N63A: A Supernova Remnant in a Cloudy Medium

N63A is a supernova remnant (SNR) associated with the OB association LH83 in the Large Magellanic Cloud. The progenitor of the supernova probably had a mass >= 30 Mo, the inferred mass of the most luminous star in LH83 (van den Bergh & Dufour 1980, PASP, 92, 32). The optical image of N63A shows a three-lobed, clover-shaped nebulosity about 20" (~ 5 pc) across. The two eastern lobes exhibit a high [S II]/H alpha ratio, indicating a shock excitation, while the western lobe shows spectra characteristic of photoionization. It has been puzzling that the optical size of the SNR N63A is much smaller than ~ 70", the extent of radio and X-ray emission. Our recent HST WFPC2 images of N63A resolved the filamentary structure in the two eastern lobes and diffuse emission in the western lobe, consistent with the excitation mechanisms diagnosed from their spectral properties. Additionally, the WFPC2 images reveal a number of cloudlets as small as 0.1 pc across, within the X-ray-emitting regions of the SNR. The [S II]/H alpha ratio and location of these cloudlets suggest that these are shocked cloudlets lagging behind the shock front. The morphology of the cloudlets indicates that they are evaporating isotropically; therefore, the magnetic field is either too weak to significantly prohibit evaporation across the field lines, or not organized enough to produce a noticeable directional dependence in the evaporation. The evaporating cloudlets are probably responsible for injecting mass into the hot SNR interior to produce the high X-ray surface brightness. Thus N63A provides a clear example of a SNR evolving in a cloudy medium, such as investigated by White and Long (1991, ApJ, 373, 543). This is also the first clear evidence of engulfed cloudlets within a SNR.