al. 2011; Vidotto et al. 2010, 2011a,b) for known transiting systems. Once the signature of a bow-shock is observed, one can infer the magnetic field intensity of the transiting planet. Here, we investigate the potential to use this model to detect magnetic fields of (hypothetical) planets orbiting inside the habitable zone of M-dwarf stars. For these cases, we show, by means of radiative transfer simulations, that the detection of bow-shocks of planets surrounding M-dwarf stars may be more difficult than for the case of close-in giant planets orbiting solar-type stars. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim