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The inner circumstellar ring of SN1987A has been observed repeatedly between March 92 and April 93 with the Faint Object Spectrograph of $HST$. The observations have been obtained as a part of the $HST$ Supernovae INtensive Study (SINS) GO program.
The line intensities were corrected for the reddening using $E(B-V)= 0.19$ and a combination of the galactic and the 30 Dor extinction laws.
To determine the physical parameters of the ring, we used all classical line ratios which are sensitive to the electron density and/or temperature.
We determined the ionic abundances using standard formulae and adopting the [NII] temperature for the lines of H, HeI, NI, NII, OI, OII, SiIII and SII, the [OIII] temperature for the lines of HeII, NIII, OIII, NeIII, NeIV and NeV. For the CIII] lines we adopted a temperature that is the geometric mean of the [NIII] and [OIII] temperatures.
Our results can be summarized as follows: 1) Conspicuous density fluctuations are revealed in the ring, at least in the range $10^3 - 3\times 10^4~cm^{-3}$. 2) The ring appears to be cooling after an initial heating and ionization produced by a UV flash. 3) About 20\% of the emitting material is neutral, suggesting that the ring is ionization bounded and/or has recombined substantially. 4) The ring consists of material enriched by low temperature CNO cycle products. 5) Its composition is similar to that of the surface layers of massive stars with mass $M~>~20~M_\odot$ at the end of their red supergiant phase (Schaller et al. 1992). 6) Its total metal content by mass is Z = 0.003 which is a factor of 6 lower than cosmic, and is typical for the LMC.
