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B.C. Dunne, R.A. Gruendl, Y.-H. Chu (UIUC)
The ultra-luminous supernova remnant (SNR) in NGC 6946 is the brightest X-ray SNR known, ~1000 times as luminous as Cas A. However, high-velocity gas with Vexp > 600 km/s has not been detected, as expected for a young remnant. HST WFPC2 images of this SNR show multiple loops. This morphology has been used as evidence of colliding SNRs of different ages, in order to explain the X-ray luminosity (Blair, Fesen, & Schlegel 1997),
We have obtained high-dispersion echelle spectra of this SNR with the KPNO 4-m telescope. The SNR is detected in H\alpha, [N II]\lambda6584, and [O III]\lambda5007 lines. The emission lines show a narrow component (FWHM ~ 40 km/s) superposed on a broad component (FWZI ~ 360 km/s). The total [N II] flux is comparable to the H\alpha flux. The broad component contains ~1.5 times as much flux as the narrow component, and the broad component has slightly higher [N II]/H\alpha ratio than the narrow component. These spectral properties are qualitatively similar to some small SNRs in M33.
The strong [N II] line in the narrow component suggests an anomalous nitrogen abundance usually associated with circumstellar material. The nitrogen abundance and small remnant size imply that the supernova progenitor was a WN star and the supernova exploded in a dense circumstellar bubble.
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