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

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+¹³CCO+C⁺ which is faster than the ¹³CO photodestruction in PN envelopes (e.g. Likkel et al. 1988). However, although the -0 and -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.

© European Southern Observatory (ESO) 2000

Online publication: March 17, 2000
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