The strong suppression of Galactic cosmic rays reaching AU Mic b, c and Prox Cen b

TL;DR

This study models Galactic cosmic ray fluxes in M dwarf systems AU Mic and Prox Cen, revealing strong suppression due to stellar winds and magnetic fields, which impacts exoplanet habitability assessments.

Contribution

First to include stellar magnetic field effects, including particle drift, in cosmic ray flux models for M dwarf systems, enhancing understanding of cosmic ray suppression in these environments.

Findings

Cosmic ray fluxes are strongly suppressed below Earth levels.

Particle drift reduces cosmic ray suppression, especially in Prox Cen.

Stellar wind strength significantly influences cosmic ray fluxes.

Abstract

The propagation of Galactic cosmic rays is well understood in the context of the solar system but is poorly studied for M dwarf systems. Quantifying the flux of cosmic rays reaching exoplanets is important since cosmic rays are relevant in the context of life. Here, we calculate the Galactic cosmic ray fluxes in AU Mic and Prox Cen planetary systems. We propagate the Galactic cosmic rays using a 1D cosmic ray transport model. We find for Prox Cen b, AU Mic b and AU Mic c that the Galactic cosmic ray fluxes are strongly suppressed and are lower than the fluxes reaching Earth. We include in our models, for the first time for a star other than the Sun, the effect of radial particle drift due to gradients and curvatures in the stellar magnetic field. For Prox Cen we find that the inclusion of particle drift leads to less suppression of Galactic cosmic rays fluxes than when it is excluded from the model. In the case of AU Mic we explore two different wind environments, with a low and high stellar wind mass-loss rate. For AU Mic, the particle drift also leads to less suppression of the Galactic cosmic ray fluxes but it is only significant for the high mass-loss rate scenario. However, both wind scenarios for AU Mic suppress the Galactic cosmic rays strongly. Overall, careful modelling of stellar winds is needed to calculate the Galactic cosmic ray fluxes reaching exoplanets. The results found here can be used to interpret future exoplanet atmosphere observations and in atmospheric models.