Interactions between exoplanets and the winds of young stars

Abstract

The topology of the magnetic field of young stars is important not only for the investigation of magnetospheric accretion, but also responsible in shaping the large-scale structure of stellar winds, which are crucial for regulating the rotation evolution of stars. Because winds of young stars are believed to have enhanced mass-loss rates compared to those of cool, main-sequence stars, the interaction of winds with newborn exoplan- ets might a ect the early evolution of planetary systems. This interaction can also give rise to observational signatures which could be used as a way to detect young planets, while simultaneously probing for the presence of their still elusive magnetic fields. Here, we investigate the interaction between winds of young stars and hypothetical planets. For that, we model the stellar winds by means of 3D numerical magnetohydrodynamic simulations. Although these models adopt simplified topologies of the stellar magnetic field (dipolar fields that are misaligned with the rotation axis of the star), we show that asymmetric field topologies can lead to an enhancement of the stellar wind power, result- ing not only in an enhancement of angular momentum losses, but also intensifying and rotationally modulating the wind interactions with exoplanets. 1 Three-dimensional numerical simulations of stellar winds The magnetic fields of young stars play fundamental roles in the Physics of accretion. Their topologies are, however, not only important for the investigation of magnetospheric accretion, but also responsi- ble for shaping the large-scale structure of stellar winds, which are crucial for regulating the rotation evolution of stars. In (1-6), we modelled stellar winds by means of three-dimensional (3D) mag- netohydrodynamics (MHD) simulations. In particular, in (2, 3) we investigate the winds of young stars and how they can interact with newborn planets. We adopte simplified topologies of the stellar magnetic field. To simulate the stellar winds, we use the 3D MHD numerical code BATS-R-US (7), which solves the coupled system of ideal MHD equations for the continuity of mass and momentum, conservation of energy and magnetic field induction. Figure 1a shows the magnetic field configura- tion adopted in the simulations at initial instant t0. The dipolar magnetic moment m is tilted with respect to the stellar rotation axis by an angle t. The stellar wind particles and magnetic field lines interact with each other self-consistently. We assume a typical young star with mass 0:8 M , radius R? = 2 R and rotation period of 1 day. The magnetic field intensity at the magnetic poles of the star