SN 1992A: Ultraviolet and Optical Studies Based on HST, IUE, and CTIO Observations

The Type Ia supernova SN 1992A in the SO galaxy NGC 1380 was observed as a target of opportunity by the International Ultraviolet Explorer (IUE) and with great alacrity by the Hubble Space Telescope (HST). Here we present the HST and IUE spectra and photometry that we obtained, as well as optical spectra obtained at the Cerro Tololo Inter-American Observatory (CTIO). The HST Faint Object Spectrograph (FOS) spectra, from 5 and 45 days past maximum light, are the best UV spectra of a Type Ia supernova and reveal for the first time with good signal-to-noise ratio the Type Ia spectral region blueward of ~2650 Å. The UV photometry taken in the F175W, F275W, and F342W bands defined by the HST filters shows light curves that resemble the Type Ia template U light curve. Using data from SN 1992A and SN 1990N, we have constructed a Type Ia template light curve for the flux region near 2750 Å that is quite detailed from 14 days before maximum light to 22 days after maximum light and that extends to 77 days after maximum light. This light curve also resembles the template U light curve. A high-resolution HST Goddard High Resolution Spectrograph (GHRS) spectrum of SN 1992A shows no evidence for any interstellar !ines attributable to NGC 1380 or to Galactic high-velocity clouds; low-velocity Galactic gas gives rise to relatively weak Mg 11 UV resonance absorption !ines. These results are consistent with low extinction along the line of sight to SN 1992A. We have done a parameterized LTE analysis of the SN 1992A spectra using a slightly modified version of Woosley's delayed-detonation model DD4. We find that the features in the region blueward of ~2650 Å in the HST spectra are P Cygni absorptions due to blends of iron peak element multiplets and the Mg II resonance multiplet. Newly synthesized magnesium, sulfur, and silicon probably extend to velocities at least as high as ~19,000 km s-ˡ. Newly synthesized Ni-Co may dominate the iron peak element abundances out to ~13,000 km s-ˡ in the ejecta of SN 1992A as in model DD4. However, further investigation of the extent of Ni-Co material in SN 1992A and other Type Ia supernovae is needed; this investigation could greatly help constrain Type Ia models. An analysis of the O I λ27773 line in SN 1992A and other Type Ia supernovae implies that the oxygen-rich layer in typical Type Ia's extends over a velocity range of at least ~11,000-19,000 km s-ˡ. None of the explosion models we considered, including model DD4, has an oxygen-rich layer that completely extends over this range. Model DD4, however, is promising, and further investigation of delayed-detonation models of its kind is merited. Spectropolarimetric observations taken by Spyromilio & Bailey of SN 1992A are consistent with the SN 1992A being spherically symmetric. We discuss these observations and the importance of taking spectropolarimetry of other Type Ia supernovae