2000A&A...358L..79V

Recent models of the envelopes of seven massive protostars are used to analyze observations of H₃⁺ infrared absorption and H¹³CO⁺ submillimeter emission lines toward these stars, and to constrain the cosmic-ray ionization rate ζ_CR. The H¹³CO⁺ gives best-fit values of ζ_CR=(2.6 ±1.8)x10^–17s^–1, in good agreement with diffuse cloud models and with recent Voyager/Pioneer data but factors of up to 7 lower than found from the H₃⁺ data. No relation of ζ_CR with luminosity or total column density is found, so that local (X-ray) ionization and shielding against cosmic rays appear unimportant for these sources. The difference between the H₃⁺ and H¹³CO⁺ results and the correlation of N(H₃⁺) with heliocentric distance suggest that intervening clouds contribute significantly to the H₃⁺ absorptions in the more distant regions. The most likely absorbers are low-density (≲10⁴cm^–3) clouds with most carbon in neutral form or in CO.