Discovery of an inner light-echo ring around SN 1987A

CCD imagery obtained on 1989 January 24 reveals a third, inner light-echo ring around SN 1987A, in addition to the two previously known outer rings. Its mean radius is 8″.2, corresponding to a projected radius of 2.1 pc from the supernova; for an optical maximum in 1987 May, the light scattering occurs off interstellar dust located only 4.4 pc in front of SN 1987A. The width of the ring is 3′.0, so that the scattering material is distributed between radii 3.1 and 5.7 pc from the supernova. The ring is brightest on the eastern side, where the average surface brightnesses in B, V, and R are 23.3 ± 0.4, 22.1 ± 0.2, and 21.4 ± 0.3 mag arcsec^-2, respectively. These values imply an average column density of 2.2 ± 10¹⁸ cm^-2 and a local density of 1.7 cm^-3. The surface brightness in the western side is about 0.5 mag arcsec^-2 fainter, indicating that the scattering material is not uniformly distributed around the supernova. Following Chevalier and Emmering, we may attribute the inner light echo to a shell of red-supergiant wind material that has been decelerated by its interaction with the surrounding medium. Paresce and Burrows failed to detect the inner ring in the B band in 1987 December, and Burrows found only a stringent upper limit to the R surface brightness in 1988 September. These earlier null results imply that there is a cavity in the dust along the line of sight toward Earth, such that the shell material outside the cavity became illuminated only at the end of 1988. The cavity might have arisen if the ambient interstellar medium had an appreciable density gradient, such that SN 1987A is located just inside the near edge of an interstellar cloud. This situation would have allowed the dense wind from the red supergiant progenitor to penetrate to larger radii in the direction toward Earth.