N2H+ depletion in the massive protostellar cluster AFGL 5142

Aims. We aim at investigating the NH3/N2H+ abundance ratio toward the high-mass star-forming region AFGL 5142 with high angular resolution in order to study whether the NH3/N2H+ ratio behaves similarly to the low-mass case, for which the ratio decreases from starless cores to cores associated with young stellar objects (YSOs). Methods. CARMA was used to observe the 3.2 mm continuum and N2H+ (1-0) emission toward AFGL 5142. We used NH3 (1, 1) and (2, 2), as well as HCO+ (1-0) and (HCO+)-C-13 (1-0) data available from the literature, to study the chemical environment. Additionally, we performed a time-dependent chemical modeling of the region. Results. The 3.2 mm continuum emission reveals a dust condensation of similar to 23 M-circle dot associated with the massive YSOs, deeply embedded in the strongest NH3 core (hereafter central core). The dense gas emission traced by N2H+ reveals two main cores, the western core of similar to 0.08 pc in size and the eastern core of similar to 0.09 pc, surrounded by a more extended and complex structure of similar to 0.5 pc, mimicking the morphology of the NH3 emission. The two cores are located to the west and to the east of the 3.2 mm dust condensation. Toward the central core the N2H+ emission drops significantly, indicating a clear chemical differentiation in the region. The N2H+ column density in the central core is one order of magnitude lower than in the western and eastern cores. Furthermore, we found low values of the NH3/N2H+ abundance ratio similar to 50-100 toward the western and eastern cores and high values up to 1000 associated with the central core. The chemical model used to explain the differences seen in the NH3/N2H+ ratio indicates that density along with temperature is a key parameter in determining the abundances of both NH3 and N2H+. The high density (n similar or equal to 106 cm(-3)) and temperature (T similar or equal to 70 K) reached in the central core allow molecules such as CO to evaporate from grain mantles. The CO desorption causes a significant destruction of N2H+, which favors the formation of HCO+. This result is supported by our observations, which show that N2H+ and HCO+ are anticorrelated in the central core. The observed values of the NH3/N2H+ ratio in the central core can be reproduced by our model for times of t similar or equal to 4.5-5.3 x 10(5) yr, while in the western and eastern cores the NH3/N2H+ ratio can be reproduced by our model for times in the range 10(4)-3 x 10(6) yr. Conclusions. The NH3/N2H+ abundance ratio in AFGL 5142 does not follow the same trend as in regions of low-mass star formation mainly because of the high temperature reached in hot cores.