Faint Object Camera Observations of the Shape and Expansion of SN 1987A

Abstract

New visible and near-UV imagery of SN 1987A obtained 1754 days after outburst with the Faint Object Camera on-board the Hubble Space Telescope is presented. A comparison between these data and previous FOC observations obtained 1278 days after outburst reveal that the supernova dimmed by a factor 0.35 +/- 0.04 between the two observations, corresponding to an exponential decay time of 450 +/- 50 days. The SN 1987A envelope is resolved in all exposures and increased in size by a factor ~1.4 between day 1278 and day 1754 in a manner consistent with linear homologous expansion in time. Detailed fits to the image profiles indicate that the SN 1987A envelope is heavily limb-darkened and well described by a surface brightness profile that decreases with angular radius ρ as φ(ρ) is proportional to 1/[1+(ρ/ρ_s_)^alpha], where α ~ 5-6. The apparent size of the SN 1987A envelope also varies significantly with wavelength. On day 1754 the inferred diameter was 2{RHO}_S_~ 120 milliarcseconds (FWHM) at visible wavelengths, but 2{RHO}_S_ ~ 190 milliarcseconds in the near-UV. The data are consistent with the envelope of SN 1987A being axially symmetrical to within 50% at a high level of confidence. Marginal evidence for a ~20% elongation of the supernova image may, however, be present in the near-UV exposures.