HST Observations of the Wolf-Rayet Nebula NGC 6888

TL;DR

This study uses Hubble Space Telescope images to analyze the ionization structure, physical conditions, and elemental abundances of NGC 6888, revealing inhomogeneities and challenging traditional nebular spectroscopy assumptions.

Contribution

First detailed HST imaging analysis of NGC 6888's ionization structure, providing new insights into its physical conditions and evolutionary history.

Findings

Photoionized clumps have higher thermal pressure than the shocked stellar wind.

Physical conditions have changed significantly over less than a few thousand years.

Inhomogeneity challenges traditional nebular spectroscopy assumptions.

Abstract

We present Hubble Space Telescope WFPC2 images of a portion of the bright northeast rim of NGC 6888, the nebular shell physically associated with the Wolf-Rayet star HD 192163. The exposures are taken in the light of Halpha 6563, [O III] 5007, and [S II] 6717,6731. The images are used to constrain models of the ionization structure of nebular features. From these models we infer physical conditions within features, and estimate elemental abundances within the nebula. The results of our analysis, together with the degree of small scale inhomogeneity apparent in the images, call into question the assumptions underlying traditional methodologies for interpretation of nebular spectroscopy. The thermal pressure of photoionized clumps is higher than the inferred internal pressure of the shocked stellar wind, implying that the current physical conditions have changed significantly over less than a few thousand years. These results are discussed within the context of published three-wind evolutionary scenarios for the formation of the nebula. We also discuss the nature of a back-illuminated radiative shock driven into the cavity surrounding NGC 6888.