N₂H⁺ depletion in the massive protostellar cluster AFGL 5142

¹ Departament d’Astronomia i Meteorologia (IEEC-UB)Institut de Ciències del Cosmos, Universitat de Barcelona, Martí i Franquès 1, 08028 Barcelona, Catalunya, Spain
e-mail: gbusquet@am.ub.es
² Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, 02138, UK
³ Department of Physics and Astronomy, University College London, Grower Street, London WC1E 6BT, UK
⁴ Institut de Ciències de l’Espai (CSIC-IEEC), Campus UAB, Facultat de Ciències, Torre C-5 parell, 08193, Bellaterra, Catalunya, Spain
⁵ Academia Sinica Institute of Astronomy and Astrophysics, Taipei, Taiwan

Received: 28 January 2010
Accepted: 22 August 2010

Abstract

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 H¹³CO⁺ (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 NH₃ core (hereafter central core). The dense gas emission traced by N₂H⁺ 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 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  ~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 ≃ 10⁶ 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 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 ≃ 4.5−5.3 × 10⁵ yr, while in the western and eastern cores the NH₃/N₂H⁺ ratio can be reproduced by our model for times in the range 10⁴−3 × 10⁶ 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.