Charting nearby stellar systems: The intensity of Galactic cosmic rays for a sample of solar-type stars

TL;DR

This study models Galactic cosmic ray fluxes in the habitable zones of nearby solar-type stars, revealing significant variability influenced by stellar wind properties and providing insights into potential radiation environments for exoplanets.

Contribution

It introduces a combined stellar wind and cosmic ray transport model to assess cosmic ray fluxes in habitable zones of nearby stars, highlighting the role of wind dominance in cosmic ray suppression.

Findings

Habitable zone of 61 Cyg A has Earth-like cosmic ray fluxes.

Other systems show fluxes much lower than Earth's.

Cosmic ray suppression depends on wind dominance and astrospheric size.

Abstract

Cosmic rays can penetrate planetary atmospheres driving the formation of prebiotic molecules, which are important for the origin of life. We calculate the Galactic cosmic ray fluxes in the habitable zone of five nearby, well-studied solar-type stars and at the orbits of 2 known exoplanets. We model the propagation of Galactic cosmic rays through the stellar winds using a combined 1.5D stellar wind and 1D cosmic ray transport model. We find that the habitable zone of 61 Cyg A has comparable Galactic cosmic ray fluxes to present-day Earth values. For the other four systems ($\epsilon$ Eri, $\epsilon$ Ind, $\xi$ Boo B and $\pi^1$ UMa), the fluxes are orders of magnitude smaller than Earth values. Thus, it is unlikely that any as-of-yet undetected Earth-like planets in their habitable zones would receive a higher radiation dose than is received on Earth. $\epsilon\,$Ind$\,$b, a Jupiter-like planet orbiting at $\sim$11au, receives higher Galactic cosmic ray fluxes than Earth. We find the suppression of Galactic cosmic rays is influenced by whether diffusion or advection dominates at GeV energies and at distances where the wind has reached its' terminal velocity. For advectively-dominated winds ($\sim$younger systems), varying the astrospheric size influences the suppression significantly. For diffusion-dominated systems ($\sim$older systems) the astrospheric size, and therefore knowledge of the ISM properties, are not very important. This reduces the Galactic cosmic ray flux uncertainties in the habitable zone for diffusion-dominated systems. Whether a system is advection- or diffusion-dominated can be determined from the stellar wind properties.