HST Observations of the Ring around SN 1987A

Hubble Space Telescope observations of SN 1987A show that the circumstellar material near the supernova forms a clumpy elliptical ring. We analyze six epochs of images made with the Faint Object Camera imaging the lines of [O III] and Hβ over the interval 1990 August-1993 October, and three epochs of observations with the Planetary Camera in the continuum and [N II] line over the interval 1992 April-1993 May. The elliptical ring in the [O III] images has a semimajor axis of 858 +/- 11 milliarcsec and a semiminor axis of 621 +/- 11 milliarcsec (6.4 x 10^17^ and 4.6 x 10^17^ +/- 8.1 x 10^15^ cm, respectively, for a distance of 50 kpc), which implies an inclination angle of 44^deg^ +/- 1^deg^, if the ring is a circle inclined to our line of sight. The apparent width of the ring in [O III] is 121 +/- 22 milliarcsec (9.0 x 10^16^ +/- 1.6 x 10^16^ cm) based on our deconvolved image, which represents an upper limit to the width of the ring. The position angle of the major axis on the sky is 89^deg^ +/- 3^deg^. One of the "clumps" on the ring is a superposed star, whose energy distribution is consistent with an A-type dwarf (V = 20.2 mag) in the Large Magellanic Cloud. The radial profile of the ring is not symmetric, with the inner boundary sharper than the outer. Based on the radial profiles and the flux interior to the ring, we concluded that the ring is indeed a tilted ring, and not a spheroidal shell. While the observed width of the ring is ~10^17^ cm, we find that the average path length through the emitting gas is ~4 x 10^15^ cm, so the filling factor is small. The ring has faded by a factor of ~3.7 in [O III] from 1990 August to 1993 October. The characteristic electron density from the [O III] λ5007 emission line averages 1.2 x 10^4^ cm^-3^, based on recombination models of the emission. Because the fading rate is uncorrelated with the surface brightness, the density appears to be independent of the brightness, which implies significant path length differences through the ring. By assuming the current structure observed in optical lines applies to the early structure in ultraviolet lines, we construct light curve models to compare with early IUE observations. The clumpy structure of the ring has relatively little effect on the light curves. The [O III] images show faint arclike nebulosity probably associated with the bipolar nebula around SN 1987A and projected inside the ring, which may have affected previous observations of the ring. Finally, we find that the fading rate provides evidence, though weak, favoring an expanding ring over a contracting one.