THE CARBON ABUNDANCE PROBLEM IN PLANETARY-NEBULAE

We have reexamined the problem of the discrepancy between carbon abund ances in planetary nebulae derived from the CII lambda 4267 and CIII] lambda 1909 line intensities. To this end, we have compiled the publis hed optical and ultraviolet data allowing to compute C/O in planetary nebulae, and constructed diagrams providing a synthetic view of the pr oblem. We have performed Monte-Carlo simulations to study the effect o f observational errors in the measurement of line intensities, and hav e found that C/O abundance ratios derived from the CII lambda 4267 lin es are strongly biased towards too large values, which explains a larg e part of the trend seen in the observational diagrams. Taking into ac count the observational errors, we discuss some other explanations whi ch have been proposed to resolve the discrepancy, such as temperature fluctuations. Adopting the CIO values from the CIII] lambda 1909 lines , we find that carbon-rich objects represent less than one half of the total number of planetary nebulae. This is significantly less than th e proportions quoted in previous studies. The percentage of carbon-ric h objects among Type I and Non Type I Galactic planetary nebulae are s imilar, contrary to what is found in the Magellanic Clouds.