  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 zeta. The H13CO+ gives best-fit values of zeta=(2.6+/- 1.8) x 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
zeta 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 (<~10^4 cm^-3) clouds with most carbon in
neutral form or in CO.
