A survey has been carried out in the CO(2-1) line to search for molecular gas in 100 planetary nebulae. Nineteen show evidence of CO emission, a few of which were known from earlier studies. The detected nebulae are mainly younger disk population objects formed from high-mass progenitors according to their morphological types and nitrogen-to-oxygen ratios. Rough estimates show a large range in the masses of the molecular envelopes from about 1 solar mass to less than .001 solar mass. For the extreme class of objects detected in CO, the ratio of molecular to ionized mass decreases with increasing radius, as expected if the nebulae grow by the ionization of previously ejected molecular envelopes. The molecular gas is the dominant observed mass component in these nebulae, at least until they expand to radii larger than about 0.1 pc, where the bulk of the CO will be dissociated.
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Huggins et al. (1989) studied this question.