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Astron. Astrophys. 328, 617-627 (1997)

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6. Discussion and conclusion

Except possibly in one single case, L134N (0, 0), no detection of 16 O18 O has been obtained up to now and the low abundance of 16 O18 O obtained by MPLC seems to reveal a general characteristic of dark clouds. For half of the observed clouds: NGC2264(IRS2), DR21, OMC3, L134N (4', -1') and TMC2, the low upper limits of the column densities obtained for 16 O18 O are significantly lower than the column densities computed with standard conditions. Our cloud model allows to explain this underabundance of molecular oxygen by an increase of the C/O elemental ratio in the gas phase with respect to the cosmic value. This decrease by a factor of 2 of the oxygen available in the gaseous phase could be due to a selective depletion of oxygen on grain ice mantles under the form of H2 O, CO, CO2 and O2 (Langer et al. 1984, Blake et al. 1987). To constrain the elemental abundances in the gas phase and, particularly the oxygen one, Ehrenfreud et al. (1992) suggest the possibility of observing solid O2 and other compounds of the interstellar mantle ices with ISO. An increase of the C/O ratio is among the simplest explanations, to which it is often referred. Whether it has some reality and is not an "ad hoc" hypothesis could be checked by comparing computed and observed abundances of several chemical species sensitive to this C/O ratio. This is planned for the future.

Another explanation could help us to understand the low abundance of molecular oxygen. A higher ionisation degree than the one obtained in most of the chemical models leads to a more efficient destruction of O2 and 16 O18 O. This ionisation can be obtained in the phase called "HIP" (High Ionization Phase) of the models of Le Bourlot et al. (1995) which show the possible existence of two stable phases in the chemical composition of interstellar clouds with a high ionization degree and a low one. Another hypothesis (Pineau des Forêts et al. 1992, Xie et al. 1995) suggests a mixing between the core and the edges of the cloud, which could be caused by turbulence and which should increase the C [FORMULA] abundance in the cloud core leading to an efficient destruction of O2 and 16 O18 O.

If one could hope to detect the 234 GHz line of 16 O18 O in molecular clouds, the introduction of 16 O18 O rotational excitation in our model clearly shows that the detection of this molecule at other frequencies (e.g. 298 and 402 GHz) is unrealistic at the moment with present millimeter receivers. Spectroscopic parameters and collisional excitation rates of 16 O18 O by He derived in this work allow to perform model calculations of the excitation of this molecule in other astrophysical environments (circumstellar envelopes, photodissociation regions...).

The explicit treatment of the 18 O chemistry shows that the selective photo-dissociation of CO and C18 O does not only affect the ratio of these two molecules but also all the oxygen-bearing species in diffuse and translucent clouds. Thus, in such clouds, the use of the standard isotopic ratio [16 O]/[18 O] could be dangerous to determine the abundance of oxygen-bearing species from the observations of their isotopic substitutes even if these species do not have any selective photodissociation. In dark clouds, however there is no isotopic fractionation for the 18 O substitutes included in the model: the low 16 O18 O abundance indicates a low abundance of the main isotope O2.

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© European Southern Observatory (ESO) 1997

Online publication: March 26, 1998