2010ApJ...717.1140M

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 ∼1400 and ∼5000 days post-explosion. We also assembled the available optical and near-infrared line fluxes from the literature between ∼300 and ∼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 1x10³ to 3x10⁴ atoms/cm3 and a total mass of the ionized gas of ∼5.8x10^–2 M_☉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 (∼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 10² atoms/cm3 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 ∼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.