An analysis of the WN shell nebula NGC 6888 using CCD imagery and spectrophotometry

Abstract

We present a model describing the morphology and physical processes in NGC 6888, a 'wind-blown' nebula around the WN6 star HD192163. CCD imagery with the Palomar 1.5m revealed distinct morphological features that were further probed with spectrophotometry using the KPNO #2 telescope + Intensified Reticon Scanner. Distinctions in morphology in (O III) compared to H$\alpha$ led to a parametrization into two physical systems: (1). An inner ionized shell observable in all the emission lines, with (N II) T\sb{\rm e} = 8000 \sp\circK, (O III) T\sb{\rm e} = 14,000 \sp\circK and (S II) N\sb{\rm e} = 400 cm\sp{-3.}, (2) The (O III) bubble and rim, visible only in (O III) and characterized by a higher (O III) T\sb{\rm e} = 50,000 \sp\circK. The nebula is found to have 5-10 M\sb\bigodot of ionized mass. An extrapolation of (O III) $\lambda$5007 fluxes to IR emission line intensities led to revised values of neutral mass $\sim$40 M\sb\bigodot for the nebula. Of this 3-6 M\sb\bigodot is found to be enriched mass, 0.5 M\sb\bigodot contributed by stellar wind mass loss and wind swept ISM mass is determined to be $\sim$20-60 M\sb\bigodot. Photoionization models demonstrate that the shell is enriched in N and He and depleted in O compared to Galactic H II regions. These results collectively indicate a scenario that is a combination of several physical processes. A slow wind ejected in an asymmetric fashion from the star is swept up by a fast stellar wind together with the ambient medium, creating the ionized shell. This interaction also creates Rayleigh Taylor instabilities which are determined to be plausible formation mechanisms for the observed knots. The hot gas ($\sim$ 10\sp7 \sp\circK) penetrates the nebular material in the NW and SE as there is less ejecta pressure opposing it along the polar axis, and creates the observed (O III) bubbles in the NW and lattice in the SE