The shock-excited P Cygni nebula

We have obtained images in the [N ii] $$\lambda 6584$$ line of the nebula around the luminous blue variable star P Cygni, using a 4 arcsec wide occulting strip. A nearly circular inner nebula is found, containing many bright condensations, with angular diameters of 21.4 arcsec in the east–west direction and 23.8 arcsec in the north–south direction. Two fainter outer arcs of [N ii] emission are also visible, with angular diameters of 1 and 1.5 arcmin. A high-resolution 4900-9000 Å UES spectrum, obtained at a position 9 arcsec west of P Cygni, shows the [N ii] $$\lambda\lambda 7378, 7412$$ doublet to be the strongest feature emitted by the nebula. High-resolution long-slit MES spectra were obtained at multiple positions across the bright inner nebula in order to investigate the density, dynamics and age of the nebula. Our [N ii] $$\lambda 6584$$ spectra show broad velocity components with an overall expansion velocity of $$140\,{\rm km \ s}^{-1}$$, while our [N ii] $$\lambda 7412$$ spectra show much narrower velocity components with an overall expansion velocity of only $$110\,{\rm km \ s}^{-1}$$. Our results are consistent with the narrow [N ii] emission originating from dense, mainly neutral knots which are being overtaken by the stellar wind from P Cyg at a relative velocity of $$100\,{\rm km \ s}^{-1}$$, with the broader irregular [N ii] and [S ii] emission arising from bow shocks around the knots. This model provides a natural explanation for the larger [N ii] expansion velocities compared to those of [N ii].