We use low-dispersion spectra obtained at the Magellan Observatory to study the broad Ha emission from the reverse shock of the infant supernova remnant SNR 1987A. These spectra demonstrate that the spatiokinematic structure of the reverse shock can be distinguished from that of the circumstellar ring and hot spots, even at ground-based spatial resolution. We measure a total dereddened H alpha flux of 1.99(+/- 0.22) x 10-(13) ergs s(-1) cm(-2) at an epoch 18.00 yr after outburst. At 50 kpc, the total reverse shock luminosity in Ha is roughly 15, which implies a L, total flux of neutral hydrogen atoms across the reverse shock of 8.9 x 10(46) s(-1), or roughly 2.3 x 10(-3) M-circle dot yr(-1). This represents an increase by a factor of similar to 4 since 1997. Lyman continuum radiation from gas shocked by the forward blast wave can ionize neutral hydrogen atoms in the supernova debris before they reach the reverse shock. If the inward flux of ionizing photons exceeds the flux of hydrogen atoms approaching the reverse shock, this preionization will shut off the broad Ly alpha and H alpha emission. The observed X- ray emission of SNR 1987A implies that the ratio of ionizing flux to hydrogen atom flux across the reverse shock is presently at least 0.04. The X-ray emission is increasing much faster than the flux of atoms, and if these trends continue, we estimate that the broad Ly alpha and H alpha emission will vanish in similar to 7 yr.

We present new proper-motion measurements of optically emitting oxygen-rich knots of supernova remnant 1E 0102.2-7219 (E0102), which are used to estimate the remnant's center of expansion and age. Four epochs of high-resolution Hubble Space Telescope images spanning 19 yr were retrieved and analyzed. We found a robust center of expansion of alpha = 1(h)04(m)02.48 and delta = -72 degrees 01 ' 53.'' 92 (J2000) with 1 sigma uncertainty of 1.'' 77 using 45 knots from images obtained with the Advanced Camera for Surveys using the F475W filter in 2003 and 2013 having the highest signal-to-noise ratio. We also estimate an upper limit explosion age of 1738 +/- 175 yr by selecting the knots with the highest proper motions and these knots are assumed to be the least decelerated. We find evidence of an asymmetry in the proper motions of the knots as a function of position angle. We conclude that these asymmetries were most likely caused by interaction between E0102's original supernova blast wave and an inhomogeneous surrounding environment, as opposed to intrinsic explosion asymmetry. The observed nonhomologous expansion suggests that the use of a free expansion model inaccurately offsets the center of expansion and leads to an overestimated explosion age. We discuss our findings as they compare to previous age and center of expansion estimates of E0102, as well as their relevance to a recently identified candidate central compact object.