Hubble Space Telescope wide field planetary camera 2 imaging of shocks in superbubbles

Bright X-ray emission has been detected in superbubbles in the Large Magellanic Cloud (LMC), and it is suggested that supernova remnants (SNRs) near the inner-shell walls are responsible for this X-ray emission. To identify SNR shocks in superbubble interiors, we have obtained Hubble Space Telescope Wide Field Planetary Camera 2 emission-line images of the X-ray-bright superbubbles DEM L152 and DEM L192 and the X-ray-dim superbubble DEM L106. We use these images to examine the shell morphology and [S II]/H alpha ratio variations in detail. Of these three superbubbles, DEM L152 has the highest X-ray surface brightness, the most filamentary nebular morphology, the largest expansion velocity (similar to 40 km s(-1)), and the highest [S II]/H alpha ratio (0.4-0.6). Its [S II]/H alpha ratio increases outward and peaks in sharp filaments along the periphery. DEM L192 has a moderate X-ray surface brightness, a complex but not filamentary morphology, a moderate expansion velocity (35 km s(-1)), and a low [S II]/H alpha ratio (similar to 0.15). DEM L106 is not detected in X-rays. Its shell structure is amorphous and has embedded dusty features; its expansion velocity is less than 10 km s(-1). None of the three superbubbles show morphological features in the shell interior that can be identified as directly associated with SNR shocks, indicating that the SNR shocks have not encountered very dense material. We find that the [S II]/H alpha ratios of X-ray-bright superbubbles are strongly dependent on the UV radiation field of the encompassed OB associations. Therefore, a tight correlation between [S II]/H alpha. ratio and X-ray surface brightness in superbubbles should not exist. We also find that the filamentary morphologies of superbubbles are associated with large expansion velocities and bright X-ray emission.