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Session 27 - Variable Stars, Novae, & Supernovae.
Display session, Tuesday, June 10
South Main Hall,

[27.06] HST/WPFC2 Images of the Ultra-Luminous Supernova Remnant in NGC 6946

W. P. Blair (JHU/CAS), R. A. Fesen (Dartmouth), E. M. Schlegel (CFA)

We report optical HST/WFPC2 images of an ultra-luminous supernova remnant (SNR), located in the northern spiral arm of the face-on Scd galaxy NGC 6946. At a distance of 5 Mpc, the remnant is \sim1000 times brighter than Cas A in X-rays, more than 10 times brighter in the optical than N49 (the brightest optical SNR in the LMC), and 3 times brighter in the radio than N132D (the brightest radio SNR in the LMC). Only the young, O-rich SNR in NGC 4449 is close to this object in total luminosity. However, optical spectra of the NGC 6946 SNR reveal no high velocities or peculiar abundances that might indicate either a young SNR or one involving a massive progenitor star.

In 1996 January, we obtained HST/WFPC2 images on the sub-arcsecond structure of this remarkable object that greatly clarify the nature of its extreme luminosity. These data show multiple and partially overlapping SNRs with a small, bright SNR shell in apparent interaction with a larger neigboring shell. The larger shell is oblong in shape (0.8\arcsec\ \times 1.2\arcsec, or 20 \times 30 pc at 5 Mpc) and consists of multiple arcs of emission in H\alpha and [S II] \lambda6725. The smaller shell is about 0.3\arcsec\ in diameter (\sim8 pc) and is much brighter on the side overlapping the larger shell. The [S II]:H\alpha ratio is high for the entire region, indicating that its optical emission arises from shocks. Roughly half of the total optical luminosity comes from the region of apparent interaction. Although potential ``multiple SNRs'' have been previously suggested, this object is perhaps the most striking and clearest example for the direct interaction of two different age supernova remnants.

This work was supported by STScI grant GO-06118.01-94A.

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