7. Prospects for future observations
One of the main goals of our study was the determination of the isotopic ratio in NGC 3242, the best object for testing the mixing hypothesis since the 3He measurements are very reliable and the progenitor mass is in the range where extra mixing should occur (see also the discussion in Rood et al. 1998). However, the high sensitivity of our observations and the fact that we searched for CO emission across the whole circumstellar shell imply that CO is absent in the nebula. We therefore conclude that the isotopic ratio cannot be determined in this important object by means of millimeter line observations. The lack of a molecular envelope around NGC 3242 can be a consequence of the low-mass of the progenitor. It is well known that the transition from the AGB phase to the PN stage is slow for low-mass stars and therefore the gas remains exposed to ionizing and dissociating radiation for a longer time than in the case of more massive stars.
This behavior is consistent with the fact that the most massive object of our sample, NGC 6720, indeed has a rich molecular envelope. Unfortunately, NGC 6720 has an estimated progenitor mass of , quite close to the limit where the nonstandard mixing mechanism is expected to significantly affect the isotopic ratio. Thus, the derived ratio is consistent with both standard and nonstandard evolutionary models. The lesson is clear: there is a trade-off between the need to select low-mass PNe in order to discriminate between theoretical models and the higher detection rate of molecular envelopes around more massive objects. The determination of isotopic ratios may be a tricky business, but future observations should take up such a challenge.
The method of using mm-wave transitions is not the only one adequate for measuring the isotopic abundance in PNe. Clegg (1985) first pointed out the possibility of using the CIII] multiplet near 1908 Å to obtain a direct estimate of the isotopic ratio in the ionized gas of PNe. Recently, Clegg et al. (1997) have successfully detected the extremely weak isotopic line 13C P-S0 in two PNe (plus a tentative detection in a third one), using the HST Goddard High Resolution Spectrograph. The derived 12C/13C ratios are 153 in NGC 3918 and 2111 in SMC N2. In either case, no measurement at mm-wavelengths has been made to independently check the derived values. However, as Clegg et al. point out, the two types of measurements are complementary since they cannot be performed on the same objects: the UV transitions require high excitation conditions, while the opposite is true for the mm-wave lines studied by us. Considering the negative results of the CO search, it would be extremely important to perform the HST observations towards NGC 3242.
© European Southern Observatory (ESO) 2000
Online publication: March 17, 2000