Physical conditions in the O++ zone from ISO and HST data. NGC 6543 revisited

Abstract

We revise the physical conditions in the O++ zone of the planetary nebula NGC 6543, obtaining two different estimates of the electron temperature (Te) and one estimate of the electron density (Ne). The electron temperature is computed by means of two independent methods, the nebular-to-auroral ratio [O III] 5007/4363, and the diagnostic diagram that combines 5007 to the [O III] infrared lines 52m and 88m. The optical and infrared fluxes have been obtained from archive HST/WFPC2 images and ISO LWS spectra respectively, and the continuum intensity in the optical has been measured from narrow-slit spectra obtained with the Isaac Newton Telescope at La Palma. The measured continuum intensity is higher than predicted by recombination theory under the hypothesis that all the Ly-alpha photons either escape or are destroyed. This fact can be explained in terms of an enhancement of the 2-photon continuum due to Ly-alpha conversion, a process that depends strongly on the local structure of the nebula. Alternative possibilities, outside the framework of recombination theory, have also been considered: e.g., the optical tail in the X-ray emission of a very hot plasma, and dust scattering of stellar radiation, but these hypotheses are not supported by quantitative estimates. While the electron temperature and density derived from the diagnostic diagram agree with the most recent determination, the temperature derived from 5007/4363,, Te(O++)_opt, is somewhat smaller than previously published values. We discuss several technical issues that contribute to the overall uncertainty in our results, focusing on the instrumental effects that might bias the [O III] 4363 intensity. We also discuss the effects of the collisional de-excitation of the O++ ground terms on the relation between Te(O++)_opt and Te(O++)_IR.