Galactic cosmic ray propagation through M dwarf planetary systems

Abstract

Quantifying the flux of cosmic rays reaching exoplanets around M dwarfs is essential to understand their possible effects on exoplanet habitability. Here, we investigate the propagation of Galactic cosmic rays as they travel through the stellar winds (astrospheres) of five nearby M dwarfs, namely: GJ 15A, GJ 273, GJ 338B, GJ 411, and GJ 887. Our selected stars each have one or two detected exoplanets and they all have wind mass-loss rates constrained by Lyman α observations. Our simulations use a combined 1D magnetohydrodynamic (MHD) Alfvén-wave-driven stellar wind model and 1D cosmic ray transport model. We find that GJ 411 and GJ 887 have Galactic cosmic rays fluxes comparable with Earth’s at their habitable zones. On the other hand, GJ 15A, GJ 273, and GJ 338B receive a lower Galactic cosmic ray flux in their habitable zones. All exoplanets in our sample, with exception of GJ 15A c and GJ 411 c, have a significantly lower flux of Galactic cosmic rays than values observed at the Earth because they orbit closer-in. The fluxes found here can be further used for chemical modelling of planetary atmospheres. Finally, we calculate the radiation dose at the surface of the habitable-zone planet GJ 273 b, assuming it has an Earth-like atmosphere. This planet receives up to 209 times less 15 MeV energy cosmic ray fluxes than values observed at Earth. However, for high-energy cosmic rays (∼GeV), the difference in flux is only 2.3 times smaller, which contributes to GJ 273 b receiving a significant surface radiation dose of 0.13 mSv yr−1 (40 per cent of the annual dose on Earth’s surface).

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

“…As Galactic cosmic rays propagate through stellar systems they are suppressed, in a energy-dependent way, by the existence of a magnetised stellar wind (e.g. Grießmeier et al 2005;Sadovski et al 2018;Herbst et al 2020;Mesquita et al 2021;Rodgers-Lee et al 2021b;Mesquita et al 2022). This is analogous to what occurs in the solar system, known as the modulation of Galactic cosmic rays (e.g.…”

“…In a series of works, we have investigated the propagation of Galactic cosmic rays in M dwarfs (Mesquita et al 2021(Mesquita et al , 2022, with the aim to derive the flux of cosmic rays that reach the habitable zone of these stars. We also calculated the radiation dose reaching an exoplanet surface (Mesquita et al 2022).…”

“…In a series of works, we have investigated the propagation of Galactic cosmic rays in M dwarfs (Mesquita et al 2021(Mesquita et al , 2022, with the aim to derive the flux of cosmic rays that reach the habitable zone of these stars. We also calculated the radiation dose reaching an exoplanet surface (Mesquita et al 2022). Here, we continue this series of works, focusing on two nearby M dwarfs whose orbiting planets are amenable to atmospheric characterisation: AU Mic and Prox Cen.…”

“…Particle drifts were also included to study the young Sun system (Cohen et al 2012). However, for other stars the effect of particle drift are usually neglected (Grießmeier et al 2005;Herbst et al 2020;Mesquita et al 2021;Rodgers-Lee et al 2021b;Mesquita et al 2022). One reason for this is that to implement particle drift it is necessary to know the 3D largescale stellar magnetic field geometry.…”

“…In this paper, we use the 3D stellar wind model results from Kavanagh et al (2021) to compute the propagation of Galactic cosmic rays through the AU Mic and Prox Cen astrospheres (analogous to the heliosphere). We use our 1D model of cosmic ray transport (Rodgers-Lee et al 2020;Mesquita et al 2021;Rodgers-Lee et al 2021b;Mesquita et al 2022) to calculate the spectrum of Galactic cosmic rays within the astrospheres. Here we implement in this model the effect of particle drift for the first time in an M dwarf system.…”