M Star Astrosphere Size Fluctuations and Habitable Planet Descreening

TL;DR

This paper models how stellar astrosphere sizes fluctuate due to interstellar medium variations, affecting habitable planets' exposure to cosmic rays, with findings indicating M star planets are rarely descreened.

Contribution

It introduces a gravitational focusing-inclusive ram-pressure model to quantify astrosphere size dependence on star mass and interstellar density, assessing descreening risks for habitable planets.

Findings

Descreening density scales as (M/M_ ext{sun})^{-2}

Descreening is rare for M star planets, occurring 10^2 to 10^9 times less frequently than for solar-type stars

M star habitable planets are virtually never exposed to severe descreening effects

Abstract

Stellar astrospheres--the plasma cocoons carved out of the interstellar medium by stellar winds--are continually influenced by their passage through the fluctuating interstellar medium (ISM). Inside dense interstellar regions, an astrosphere may be compressed to a size smaller than the liquid-water habitable zone distance. Habitable planets then enjoy no astrospheric buffering from the full flux of Galactic cosmic rays and interstellar dust and gas, a situation we call ``descreening.'' Recent papers (Yeghikyan and Fahr, Pavlov et al.) have suggested such global consequences as severe ozone depletion and glaciation. Using a ram-pressure balance model that includes gravitational focusing of the interstellar flow, we compute the size of the astrosphere in the apex direction as a function of parent star mass. We derive a dependence on the parent-star mass M due to gravitational focusing for densities larger than about 100 (M/M_\odot)^{-2} cm^{-3}. We calculate the interstellar densities required to descreen planets in the habitable zone of solar- and subsolar-mass stars and find a critical descreening density of roughly 600 (M/M_\odot)^{-2} cm^{-3} for the Sun's velocity relative to the local ISM. Finally, we estimate from ISM observations the frequency of descreening encounters as 1--10 Gyr^{-1} for solar-type stars and 10^2--10^9 times smaller for M stars. Given this disparity, we conclude that M star habitable-zone planets are virtually never exposed to the severe effects discussed by Yeghikyan and Fahr and Pavlov et al.