Deuterium chemistry in the Orion Bar PDR^*

“Warm” chemistry starring CH₂D⁺

¹ Max Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: bparise@mpifr-bonn.mpg.de
² European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
³ LUTH, Observatoire Paris-Meudon, 5 place Jules Janssen, 92195 Meudon Cedex, France
⁴ California Institute of Technology, MC 301-17, Pasadena, CA 91125, USA

Received: 26 June 2009
Accepted: 18 September 2009

Abstract

Context. High levels of deuterium fractionation in gas-phase molecules are usually associated with cold regions, such as prestellar cores. Significant fractionation ratios are also observed in hot environments such as hot cores or hot corinos, where they are believed to be produced by the evaporation of the icy mantles surrounding dust grains, and are thus remnants of a previous cold (either gas-phase or grain surface) chemistry. The recent detection of DCN towards the Orion Bar, in a clump at a characteristic temperature of 70 K, has shown that high deuterium fractionation can also be detected in PDRs. The Orion Bar clumps thus appear to be a good environment for the observational study of deuterium fractionation in luke warm gas, allowing us to validate chemistry models for a different temperature range, where dominating fractionation processes are predicted to differ from those in cold gas (<20 K).

Aims. We aimed to study observationally in detail the chemistry at work in the Orion Bar PDR, to understand whether DCN is either produced by ice mantle evaporation or is the result of warm gas-phase chemistry, involving the CH₂D⁺ precursor ion (which survives higher temperatures than the usual H₂D⁺ precursor).

Methods. Using the APEX and the IRAM 30 m telescopes, we targeted selected deuterated species towards two clumps in the Orion Bar.

Results. We confirmed the detection of DCN and detected two new deuterated molecules (DCO⁺ and HDCO) towards one clump in the Orion Bar PDR. Significant deuterium fractionations are found for HCN and H₂CO, but we measured a low fractionation in HCO⁺. We also provide upper limits to other molecules relevant to deuterium chemistry.

Conclusions. We argue that grain evaporation in the clumps is unlikely to be a dominant process, and we find that the observed deuterium fractionation ratios are consistent with predictions of pure gas-phase chemistry models at warm temperatures (T ~ 50 K). We show evidence that warm deuterium chemistry driven by CH₂D⁺ is at work in the clumps.