Powerful winds from low-mass stars: V374 Peg

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(infinity) similar or equal to (1500-2300) n-1/2(12) km s-1, where n(12) is the coronal wind base density in units of 1012 cm-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 tau similar or equal to 28 n-1/2(12) 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 (less than or similar to 1011 cm-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 (similar to 0.1 G) is able to shield a planet orbiting at 1 au 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.