HST STIS observations of the mixing layer in the Cat's eye Nebula

Planetary nebulae (PNe) are expected to have a ∼10⁵ K interface layer between the ≥10⁶ K inner hot bubble and the ∼10⁴ K optical nebular shell. The PN structure and evolution, and the X-ray emission, depend critically on the efficiency of the mixing of material at this interface layer. However, neither its location nor its spatial extent have ever been determined. Using high-spatial resolution HST STIS spectroscopic observations of the N v λλ 1239,1243 lines in the Cat's Eye Nebula (NGC 6543), we have detected this interface layer and determined its location, extent, and physical properties for the first time in a PN. We confirm that this interface layer, as revealed by the spatial distribution of the N v λ1239 line emission, is located between the hot bubble and the optical nebular shell. We estimate a thickness of 1.5 × 10¹⁶ cm and an electron density of ∼200 cm^-3 for the mixing layer. With a thermal pressure of ∼2 × 10^-8 yncm^-2, the mixing layer is in pressure equilibrium with the hot bubble and ionized nebular rim of NGC 6543.