P Rimmer

Paul B Rimmer, Aline Vidotto and Christiane Helling

University of St Andrews

Cosmic ray modulation in the habitable zone around Tau Boo

Cosmic rays have an important effect on atmospheric ionization and chemistry in planetary atmospheres. They may play a role in cloud formation and may also provide a mechanism for mutations in organisms. The modulation of cosmic ray flux on planets around stars unlike our sun may therefore have important implications for both characterization (atmospheric chemistry) and habitability (biological mutations).


How does a star more magnetically active than our sun affect the cosmic ray flux? In order to explore this question, we model the galactic cosmic ray modulation within the habitable zone of Tau Boo during four phases of its magnetic activity. In order to calculate the modulation of the galactic cosmic rays, we numerically solve the 2D collisional Vlasov equation for transport of cosmic rays with energy < 109 eV, accounting for the effects of the stellar magnetic field, stellar wind velocities and inelastic collisions between the cosmic rays and the stellar wind. For our boundary conditions, we assume the interstellar values for our galactic cosmic ray flux-spectrum at a radius of 2 AU from Tau Boo. The magnetic field, wind velocities and densities are computed by a 3D MHD model of the stellar wind, which incorporates the observationally reconstructed magnetic field of Tau Boo during four different phases of its magnetic cycle. The cosmic ray transport model is run until steady state is achieved.


We find that, when Tau Boo is less magnetised, the cosmic ray flux varies by orders of magnitude at different regions within the habitable zone, ranging from ~5x the interstellar flux to complete exclusion. When Tau Boo is most magnetized, virtually all cosmic rays < 109 eV are excluded from the entire habitable zone. The close-in planet observed near Tau Boo is shielded from < 109 eV cosmic rays over all four phases of the magnetic cycle. We discuss some of the implications our results have for the number of free electrons in the atmosphere of a hypothetical planet within the habitable zone of Tau Boo, and will speculate on how cosmic ray exclusion might affect the planet's overall habitability. We furthermore consider the implications of our results for planets in more extreme environments such as around a neutron star with a strong magnetic field.

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