2011A&A...525A.141B

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). CARMA was used to observe the 3.2mm continuum and N₂H⁺(1-0) emission toward AFGL 5142. We used NH₃(1, 1) and (2, 2), as well as HCO⁺(1-0) and H¹³CO⁺(1-0) data available from the literature, to study the chemical environment. Additionally, we performed a time-dependent chemical modeling of the region. The 3.2mm continuum emission reveals a dust condensation of ∼23M_☉ 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 ∼0.08pc 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 NH₃ emission. The two cores are located to the west and to the east of the 3.2mm 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 ∼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≃10⁶cm^–3) and temperature (T≃70K) 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≃4.5-5.3x10⁵yr, while in the western and eastern cores the NH₃/N₂H⁺ ratio can be reproduced by our model for times in the range 10⁴-3x10⁶ yr. 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.