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.
