THE HIGHLY IONIZED REGIONS OF NGC-6302 AND NGC-6537

We present [Ne V] 24.32 mum and 3426 angstrom emission-line fluxes for NGC 6302 and NGC 6537. These fluxes along with those of [Ne V] 14.3 m um allow us to determine the physical conditions within the highly ion ized regions of these nebulae. The electron densities found via the [N e V] line ratios are similar to those found in studies of the low-ioni zation regions. However, the electron temperatures determined from the [Ne V] line ratios are much greater than those in the low-ionization regions. We use photoionization models to show that these high derived electron temperatures are inconsistent with the assumption that the n ebular ionization is produced solely by radiation from the central sta r. Much of the intense [Ne V] 3426 angstrom emission from these object s must arise from highly ionized shocked material; a conclusion which is in accord with other evidence. The fact that the [Ne V] infrared li nes are less affected by the shock means that we can estimate the rela tive volumes of the shocked and radiatively excited portions of these nebulae. In the most likely scenario, the volume of the highly ionized shocked region is less than 10% that of the highly ionized region for med by photoionization. We note that under such conditions, infrared e mission lines are more useful than optical or UV emission lines for ab undance studies because they are less affected by shocks. With appropr iate corrections to the optical and UV emission-line fluxes, these neb ulae can still be described by photoionization models.