Fabry-Perot imaging spectroscopy of the Crab Nebula, Cassiopeia A, and Nova GK Persei.

Abstract

Three-dimensional spatial models of the line-emitting gas in the Crab Nebula, Cassiopeia A, and Nova GK Persei have been developed from imaging spectroscopy taken with the Goddard Fabry-Perot Imager (GFPI) at Michigan-Dartmouth-MIT Observatory (MDM). The spatial model of the Crab Nebula in (O III) $\lambda$5007 reveals the relationship between the "high-helium torus" and the north-south bipolar asymmetry of the remnant shell, the size of the inner synchrotron cavity, and holes near the ends of the major optical axis. Images of the fast-moving knots of Cas A in (S II) $\lambda\lambda$6716, 6731 provide a more complete view of the three-dimensional morphology of the remnant. The bright northern arch is actually two kinematically distinct rings; the larger contains the most highly redshifted material in the nebula and many of the newly-formed (O III) knots. The most blueshifted material occurs in patchy nebulosity to the southeast. The data for the quasi-stationary flocculi of Cas A in H$\sb\alpha$ + (N II) $\lambda\lambda$6548, 6583 reveal new nebular features and show an elliptical distribution on the sky. Isophotes of GK Per in H$\sb\alpha$ + (N II) $\lambda\lambda$6548, 6583 show a lack of emission in the forward, northeastern zone, suggest the presence of a bulge-like structure in the south-southeast, and indicate recent strengthening in the northeastern limb. Extraordinary, compact, line-emitting knots are apparent in images of the Crab Nebula obtained with the GFPI at MDM. The knots appear most prominently in (O III) $\lambda$5007 through a 5.3 A (FWHM) bandpass at 5015.3 A, with fluxes of order 10$\sp{-14}$ erg cm$\sp{-2}$ s$\sp{-1}$. They are aligned in arcs, seven to the north and four to the south, from the pulsar. Measurements over a two-year baseline yield proper motions $\sbsp{\sim}{<}0\sbsp{.}{\prime\prime}$1 yr $\sp{-1}$ ($\sbsp{\sim}{<}$900 km s$\sp{-1}$ at 1800 pc). Spectroscopy from MDM and the Multiple Mirror Telescope shows that the knots are characterized by remarkably strong (Ar III) emission, possibly indicating high argon abundances or unusual excitation mechanisms; high gas temperatures are excluded. The relative intensities of (Ar III) and (Ni II) appear to be stronger from the sides of the knots closer to the pulsar.