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Astron. Astrophys. 355, 69-78 (2000)

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3. Measuring the carbon isotopic ratio from millimeter wave observations

Molecular line observations at mm-wavelengths provide a powerful method to estimate the 12C/13C  ratio in PNe. In particular, the 12CO/13CO ratio should faithfully reflect the atomic 12C/13C  ratio, since the mechanisms which could alter the 12CO/13CO ratio are not expected to be at work in PNe. In fact, the kinetic temperature in PN envelopes (25-50 K) is high enough that isotopic fractionation should not operate. Also, selective photodissociation is expected to be compensated by the isotope exchange reaction 12CO+13C[FORMULA]CO+C+ which is faster than the 13CO photodestruction in PN envelopes (e.g. Likkel et al. 1988). However, although the [FORMULA]-0 and [FORMULA]-1 lines of 12CO have been extensively observed in PNe (e.g. Huggins et al. 1996), very few observations of the 13CO lines are available and the value of the isotopic ratio is presently poorly known.

In order to estimate the 12CO/13CO isotopic ratio, one needs to make a number of approximations. First, we assume that the emitting regions fill the antenna beams in the lines of both molecules, or that the filling factor is the same (in the case of an extended clumpy medium). Second, we assume that the rotational levels are thermalized at a representative uniform temperature of 25 K (see e.g. Bachiller et al. 1997). Thermalization is indeed a reasonable assumption for 12CO and 13CO, since the dipole moment is quite small (about 0.1 Debye). Third, if we assume that the emission is optically thin for both the 12CO and 13CO lines, then the 12CO/13CO column density ratio is given by the ratio of the integrated intensities. We will discuss below the uncertainties introduced by this approach in the derived isotopic ratios.

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© European Southern Observatory (ESO) 2000

Online publication: March 17, 2000