The Earth-like Galactic cosmic ray intensity in the habitable zone of the M dwarf GJ 436

TL;DR

This study models Galactic cosmic ray fluxes in the habitable zone of the M dwarf GJ 436, revealing that cosmic ray intensities there are similar to Earth's, but significantly lower near the planet GJ 436 b, influenced by stellar wind properties.

Contribution

It provides the first detailed simulations of cosmic ray propagation in GJ 436's astrosphere, considering different stellar wind regimes and their effects on cosmic ray fluxes at various distances.

Findings

Cosmic ray fluxes in GJ 436's habitable zone are comparable to Earth's.

Stellar wind properties significantly affect cosmic ray modulation.

Cosmic ray flux near GJ 436 b is about 10,000 times lower than at Earth.

Abstract

Galactic cosmic rays are energetic particles important in the context of life. Many works have investigated the propagation of Galactic cosmic rays through the Sun's heliosphere. However, the cosmic ray fluxes in M dwarf systems are still poorly known. Studying the propagation of Galactic cosmic rays through the astrospheres of M dwarfs is important to understand the effect on their orbiting planets. Here, we focus on the planetary system GJ 436. We perform simulations using a combined 1D cosmic ray transport model and 1D Alfv\'en-wave-driven stellar wind model. We use two stellar wind set-ups: one more magnetically-dominated and the other more thermally-dominated. Although our stellar winds have similar magnetic field and velocity profiles, they have mass-loss rates two orders of magnitude different. Because of this, they give rise to two different astrosphere sizes, one ten times larger than the other. The magnetically-dominated wind modulates the Galactic cosmic rays more at distances < 0.2 au than the thermally-dominated wind due to a higher local wind velocity. Between 0.2 and 1 au the fluxes for both cases start to converge. However, for distances > 10 au, spatial diffusion dominates, and the flux of GeV cosmic rays is almost unmodulated. We find, irrespective of the wind regime, that the flux of Galactic cosmic rays in the habitable zone of GJ 436 (0.2 - 0.4 au) is comparable with intensities observed at Earth. On the other hand, around GJ 436 b (0.028 au), both wind regimes predict Galactic cosmic ray fluxes that are approximately $10^4$ times smaller than the values observed at Earth.