Infrared [fe Ii] Emission from P Cygni's Nebula: Atomic Data, Mass, Kinematics, and the 1600 A.d. Outburst

We present moderate and high-dispersion 1–2.5 µm spectra of the ∼10 ′′-radius 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] λ12567/λ16435, often used as a reddening indicator, for which we empirically derive an intrinsic value of 1.49, which is 10–40% higher than previous estimates. High-dispersion spectra of [Fe ii] λ16435 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 M ⊙ ; 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 ∼10 46.3 ergs, far less than the kinetic energy released during the giant eruption of η Car in the 1840s, but close to the value for η Car's smaller 1890 outburst. In this respect, it is interesting that the infrared spectrum of P Cygni's nebula resembles that of the " Little Homunculus " around η Car, ejected in that star's 1890 eruption. The mass and kinetic energy in the nebulae of η Car and P Cygni give insight to the range of parameters expected for extragalactic η Car-like eruptions.