Cosmic Ray Compaction of Porous Interstellar Ices
نویسندگان
چکیده
We studied the compaction of microporous vapor-deposited ice films under irradiation with different ions in the 80Y400 keVenergy range.We found that porosity decreases exponentially with irradiation fluence, with amean compaction area per ion that scales linearlywith the stopping power of the projectile S above a threshold St 1⁄4 4 eV8 . The experiments roughly follow a universal dependence of ion-induced compaction with restricted dose (eV molecule ). This behavior can be used to extrapolate our results to conditions of the interstellar medium. Relating our results to ionization rates of interstellar H2, we estimate that porous ice mantles on grains in dense molecular clouds are compacted by cosmic rays in 10Y50 million years. Subject headinggs: cosmic rays — ISM: molecules — methods: laboratory — radiation mechanisms: nonthermal Online material: color figures
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