Probing the initial conditions of high mass star formation^*

I. Deuteration and depletion in high mass pre/protocluster clumps

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: tpillai@cfa.harvard.edu
² School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, UK
³ Department of Physics & Astronomy, University of British Columbia, Vancouver, BC, V6T 1Z1, Canada
⁴ Centre for Astrophysics Research, Science & Technology Research Institute, University of Hertfordshire, College Lane, Hatfield, AL10 9AB, UK

Received: 23 May 2006
Accepted: 5 February 2007

Abstract

Aims.UltraCompact Hii regions are signposts of high-mass star formation. Since high-mass star formation occurs in clusters, one expects to find even earlier phases of massive star formation in the vicinity of UltraCompact Hii regions. Here, we study the amount of deuteration and depletion toward pre/protocluster clumps found in a wide-field (1010 arcmin) census of clouds in 32 massive star-forming regions that are known to harbour UCHii regions.

Methods.We determine the column density of NH₃, NH₂D, CO, H¹³CN, and HC¹⁵N lines. We used the (J,K) = (1, 1) and (2, 2) inversion transitions of NH₃ to constrain the gas temperatures.

Results.We find that 65% of the observed sources have strong NH₂D emission and more than 50% of the sources exhibit a high degree of deuteration, (0.1 ≤ NH₂D/NH₃ ≤ 0.7), 0.7 being the highest observed deuteration of NH₃ reported to date. Our search for NHD₂ in two sources did not result in a detection. The enhancement in deuteration coincides with moderate CO depletion onto dust grains. There is no evidence of a correlation between the two processes, though an underlying correlation cannot be ruled out as the depletion factor is very likely to be only a lower limit. Based on simultaneously observed H¹³CN and HC¹⁵N (J = 1-0) lines, we derive a high abundance ratio of H¹³CN to HC¹⁵N, which might indicate anomalous ratios of C and N isotopes relative to those derived toward the local ISM.

Conclusions.We find CO depletion and high deuteration towards cold cores in massive star forming regions. Therefore, these are good candidates for sources at the early phases of massive star formation. While our sensitive upper limits on NHD₂ do not prove the predictions of the gas-phase and grain chemistry models wrong, an enhancement of ≈10⁴ over the cosmic D/H ratio from NH₂D warrants explanation.