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C.H.R. Chen, Y.-H. Chu, S.D. Points (UIUC)
Bright X-ray emission has been detected in superbubbles in the Large Magellanic Cloud (LMC). As their X-ray luminosities are much higher than those expected in superbubble models by, e.g., Weaver et al. (1977), it is suggested that supernova remnants (SNRs) hitting the inner shell walls are responsible for the X-ray emission (Chu & Mac Low 1990). To identify SNR shocks in superbubble interiors, we have obtained HST WFPC2 images of the X-ray-bright superbubbles N44 and N51D in H\alpha and [S II] lines, and the X-ray-dim superbubble DEM106 in H\alpha. We expect to use the filamentary morphology and [S~II]/H\alpha ratio to identify SNR shocks.
Of these three superbubbles, N44 has the highest X-ray surface brightness. N44 also has the most filamentary nebular morphology, the largest expansion velocity (~45 km/s), and the highest [S II]/H\alpha ratio (0.4--0.6). The [S II]/H\alpha ratio increases outwards and peaks in sharp filaments along the periphery. N51D has a moderate X-ray surface brightness. N51D has a turbulent, but not filamentary, morphology, a moderate expansion velocity (35 km/s), and a low [S II]/H\alpha ratio (~0.15). DEM106 is not detected in X-rays. Its shell structure is amorphous and has embedded dusty features. Its expansion velocity is < 10 km/s.
In order to determine whether the morphological differences among N44, N51D, and DEM106 are statistically significant, we have examined all archival WFPC2 H\alpha images of LMC fields. We find a good correlation between X-ray surface brightness and sharp filamentary nebular morphology, suggesting that SNR shocks are responsible for the filamentary morphology. Therefore, high-resolution imaging is an effective way to identify SNR shocks. The [S II]/H\alpha ratio is not an effective indicator of SNR shocks near OB associations, unless its spatial variation is taken into account.
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