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

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5. The isotopic ratio 16O/ 18O

Taking into account the chemistry of 13 C and 18 O bearing species, our model allows to compute the isotopic abundance ratio of oxygen-bearing species, X16 O/X18 O, and to determine whether isotopic fractionation occurs for some of them, and in particular for O2.

This is illustrated in Fig. 7 for the oxygen-bearing species O, CO, O2, OH and H2 O where are plotted the normalized column density ratios R defined by:

[EQUATION]

except for O2 where:

[EQUATION]

The ratio R (X16 O/X18 O) should be about 1 if there is no isotopic fractionation. The effect of the selective photo-dissociation of CO and its isotopic variants on the C16 O/C18 O ratio in interstellar clouds has already been studied by several authors (Bally & Langer 1982, Chu & Watson 1983, Glassgold et al. 1985, Warin et al. 1996). It is briefly recalled here: for very low visual extinctions, there is no efficient protection against the ultraviolet radiation field and the photo-destruction rates of CO and C18 O are similar, so that R (CO/C18 O) is close to unity. As the visual extinction increases, CO protects itself more efficiently than C18 O because of its larger abundance, so that R (CO/C18 O) increases. The maximum enhancement occurs for [FORMULA] = 2-4 and increases with increasing density as a consequence of a more efficient CO formation. When the UV radiation field becomes too attenuated to photodissociate CO or C18 O, R (CO/C18 O) decreases back to unity ([FORMULA] 7) and becomes constant again. The photodissociation of the other oxygen-bearing species occurs through a continuum, so that selective photo-dissociation does not occur for these species. For the three other oxygenated molecules included in the model: OH, H2 O and O2 the normalized column density ratio is anti-correlated with that of CO: it decreases and reaches a minimum for [FORMULA] = 2-4. In this region, where R (CO/C18 O) is higher than unity, C18 O is more easily photodissociated than CO and the medium is enriched into 18 O, which allows a larger formation of the other 18 O-substituted molecules to occur.

[FIGURE] Fig. 7. The influence of the isotopic fractionation on the column densities of the principal oxygen-bearing species for cloud models at thermal equilibrium

Isotopic fractionation does not affect dark clouds, where O2 has a better chance to be detected, so that the low value of 16 O18 O abundance derived from the observations also indicates a low abundance for the main isotope. The effect of isotopic fractionation must be taken into account in diffuse and translucent clouds. When the fractionation is maximum, neglecting the isotopic fractionation may let one underestimate the CO column density by a factor of 6 and overevaluate the abundance of other oxygen-bearing species by a factor of 2.5 if the column densities of the main isotopes are deduced from the observations of the 18 O-bearing species. For the O2 /CO ratio, the overestimate can reach a factor of 15 for [FORMULA] 2 but this has no consequence since O2 should be undetectable in such clouds.

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© European Southern Observatory (ESO) 1997

Online publication: March 26, 1998

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