Star formation in extreme environments: the effects of cosmic rays and mechanical heating

Context. The molecular interstellar medium in extreme environments, such as Arp 220, but also NGC 253 appears to have extremely high cosmic ray (CR) rates (103−104 × Milky Way) and substantial mechanical heating from supernova driven turbulence. Aims. We explore the consequences of high CR rates and mechanical heating on the chemistry of the clouds. Methods. PDR model predictions are made for low, n = 103, and high, n = 105.5 cm-3, density clouds using well-tested chemistry and radiation transfer codes. Column densities of relevant species are discussed, and special attention is given to water-related species. Fluxes are shown for fine-structure lines of O, C+, C, and N+, and molecular lines of CO, HCN, HNC, and HCO+. A comparison is made with an X-ray dominated region model. Results. Fine-structure lines of [CII], [CI], and [OI] are remarkably similar for different mechanical heating and CR rates, when already exposed to large amounts of UV. Both HCN and H2O abundances are boosted for very high mechanical heating rates, while ionized species are relatively unaffected. Both OH+ and H2O+ are enhanced for very high CR rates ζ ≥ 5 × 10-14 s-1. A combination of OH+, OH, H2O+, H2O, and H3O+ traces the CR rates, and is able to distinguish between enhanced cosmic rays and X-rays.