Powerful winds from low-mass stars: V374 Peg

Abstract

The M dwarf V374 Peg (M4) is believed to lie near the theoretical mass threshold for fully convective interiors. Its rapid rotation (P= 0.44 d) along with its intense magnetic field point towards magnetocentrifugal acceleration of a coronal wind. In this work, we investigate the structure of the coronal wind of V374 Peg by means of three-dimensional magnetohydrodynamical (MHD) numerical simulations. For the first time, an observationally derived surface magnetic field map is implemented in MHD models of stellar winds for a low-mass star. By self-consistently taking into consideration the interaction of the outflowing wind with the magnetic field and vice versa, we show that the wind of V374 Peg deviates greatly from a low-velocity, low-mass-loss rate solar-type wind. We have found general scaling relations for the terminal velocities, mass-loss rates and spin-down times of highly magnetized M dwarfs. In particular, for V374 Peg, our models show that terminal velocities across a range of stellar latitudes reach u∞≃ (1500-2300) n-1/212kms-1, where n12 is the coronal wind base density in units of 1012cm-3, while the mass-loss rates are about We also evaluate the angular momentum loss of V374 Peg, which presents a rotational braking time-scale τ≃ 28 n-1/212 Myr. Compared to observationally derived values from period distributions of stars in open clusters, this suggests that V374 Peg may have low coronal base densities (≲1011cm-3). We show that the wind ram pressure of V374 Peg is about 5 orders of magnitude larger than for the solar wind. Nevertheless, a small planetary magnetic field intensity (∼0.1 G) is able to shield a planet orbiting at 1au against the erosive effects of the stellar wind. However, planets orbiting inside the habitable zone of V374 Peg, where the wind ram pressure is higher, might be facing a more significant atmospheric erosion. In that case, higher planetary magnetic fields of, at least, about half the magnetic field intensity of Jupiter are required to protect the planet's atmosphere. © 2010 The Authors Monthly Notices of the Royal Astronomical Society © 2010 RAS.