Characterisation of the HD219134 multi-planet system II. Stellar-wind sputtered exospheres in rocky planets b & c

TL;DR

This study models the formation of refractory-rich exospheres around rocky planets HD219134b and c caused by stellar wind sputtering, providing detailed predictions of their composition, density, and potential observability.

Contribution

It presents a comprehensive 3D model of exosphere formation driven by stellar wind sputtering, constrained by observational data, and assesses the detectability of these exospheres.

Findings

Refractory-rich exospheres can form around close-in rocky planets due to stellar wind sputtering.

The exospheres of planets b and c are dense and extended, with elements like O and Mg.

The predicted transit signal of the exosphere of planet b is too small for current UV instruments to detect.

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-alpha 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/cm3, extending to several planetary radii. For planet b, the column density of OI along the line of sight reaches 10^{13}/cm2, 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 OI exosphere of planet b. We find that the transit depth in the OI 1302.2A line is 0.042%, which is a small increase relative to the continuum transit (0.036%). This implies that the sputtered exosphere of HD219134b is unlikely to be detectable with our current UV instruments.