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

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 ζCR. The H13CO+ gives best-fit values of ζCR= (2.6 ± 1.8) × 10−17 s−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 ζCR 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 H 13CO+ 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 (∼ 104 cm−3) clouds with most carbon in neutral form or in CO.