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

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2. The chemistry and spectroscopy of [FORMULA] O [FORMULA] O

2.1. The chemistry

The chemical reaction scheme involving isotopic substituted 13 C- and 18 O- molecules is merely obtained by duplicating the reactions between C- and O- bearing species and replacing the main isotopes by 13 C and 18 O. Four isotopic exchange reactions have also been included in the model (Langer et al. 1984) and we consider both forward and backward reactions:

[EQUATION]

The library of reactions contains about 2100 reactions among 111 chemical species. As in MVB, it is extracted from Viala (1986) where only C and O compounds have been kept; rate coefficients have been up-dated according to the compilations by Anicich (1993) for ion-neutral reactions and by Millar et al. (1996) for neutral-neutral reactions.

The molecule 16 O18 O is mainly produced through two neutral-neutral reactions, instead of only one for 16 O2:

[EQUATION]

with both reaction rates assumed equal to that of the corresponding reaction involving the principal isotope 16 O, i.e. k = 3.3 10 [FORMULA] exp(-6.0/T) cm3 s-1 (Davidsson & Stenholm 1990).

Photoionisation and photodissociation by UV photons are very efficient destruction processes of 16 O18 O. In the regions where UV is excluded, 16 O18 O is mainly destroyed by reactive collisions with [FORMULA], [FORMULA] and [FORMULA], like 16 O2.

The 16 O18 O formation rate is twice more efficient than that of 16 O2 so that one can expect the 16 O2 /16 O18 O ratio to be equal to half the isotopic ratio [16 O]/[18 O] (=500). The isotopic ratio of the other oxygen-bearing species X16 O/X18 O can differ from the isotopic elemental ratio due to the isotopic exchange reactions and the selective photodissociation of CO and C18 O. This is discussed in Sect. 5 below.

2.2. The spectroscopy

To determine the rotational population of 16 O18 O, we take into account the following processes: spontaneous emission, stimulated emission and absorption of the ambient background radiation and collisional excitation and de-excitation between the first 46 rotational levels of 16 O18 O (E [FORMULA] [FORMULA] 500 K).

As O2, 16 O18 O is a [FORMULA] molecule. It however contains two different atoms, so that, contrarily to 16 O2, even N levels do exist (except the level N =J =0 which is forbidden for symmetry reasons). The energies of the levels are obtained from the same set of equations that was used for 16 O2 (eqs. (2)-(3) in MVB) with the spectroscopic constants listed in Table 1.


[TABLE]

Table 1. Spectroscopic constants for the ground vibrational level of 16 O18 O (Gordy & Cook 1984)


The electric dipole moment of 16 O18 O is too small to be taken into account and we consider only radiative magnetic dipole transitions between the rotational levels which obey to the selection rules: [FORMULA] = 0, [FORMULA] 2 and [FORMULA] = 0, [FORMULA] 1. For all transitions considered in the model, the upper level energies, the frequencies and the Einstein coefficients are listed in Table 2


[TABLE]

Table 2. Upper level energies, frequencies and spontaneous emission coefficients of the first 71 rotational lines of 16 O18 O


and are obtained by using equations (5)-(7) of MVB and the line strengths listed in Table XIII of Steinbach's thesis (1974).

The excitation and de-excitation of 16 O18 O by collision with H2 and He are considered for [FORMULA] =0, 1, 2, 3, 4, 5 and 6 using the collision rates of 16 O2 (Corey 1984, Orlikowsky 1986, Corey et al. 1986). Collision rates for transitions with odd values of [FORMULA] and for levels with even values of N which do not exist for O2, have been obtained by interpolation, as explained in Appendix 1 where the de-excitation rates of 16 O18 O-He are listed (Table 4).

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© European Southern Observatory (ESO) 1997

Online publication: March 26, 1998

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