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∞ (1500–2300) n−1/2 12 km s−1, where n12 is the coronal wind base density in units of 1012 cm−3, while the mass-loss rates are about Ṁ 4 × 10−10n 12 M yr−1. We also evaluate the angular momentum loss of V374 Peg, which presents a rotational braking time-scale τ 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 ( 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 (∼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.