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Context. Ionization plays a central role in the gas-phase chemistry ofmolecular clouds. Since ions are coupled with magnetic fields, which can inturn counteract gravitational collapse, it is of paramount importance tomeasure their abundance in star-forming regions. Aims. We use spectral lineobservations of the high-mass star-forming region NGC 6334 I to derive theabundance of two of the most abundant molecular ions, HCO+ and N2H+, andconsequently, the cosmic ray ionization rate. In addition, the line profilesprovide information about the kinematics of this region. Methods. We presenthigh-resolution spectral line observations conducted with the HIFI instrumenton board the Herschel Space Observatory of the rotational transitions with Jup> 5 of the molecular species C17O, C18O, HCO+, H13CO+, and N2H+. Results. TheHCO+ and N2H+ line profiles display a redshifted asymmetry consistent with aregion of expanding gas. We identify two emission components in the spectra,each with a different excitation, associated with the envelope of NGC 6334 I.The physical parameters obtained for the envelope are in agreement withprevious models of the radial structure of NGC 6334 I based on submillimetercontinuum observations. Based on our new Herschel/HIFI observations, combinedwith the predictions from a chemical model, we derive a cosmic ray ionizationrate that is an order of magnitude higher than the canonical value of 10^(-17)s-1. Conclusions. We find evidence of an expansion of the envelope surroundingthe hot core of NGC 6334 I, which is mainly driven by thermal pressure from thehot ionized gas in the region. The ionization rate seems to be dominated bycosmic rays originating from outside the source, although X-ray emission fromthe NGC 6334 I core could contribute to the ionization in the inner part of theenvelope.

Ionization toward the high-mass star-forming region NGC 6334 I