2004MNRAS.351.1026W

We present analysis of spectra and Hubble Space Telescope (HST) images of NGC 6543, the Cat's Eye Nebula. Using HST WFPC2 images taken in narrow-band filters isolating Hα and Hβ, we have produced an extinction map towards the nebula, and find that c(Hβ) is low, at ∼0.1, and exhibits only very small spatial variations across the surface of the nebula. Using filters isolating [Oiii] emission lines, we have produced a temperature map of NGC 6543, accounting carefully for contamination due to continuum emission and other lines within the filter bandpasses. We find that the temperature over most of the nebula is between 7000 and 9000K, with higher values seen towards the edge of the bright core. The value of t²_A, the temperature fluctuation parameter, is low at around 0.005 in the central regions.

We have obtained long-slit scanning spectra of NGC 6453, which, when combined with the total Hβ flux, give integrated fluxes for all the lines in our spectra. We determine ionic and total abundances within the nebula from both recombination lines and forbidden lines, and we find that recombination lines yield a higher abundance by a factor of approximately three. A temperature fluctuation parameter of 0.053 would resolve the discrepancy, but several factors argue against temperature fluctuations as the main cause: the value of t² required is inconsistent with the results derived from the temperature map; the agreement of the temperatures derived from the hydrogen Balmer jump and [Oiii] line diagnostics also implies a low value of t²; and the agreement of abundances derived from temperature-insensitive infrared fine-structure lines and temperature-sensitive optical forbidden lines is inconsistent with the temperature fluctuation scenario.

We argue that a more likely explanation for the observed temperature and abundance discrepancies is the presence within the nebula of some hydrogen-deficient material at low temperatures. Weakly temperature-dependent line ratios of Hei and Oii imply lower temperatures than those found from forbidden-line temperature diagnostics and the hydrogen Balmer jump, as is expected in the presence of cold hydrogen-deficient material.