THE IONIZATION STRUCTURE OF PLANETARY-NEBULAE .10. NGC-2392

Spectrophotometric observations of emission-line intensities over the spectral range 1400-7200 angstrom have been made in six positions in the planetary nebula NGC 2392. The O++ electron temperature averages 14,500 K, which is 4100 K higher than the average N+ electron temperature; this is an unusually large difference. The Balmer continuum electron temperature averages 1500 K higher than the O++ electron temperature, but this difference is only slightly greater than the measurement errors. As found for most of the other planetaries in this series, the lambda-4267 C II line intensity implies a C++ abundance that is several times higher than that determined from the lambda-1906, 1909 C III] lines. The discrepancy disappears if one adopts the N+ electron temperature, rather than the O++ electron temperature for the C++ region, but both theoretical and observational evidence support the use of the O++ temperature. As for the other papers in this series, it is argued that the lambda-4267 C II line intensity is not being interpreted correctly, perhaps because it is blended with a line from an unknown high-excitation ion. Standard equations used to correct for the existence of elements in other than the optically observable ionization stages give consistent results for the different positions that are in excellent agreement with abundances calculated using ultraviolet lines, and there is no evidence for any abundance gradient in the nebula. The logarithmic abundances (relative to H = 12.00) are He = 10.99, O = 8.53, N = 8.04, Ne = 7.88, C = 7.62:, Ar = 6.15, and S = 6.63. These abundances agree well with determinations by Aller & Keyes, except that the C abundance is a factor of 5 lower than theirs. The abundances are very similar to those found for NGC 1535 and NGC 6826 (previous papers in this series). As for NGC 1535 and NGC 6826, the rather low abundances of He, N, and C suggest that there was little if any mixing of CNO-processed material into the nebular shell in the progenitor to NGC 2392. The O, Ne, and Ar abundances also appear to be somewhat low, suggesting that the progenitor to NGC 2392 may have formed out of somewhat metal-poor material.