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We aim at investigating with high angular resolution the NH3/N2H+ ratiotoward the high-mass star-forming region AFGL 5142 in order to study whetherthis ratio behaves similarly to the low-mass case, for which the ratiodecreases from starless cores to cores associated with YSOs. CARMA was used toobserve the 3.2 mm continuum and N2H+(1-0) emission. We used NH3(1,1) and(2,2), HCO+(1-0) and H13CO+(1-0) data from the literature and we performed atime-dependent chemical modeling of the region. The 3.2 mm continuum emissionreveals a dust condensation of ~23 Msun associated with the massive YSOs,deeply embedded in the strongest NH3 core (hereafter central core). The N2H+emission reveals two main cores, the western and eastern core, located to thewest and to the east of the mm condensation, and surrounded by a more extendedand complex structure of ~0.5 pc. Toward the central core the N2H+ emissiondrops significantly, indicating a clear chemical differentiation in the region.We found low values of the NH3/N2H+ ratio ~50-100 toward the western/easterncores, and high values up to 1000 in the central core. The chemical modelindicates that density, and in particular temperature, are key parameters indetermining the NH3/N2H+ ratio. The high density and temperature reached in thecentral core allow molecules like CO to evaporate from grain mantles. The COdesorption causes a significant destruction of N2H+, favoring the formation ofHCO+. This result is supported by our observations, which show that N2H+ andHCO+ are anticorrelated in the central core. The observed values of theNH3/N2H+ ratio in the central core can be reproduced by our model for timest~4.5-5.3x10^5 yr (central) and t~10^4-3x10^6 yr (western/eastern). TheNH3/N2H+ ratio in AFGL 5142 does not follow the same trend as in regions oflow-mass star formation mainly due to the high temperature reached in hotcores.

N2H+ depletion in the massive protostellar cluster AFGL 5142