| Abstract
| - Aims. We aim at investigating the NH 3/N 2H + abundance ratio toward the high-mass star-forming region AFGL 5142 with high angular resolution in order to study whether the NH 3/N 2H + 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 N 2H + (1-0) emission toward AFGL 5142. We used NH 3 (1, 1) and (2, 2), as well as HCO + (1-0) and H 13CO + (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 ~23 M⊙ associated with the massive YSOs, deeply embedded in the strongest NH 3 core (hereafter central core). The dense gas emission traced by N 2H + reveals two main cores, the western core of ~0.08 pc in size and the eastern core of ~0.09 pc, surrounded by a more extended and complex structure of ~0.5 pc, mimicking the morphology of the NH 3 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 N 2H + emission drops significantly, indicating a clear chemical differentiation in the region. The N 2H + 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 NH 3/N 2H + abundance ratio ~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 NH 3/N 2H + ratio indicates that density along with temperature is a key parameter in determining the abundances of both NH 3 and N 2H +. The high density ( n ≃ 10 6 cm -3) and temperature ( T ≃ 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 N 2H +, which favors the formation of HCO +. This result is supported by our observations, which show that N 2H + and HCO + are anticorrelated in the central core. The observed values of the NH 3/N 2H + ratio in the central core can be reproduced by our model for times of t ≃ 4.5−5.3 × 10 5 yr, while in the western and eastern cores the NH 3/N 2H + ratio can be reproduced by our model for times in the range 10 4−3 × 10 6 yr. Conclusions. The NH 3/N 2H + 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.
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