ABUNDANCES AND DENSITY STRUCTURE OF THE INNER CIRCUMSTELLAR RING AROUND SN 1987A

Abstract

We present optical spectroscopic data of the inner circumstellar ring around supernova (SN) 1987A from the Anglo-Australian Telescope and the Very Large Telescope (VLT) between similar to 1400 and similar to 5000 days post-explosion. We also assembled the available optical and near-infrared line fluxes from the literature between similar to 300 and similar to 2000 days. These line light curves were fitted with a photoionization model to determine the density structure and the elemental abundances for the inner ring. We found densities ranging from 1 x 10(3) to 3 x 10(4) atoms cm(-3) and a total mass of the ionized gas of similar to 5.8 x 10(-2) M(circle dot) within the inner ring. Abundances inferred from the optical and near-infrared data were also complemented with estimates of Lundqvist & Fransson based on ultraviolet lines. This way we found an He/H ratio (by number of atoms) of 0.17 +/- 0.06 which is roughly 30% lower than previously estimated and twice the solar and the Large Magellanic Cloud (LMC) value. We found an N/O ratio of 1.5 +/- 0.7, and the total (C+N+O)/(H+He) abundance about 1.6 times its LMC value or roughly 0.6 times the most recent solar value. An iron abundance of 0.20 +/- 0.11 times solar was found which is within the range of the estimates for the LMC. We also present late time (similar to 5000-7500 days) line light curves of [O III], [Ne III], [Ne IV], [Ar III], [Ar IV], and [Fe VII] from observations with the VLT. We compared these with model fluxes and found that an additional 10(2) atoms cm(-3) component was required to explain the data of the highest ionization lines. Such low-density gas is expected in the H II-region interior to the inner ring which likely extends also to larger radii at higher latitudes (out of the ring plane). At epochs later than similar to 5000 days, our models underproduce the emission of most of these lines as expected due to the contribution from the interaction of the SN ejecta with the ring.