On the Nature of the Outer Rings around SN 1987A

Abstract

We present recent Hubble Space Telescope (HST) Faint Object Spectrograph and ground-based spectroscopic observations of SN 1987A and its surroundings and discuss them in conjunction with HST Faint Object Camera and Wide Field Planetary Camera 2 narrowband imaging. We have determined the following properties of the outer rings around SN 1987A: (1) the ionization and the temperature in the northern outer ring of SN 1987A are similar to those measured in the inner ring; (2) the [O II] electron density in the outer rings, both the northern and the southern, is ~800 cm-3, less than a third of that in the inner ring; (3) the N/O and N/C abundance ratios measured in the northern ring are ~3 times lower than in the inner ring, whereas the N/H abundance ratio is about half the value measured in the inner ring. Moreover, narrowband imaging indicates that the prevailing conditions in both the northern and the southern outer rings are quite similar to each other and to those measured in the portion of the northern outer ring that projects on the ejecta. These findings indicate that the gas in the outer rings consists of material that was ejected by the supernova progenitor at an earlier stage of evolution than the inner ring, when CNO processing was less advanced. Comparison with evolutionary models suggests that the material that constitutes the outer rings was ejected from the supernova progenitor ~104 yr before the bulk of the inner-ring material was ejected.