Astron. Astrophys. 347, L19-L22 (1999)

1. Introduction

Observations of deuterated molecules in the gas phase towards cold clouds (TMC-1), warm clouds and "hot cores" have shown significant fractionation effects (e.g. Millar & Hatchell 1998 and references therein). In particular, in "hot cores" the abundances of deuterated species can reach factors of to larger than those expected on the basis of the cosmic [D]/[H] ratio of (Dring et al. 1997). The detection of such high abundances of deuterated molecules in "hot cores" is surprising, as it is difficult to produce and maintain deuterium enrichment through gas-phase chemistry at the high temperatures () of those regions (Millar et al. 1989; Jacq et al. 1990; Rodgers & Millar 1996). The current interpretation of this deuteration enhancement in "hot cores" involves gas-grain chemistry during the cold, dense cloud core stage, followed by evaporation from the grain mantles of the deuterated molecules when a star is formed at the centre of the core (e.g. Millar & Hatchell 1998), or by the passage of a shock (Bergin et al. 1999).

The importance of reactions involving deuterium in grain mantles for the enhancement of the abundance of deuterated molecules, was originally noted by Tielens (1983). Brown & Millar (1989) produced time-dependent chemical models of a dense quiescent cloud and of a "hot core", including deuterium-bearing species and accretion onto the grains. These models produced a high abundance of deuterated molecules in the grain mantles during the cold, dense cloud stage, which is reflected in the expected enhanced fractionation of deuterated species by evaporation during the "hot core" phase.

Presently, the enrichment in deuterium of the icy mantles, although predicted by theoretical models, has not been observed. The origin of the high levels of fractionation of deuterated gaseous species in "hot cores" is therefore still only a supposition. The observation of deuterated species in the grain mantles would be the first demonstration that mantle chemistry in fact generates deuterium enrichment, and a crucial test for the models of cloud chemistry.

Water-ice has been shown to be ubiquitous in grain mantles: H₂O is by far the most abundant molecule in the icy mantles, with an abundance comparable to that of gaseous CO (Tielens et al. 1991). Taking the models of Tielens (1983) and Brown & Millar (1989), the estimated abundance of HDO in the mantles is of the order of 1% and 6%, respectively, of the abundance of H₂O-ice at the densities characteristic of dense cloud cores. The infrared feature due to OD stretch of HDO in amorphous H₂O ice deposits produces a broad absorption around 2450 cm^-1 (4.1 µm) (Mayer & Pletzer 1985) which is inaccessible from the ground but accessible to the Infrared Space Observatory (ISO, Kessler et al. 1996). Because of the high abundance of water-ice in grain mantles and of the infrared spectrum of HDO, this species is the best candidate for the detection of deuterated molecules in the solid state in molecular clouds. Its detection may provide the missing link between theory and observations in dark cloud chemistry.

© European Southern Observatory (ESO) 1999

Online publication: June 30, 1999
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