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

Two-dimensional spectra of Supernova 1987A were obtained on 1998 November14-15 (4282 days after outburst) with the Space Telescope Imaging Spectrograph(STIS) on board the Hubble Space Telescope (HST). The slit sampled portions ofthe inner circumstellar ring at the east and west ansae as well as smallsections of both the northern and southern outer rings. The temperature anddensity at these locations are estimated by nebular analysis of [N II], [OIII], and [S II] emission line ratios, and with time-dependentphotoionization/recombination models. The results from these two methods aremutually consistent. The electron density in the inner ring is ~ 4000 cm-3 forS II, with progressively lower densities for N II and O III. The electrontemperatures determined from [N II] and [O III] line ratios are ~11,000 K and\~22,000 K, respectively. These results are consistent with evolutionary trendsin the circumstellar gas from similar measurements at earlier epochs. We findthat emission lines from the outer rings come from gas of lower density (n_e\la 2000 cm-3) than that which emits the same line in the inner ring. The N/Oratio appears to be the same in all three rings. Our results also suggest thatthe CNO abundances in the northern outer ring are the same as in the innerring, contrary to earlier results of Panagia et al. (1996). Physical conditionsin the southern outer ring are less certain because of poorer signal-to-noisedata. The STIS spectra also reveal a weak Ha emission redshifted by ~100 km s-1at p.a. 103\arcdeg that coincides with the recently discovered new regions thatare brightening (Lawrence et al. 2000). This indicates that the shockinteraction in the SE section of the inner ring commenced over a year before itbecame apparent in HST images.Comment: 25 pages, 6 figures, to appear in December 1, 2000 Astrophysical Journa