THE EFFECT OF VARYING COSMIC-RAY IONIZATION RATES ON DARK CLOUD CHEMISTRY

We investigate the effects of varying the cosmic ray ionization rate i n chemical models of dense interstellar clouds. In the absence of such ionization, a scenario which may be applicable to dark cloud cores, w e find that chemi-ionization is able to drive a limited ion-neutral ch emistry. Models of clouds in starburst galaxies, which may have enhanc ed cosmic ray fluxes, are also investigated and enable an upper limit to be derived for the cosmic ray ionization rate in M82. The derived v alue, which is about 700 times the typical value for Galactic molecula r clouds, is in good agreement with that necessary to explain the rece nt observations of C I in this galaxy.