Optically Derived Carbon Abundances in Planetary Nebulae

Extract

Gas phase carbon abundances have been determined for a number of bright planetary nebulae from new photoelectric measurements of optical recombination lines (C II λ 4267, C III λ 4650 and C IV λ 4659). Because of blending problems for the latter two features, the abundance for any object may have substantial errors, but the average abundance should be reliable. For the twelve best observed planetaries, this average is C/H = (8.4 ± 2.9 s.d.) x 10⁻⁴ by number. If it is assumed that the planetary progenitors had essentially solar abundances, then, based on Cameron's most recent results (C/H = 4.2 × 10⁻⁴, O/H = 6.9 × 10⁻⁴), it appears that the planetary ejecta have been enriched in carbon, presumably because of dredging of newly synthesized triple α carbon by helium shell flashes during the late evolution of the progenitor. The helium abundance is also slightly high, as would be expected in this interpretation. Since the mean planetary carbon abundance exceeds the solar oxygen abundance, it is possible that the progenitor became a carbon star prior to the ejection of the planetary; that may even have caused the ejection. Because the planetaries in this study were drawn from Peimbert's samples with relatively low mass progenitors (1 – 2.5 M_⊙; these are not significantly helium- and nitrogen-rich objects), such a process might be a general feature of late double shell source evolution.