Evolution and Geometry of Hot Spots in Supernova Remnant 1987A
  We present ground-based near-infrared imaging and {\em HST} optical imaging and spectroscopy of the interaction between the ejecta of SN 1987A and its equatorial circumstellar ring. This interaction has made a transition, from emission originating in just a few ``hot spots'' at restricted locations in position angle around the ring, to a collision producing optical emission over a nearly continuous distribution, with few breaks larger than 45 degrees. The centroids of the first three spots are measured to move at 2000-3000 km/s, which we interpret as a lower limit of the velocity of the forward blast front. Multi-wavelength light curves of the spots show that they do not evolve uniformly, and change significantly on timescales as short as one month; in particular the first spot shows a significant break in its lightcurve. Implications of observed delays between spots appearances are discussed, which leads to a generalized model of hot spot evolution, and suggests that the early appearance of the first hot spot is explained by its inward radial position and a fairly uniform forward blast wave, rather than extraordinary physical circumstances. Data further suggest that the forward blast is reaching the bulk of the inner ring material to the east, the density of which appears higher than elsewhere in the ring. We study the ring geometry, finding evidence suggestive of an intrinsic ellipticity of 0.95, and find lower and upper distance limits of 47.9 +/- 0.92 kpc and 54.4 +/- 2.1 kpc, respectively. 
Ben E. K. Sugerman, Stephen S. Lawrence, Arlin P. S. Crotts, Patrice   Bouchet, and Steve R. Heathcote