The shock-excited P-Cygni nebula

Abstract

We have obtained images in the [N II] λ6584 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] λλ7378, 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] λ6584 spectra show broad velocity components with an overall expansion velocity of 140 km s-1, while our [N II] λ7412 spectra show much narrower velocity components with an overall expansion velocity of only 110 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 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].