Physical Conditions in Circumstellar Gas Surrounding SN 1987A 12 Years after Outburst

Two-dimensional spectra of SN 1987A were obtained on 1998 November 14-15 (4282 days after outburst) with the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope (HST). The slit sampled portions of the inner circumstellar ring at the east and west ansae as well as small sections of both the northern and southern outer rings. The temperature and density at these locations are estimated by nebular analysis of [N II], [O III], and [S II] emission-line ratios, and with time-dependent photoionization/recombination models. The results from these two methods are mutually consistent. The electron density in the inner ring is ~4000 cm-3 for S II, with progressively lower densities for N II and O III. The electron temperatures determined from [N II] and [O III] line ratios are ~11,000 and ~22,000 K, respectively. These results are consistent with evolutionary trends in the circumstellar gas from similar measurements at earlier epochs. We find that emission lines from the outer rings come from gas of lower density (ne ≲ 2000 cm-3) than that which emits the same line in the inner ring. The N/O ratio appears to be the same in all three rings. Our results also suggest that the CNO abundances in the northern outer ring are the same as in the inner ring, contrary to earlier results. Physical conditions in the southern outer ring are less certain because of poorer signal-to-noise data. The STIS spectra also reveal a weak Hα emission redshifted by ~100 km s-1 at P.A. 103° that coincides with the recently discovered new regions that are brightening. This indicates that the shock interaction in the southeast section of the inner ring commenced over a year before it became apparent in HST images.