The giant halos of NGC 6543 and 6826

Long-slit spectra at several positions on the large, faint haloes of NGC 6543 and 6826, from IPCS and CCD detectors, are analysed to give average properties of these two regions. Comparison of the halo and bright core spectra shows that the halo emission is thermal and not reflection. Average halo properties and masses are estimated. The [O III] electron temperature is higher in each halo than in its central nebula; we find Te (halo) = 14 700 and 13 000 K for NGC 6543 and 6826, respectively. The mass ratios (halo/core) are in the range 2.9–10.2 and 0.07–2.24 for NGC 6543 and 6826, respectively, from absolute Hβ flux measurements at representative slit positions. Both haloes appear to have lower abundances than their bright cores. Photo-ionization models are used with limited success to reproduce the conditions of the haloes. ‘Pre-hardening’ of the ionizing radiation field by optically thick inner cores does not produce sufficient heating to explain the observed high Te values. It is suggested that the hot fast stellar wind in NG 6543 has escaped beyond the central nebula and has shocked the filamentary halo.