Central star temperatures of optically thick planetary nebulae and a distance-independent test of dredge-up theory

The effective temperatures are calculated and the V magnitudes are predicted for the central stars of 62 optically thick planetary nebulas by forcing agreement between the hydrogen and ionized helium Zanstra temperatures. Comparison to the measured V for 34 of the stars shows good agreement and validates the method for the other 28, for which no Zanstra analysis is available. In many cases the predicted V magnitudes are actually likely to be better than the currently measured values. The comparison suggests that optical depth and not an ultraviolet excess is the dominant cause of the Zanstra discrepancy. Nebular N/O correlates positively with effective temperature, demonstrating that the N/O-core mass relation rises more steeply than predicted by dredge-up theory.