Characterization of the HD 219134 multi-planet system II. Stellar-wind sputtered exospheres in rocky planets b & c

Abstract

We present a 3D study of the formation of refractory-rich exospheres around the rocky planets HD219134b and c. These exospheres are formed by surface particles that have been sputtered by the wind of the host star. The stellar wind properties are derived from magnetohydrodynamic simulations, which are driven by observationally-derived stellar magnetic field maps, and constrained by Ly-α observations of wind mass-loss rates, making this one of the most well-constrained model of winds of low-mass stars. The proximity of the planets to their host star implies a high flux of incident stellar wind particles, thus the sputtering process is sufficiently effective to build up relatively dense, refractory-rich exospheres. The sputtering releases refractory elements from the entire dayside surfaces of the planets, with elements such as O and Mg creating an extended neutral exosphere with densities larger than 10 cm−3, extending to several planetary radii. For planet ‘b’, the column density of O I along the line of sight reaches 1013 cm−2, with the highest values found ahead of its orbital motion. This asymmetry would create asymmetric transit profiles. To assess its observability, we use a ray tracing technique to compute the expected transit depth of the O I exosphere of planet ‘b’. We find that the transit depth in the O I 1302.2 Å line is 0.042 per cent, which is a small increase relative to the continuum transit (0.036 per cent). This implies that the sputtered exosphere of HD219134b is unlikely to be detectable with our current UV instruments.