Infrared [Fe II] emission from P Cygni's nebula: Atomic data, mass, kinematics, and the 1600 A.D. outburst

Abstract

We present moderate and high-dispersion 1-2.5 micron spectra of the 10 arcsec nebula around P Cygni, dominated by bright emission lines of [Fe II]. Observed [Fe II] line ratios disagree with theoretical transition rates in the literature, so we use the spectrum of P Cygni's nebula to constrain the atomic data for low-lying levels of [Fe II]. Of particular interest is the ratio [Fe II] 12567/16435, often used as a reddening indicator, for which we empirically derive an intrinsic value of 1.49, which is 10--40 per cent higher than previous estimates. High-dispersion spectra of [Fe II] 16435 constrain the geometry, detailed structure, and kinematics of P Cygni's nebula, which is the major product of P Cygni's outburst in 1600 A.D. We use the [N II]/[N I] line ratio to conclude that the nebula is mostly ionized with a total mass of ~0.1 Msun; more than the mass lost by the stellar wind since the eruption. For this mass, we would expect a larger infrared excess than observed. We propose that the dust which obscured the star after the outburst has since been largely destroyed, releasing Fe into the gas phase to produce the bright [Fe II] emission. The kinetic energy of this shell is ~2e46 ergs, far less than the kinetic energy released during the giant eruption of eta Car in the 1840s. The mass and kinetic energy in the nebulae of eta Car and P Cygni give insight to the range of parameters expected for extragalactic eta Car-like eruptions.