ALBERT

Description: We present new Hubble Space Telescope images of high-velocity H-alpha and Lyman-alpha emission in the outer debris of SN 1987A. The H-alpha images are dominated by emission from hydrogen atoms crossing the reverse shock. For the first time we observe emission from the reverse shock surface well above and below the equatorial ring, suggesting a bipolar or conical structure perpendicular to the ring plane. Using the H-alpha imaging, we measure the mass flux of hydrogen atoms crossing the reverse shock front, in the velocity intervals (7,500 〈 V(sub obs) 〈 2,800 km/s) and (1,000 〈 V(sub obs) 〈 7,500 km/s), M(sub H) = 1.2 10(exp 3) M/ y. We also present the first Lyman-alpha imaging of the whole remnant and new Chandra X-ray observations. Comparing the spatial distribution of the Lyman-alpha and X-ray emission, we observe that the majority of the high-velocity Lyman-alpha emission originates interior to the equatorial ring. The observed Lyman-alpha/H-alpha photon ratio, R(L-alpha/H-alpha) approx. = 17, is significantly higher than the theoretically predicted ratio of approx. = 5 for neutral atoms crossing the reverse shock front. We attribute this excess to Lyman-alpha emission produced by X-ray heating of the outer debris. The spatial orientation of the Lyman-alpha and X-ray emission suggests that X-ray heating of the outer debris is the dominant Lyman-alpha production mechanism in SN 1987A at this phase in its evolution.

Description: Multiwavelength observations of supernova remnant (SNR) 1987A show that its morphology and luminosity are rapidly changing at X-ray, optical, infrared, and radio wavelengths as the blast wave from the explosion expands into the circumstellar equatorial ring, produced by mass loss from the progenitor star. The observed infrared (IR) radiation arises from the interaction of dust grains that formed in mass outflow with the soft X-ray emitting plasma component of the shocked gas. Spitzer IRS spectra at 5 - 30 microns taken on day 6190 since the explosion show that the emission arises from approx. 1.1 x 10(exp -6) solar mass of silicate grains radiating at a temperature of approx. 180+/-(15-20) K. Subsequent observations on day 7137 show that the IR flux had increased by a factor of 2 while maintaining an almost identical spectral shape. The observed IR-to-X-ray flux ratio (IRX) is consistent with that of a dusty plasma with standard LMC dust abundances. This flux ratio has decreased by a factor of approx. 2 between days 6190 and 7137, providing the first direct observation of the ongoing destruction of dust in an expanding SN blast wave on dynamic time scales. Detailed models consistent with the observed dust temperature, the ionization fluence of the soft X-ray emission component, and the evolution of IRX suggest that the radiating si1icate grains are immersed in a 3.5 x 10(exp 6) K plasma with a density of (0.3 - 1) x 10(exp 4)/cu cm, and have a size distribution that is confined to a narrow range of radii between 0.02 and 0.2 microns. Smaller grains may have been evaporated by the initial UV flash from the supernova.

Description: During the first several weeks after the explosion of SN 1987A, Fe/Co/Ni clumps, containing approx. 1% of the mass of the supernova envelope, absorbed most of the energy release by Ni-56 and Co-56 decay. As a result, the clumps expanded relative to the substrate, forming a 'nickel bubble' of low-density Fe/Co/Ni. Later the clumps captured approx. 10% of the radioactive luminosity of gamma rays and positrons. Assuming that these elements are not mixed microscopically with other elements, we find that the clumps must occupy approx. greater than 30% of the volume of the emitting region (radial velocity approx. less than 2500 km/s). The result indicates that the emission at late times is dominated by an extra source of heating and ionization, most likely photoionization by two-photon continuum from metastable helium in the gas surrounding the clumps. The resulting 'frothy' structure, consisting of bubbles of low-density Fe surrounded by higher-density filaments of H, He, and other elements, will persist and may be seen in the spectra and structure of supernova remnants.

Description: Following is final report on the study "The Impact of SN1987A with its Circumstellar Ring", which is now complete. In 1994, it was predicted that the blast wave from SN1987A would strike the circumstellar ring in AD1999+/-3, and that the ring would brighten by several hundreds optically as the shock entered the ring. It was also predicted that the emission lines from the shocked ring would have linewidths of a few hundred km/s. Today, we see a "hot spot" on the ring that first appeared in Hubble Space Telescope (HST) images in 1995 and has doubled in brightness between August 1997 and February 1998. Moreover, spectra from STIS show that the emission lines from the hot spot have widths and blue shifts of order 200 km/s, just as we predicted in 1994. Our guess of 1999+/-3 for the impact time was lucky, because we assumed that the hot gas in the bubble between the supernova and the ring had a low density (approximately 10 cm(exp -3)). But it was pointed out that the ROSAT observations of soft X-rays from SN 1987A implied that the intervening gas had a higher density, (approximately 100 cm(exp -3)), which would delay the impact until ca. 2007. Others developed hydrodynamic models to fit the ROSAT X-ray emission spectrum and came to the same conclusion. But these models were oversimplified in that they assumed that the ring was perfectly round. Now we see clearly that the hot spot is a peninsula that protrudes inward from the ring -- the first spot on the ring to be struck by the blast wave. No doubt there are other protrusions on the ring, which we may expect to light up in the next few years until they finally merge to set the entire ring ablaze, probably within 5 - 10 years. It was also predicted that the X-ray emission must be accompanied by optical and ultraviolet emission from atoms in the supernova debris and in the circumstellar gas that cross the reverse shock and the blast wave, respectively, and that the Lyman-alpha and NV emission lines should be bright enough to see with the STIS.

Description: By the time the expanding envelope of a Type 2 supernova becomes transparent in the optical continuum, most of the gamma-ray luminosity produced by radioactive Fe/Co/Ni clumps propagates into the hydrogen/helium envelope and is deposited there, if at all. The resulting fast electrons excite He 1 and H 1, the two- photon continua of which are the dominant internal sources of ultraviolet radiation. The UV radiation is blocked by scattering in thousands of resonance lines of metals and converted by fluorescence into optical and infrared emission lines that escape freely. We describe results of Monte Carlo calculations that simulate non-LTE scattering and fluorescence in more than five million allowed lines of Ca, Sc, Ti, V, Cr, Mn, Fe, Co, and Ni. For a model approximating conditions in the envelope of SN 1987A, the calculated emergent spectrum resembles the observed one. For the first 2 yr after explosion, the ultraviolet radiation (lambda less than or approximately equals 3000) is largely blocked and converted into a quasi continuum of many thousands of weak optical and infrared emission lines and some prominent emission features, such as the Ca 2 lambdalambda8600 triplet. Later, as the envelope cools and expands, it becomes more transparent, and an increasing fraction of the luminosity emerges in the UV band.