2006A&A...454L..99V

The cosmic-ray ionization rate ζ_CR of dense molecular clouds is a key parameter for their dynamics and chemistry. Variations of ζ_CR are well established, but it is unclear if these are related to source column density or to Galactic location. Using the APEX telescope, we have mapped the 364GHz line of H₃O⁺ in the Sgr B2 region and observed the 307GHz line at selected positions. With the IRAM 30-m telescope we have observed the H₂¹⁸O 203GHz line at the same positions. Strong H₃O⁺ emission is detected over a ∼3x2pc region, indicating an H₃O⁺ column density of 10¹⁵-10¹⁶cm^–2 in an 18'' beam. The H₃O⁺ abundance of ∼3x10^–9 and H₃O⁺/H₂O ratio of ∼1/50 in the Sgr B2 envelope are consistent with models with ζ_CR∼4x10^–16s^–1, 3xlower than derived from H₃⁺ observations toward Sgr A, but 10xthat of local dense clouds. The ionization rates of interstellar clouds thus seem to be to first order determined by the ambient cosmic-ray flux, while propagation effects cause a factor of ∼3 decrease from diffuse to dense clouds.