un 2 00 9 M Star Astrosphere Size Fluctuations and Habitable Planet Descreening ∗

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 liquidwater 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⊙) cm−3. We calculate the interstellar densities required to descreen planets in the habitable zone of solarand subsolar-mass stars and find a critical descreening density of roughly 600 (M/M⊙) 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 102–109 times smaller for M ∗To be published in Astrobiology †Corresponding author: [email protected]; Present affiliation: Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721