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

TL;DR

This study uses high-resolution HST STIS spectroscopy to identify and characterize the mixing layer in the Cat's Eye Nebula, revealing its location, size, and physical properties for the first time in a planetary nebula.

Contribution

First detection and detailed characterization of the mixing layer in a planetary nebula using spatially resolved spectroscopy.

Findings

The mixing layer is located between the hot bubble and optical shell.

Its thickness is approximately 1.5×10^16 cm.

The electron density of the layer is about 200 cm^-3.

Abstract

Planetary nebulae (PNe) are expected to have a ~10$^{5}$ K interface layer between the $\geq$10$^{6}$ K inner hot bubble and the ~10$^{4}$ K optical nebular shell. The PN structure and evolution, and the X-ray emission depend critically on the efficiency of mixing of material at this interface layer. However, neither its location nor its spatial extent has ever been determined so far. Using high-spatial resolution HST STIS spectroscopic observations of the N V 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 1239 line emission, is located between the hot bubble and the optical nebular shell. We estimate a thickness of 1.5$\times$10$^{16}$ cm and an electron density of ~200 cm$^{-3}$ for the mixing layer. With a thermal pressure of ~2$\times$10$^{-8}$ dyn cm$^{-2}$, the mixing layer is in pressure equilibrium with the hot bubble and ionized nebular rim of NGC 6543.