Hubble space telescope observations of oxygen-rich supernova remnants in the magellanic clouds. III. WFPC2 imaging of the young, Crab-like supernova remnant SNR 0540-69.3

HST images with WFPC2 of the young, oxygen-rich, Crab-like supernova remnant SNR 0540-69.3 in the Large Magellanic Cloud (LMC) reveal details of the emission distribution and the relationship between the expanding ejecta and synchrotron nebula. The emission distributions appear very similar to those seen in the Crab Nebula, with the ejecta located in a thin envelope surrounding the synchrotron nebula. The [O III] emission is more extended than other tracers, forming a faint "skin" around the denser filaments and synchrotron nebula, as also observed in the Crab. The [O III] exhibits somewhat different kinematic structure in long-slit spectra, including a more extended high-velocity emission halo not seen in images. Yet even the fastest expansion speeds in SNR 0540-69.3's halo are slow when compared to most other young supernova remnants, although the Crab Nebula has similar slow expansion speeds. We show a striking correspondence between the morphology of the synchrotron nebula observed in an optical continuum filter with that recently resolved in Chandra X-ray images. We argue that the multicomponent kinematics and filamentary morphology of the optical emission-line features likely result from magnetic Rayleigh-Taylor instabilities that form as the synchrotron nebula expands and sweeps up ejecta, as seen in the Crab Nebula. Our images and spectra help to refine our understanding of SNR 0540-69.3 in several more detailed respects: they confirm the identification of Hα + [N II] in the red spectrum, show that the systemic velocity of SNR 0540-69.3 is not significantly different from that of the LMC, and hint at a lower Ne abundance than the Crab (potentially indicating a more massive progenitor star).