Flash Ionization of the Partially Ionized Wind of the Progenitor of SN 1987A

The H II region created by the progenitor of SN 1987A was further heated and ionized by the supernova flash. Prior to the flash, the temperature of the gas was ~4000-5000 K, and helium was neutral, while the postflash temperature was only slightly less than ~10⁵ K, the gas being ionized to helium-like ionization stages of C, N, and O. We have followed the slow postflash cooling and recombination of the gas, as well as its line emission, and find that the strongest lines should be N V λ1240 and O VI λ1034. Both of these lines are good probes for the density of the gas, and suitable instruments for detecting the lines are the space telescope imaging spectrographs (STIS's) on the Hubble Space Telescope (HST) and Far-Ultraviolet Spectroscopic Explorer (FUSE). Other lines that may be detectable are [N IV] λ1486 and [O III] λ5007, although they are expected to be substantially weaker. The relative strength of the oxygen lines is found to be a good tracer of the color temperature of the supernova flash. From previous observations, we put limits on the hydrogen density, n_H, of the H II region. The early N V λ1240 flux measured by IUE gives an upper limit of n_H~180 η^-0.40 cm^-3, where η is the filling factor of the gas. The recently reported emission in [O III] λ5007 at 2500 days requires n_H=(160+/-12)η^-0.19 cm^-3 for a supernova burst similar to that in the 500full1 model of Ensman & Burrows. For the more energetic 500full2 burst, the density is n_H=(215+/-15)η^-0.19 cm^-3. These values are much higher than in models of the X-ray emission from the supernova (n_H~75 cm^-3), and it seems plausible that the observed [O III] emission is produced primarily elsewhere than in the H II region. We also discuss the type of progenitor consistent with the H II region. In particular, it seems unlikely that its spectral type was much earlier than B2 Ia.