Limits on the cosmic-ray ionization rate toward massive young stars

Recent models of the envelopes of seven massive protostars are used to anal yze observations of H-3(+) infrared absorption and (HCO+)-C-13 submillimete r emission lines toward these stars, and to constrain the cosmic-ray ioniza tion rate zeta(CR) The (HCO+)-C-13 gives best-fit values of zeta(CR)= (2.6 +/- 1.8) x 10(-17) S-1, in good agreement with diffuse cloud models and wit h recent Voyager/Pioneer data but factors of up to 7 lower than found from the H-3(+) data. No relation of zeta(CR) with luminosity or total column de nsity is found, so that local (X-ray) ionization and shielding against cosm ic rays appear unimportant for these sources. The difference between the H- 3(+) and (HCO+)-C-13 results and the correlation of N (H-3(+)) with helioce ntric distance suggest that intervening clouds contribute significantly to the H-3(+) absorptions in the more distant regions. The most likely absorbe rs are low-density (less than or similar to 10(4) cm(-3)) clouds with most carbon in neutral form or in CO.