Volume 467, Number 1, May III 2007
|Page(s)||207 - 216|
|Section||Interstellar and circumstellar matter|
|Published online||20 February 2007|
Probing the initial conditions of high mass star formation*
I. Deuteration and depletion in high mass pre/protocluster clumps
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: email@example.com
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
Accepted: 5 February 2007
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.
Key words: stars: formation / astrobiology / molecular data / methods: observational / ISM: clouds / radio lines: ISM
© ESO, 2007
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