SN-1992A - ULTRAVIOLET AND OPTICAL STUDIES BASED ON HST, IUE, AND CTIO OBSERVATIONS

The Type Ia supernova SN 1992A in the S0 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, a s well as optical spectra obtained at the Cerro Tololo Inter-American Observatory (CTIO). The HST Faint Object Spectrograph (FOS) spectra, f rom 5 and 45 days past maximum light, are the best UV spectra of a Typ e Ia supernova and reveal for the first time with good signal-to-noise ratio the Type Ia spectral region blueward of approximately 2650 angs trom. The UV photometry taken in the F175W, F275W, and F342W bands def ined by the HST filters shows light curves that resemble the Type Ia t emplate 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 27 50 angstrom that is quite detailed from 14 days before maximum light t o 22 days after maximum light and that extends to 77 days after maximu m light. This light curve also resembles the template U light curve. A high-resolution HST Goddard High Resolution Spectrograph (GHRS) spect rum of SN 1992A shows no evidence for any interstellar lines attributa ble to NGC 1380 or to Galactic high-velocity clouds; low-velocity Gala ctic gas gives rise to relatively weak Mg II UV resonance absorption l 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 delay ed-detonation model DD4. We find that the features in the region bluew ard of approximately 2650 angstrom in the HST spectra are P Cygni abso rptions due to blends of iron peak element multiplets and the Mg II re sonance multiplet. Newly synthesized magnesium, sulfur, and silicon pr obably extend to velocities at least as high as approximately 19,000 k m s-1. Newly synthesized Ni-Co may dominate the iron peak element abun dances out to approximately 13,000 km s-1 in the ejecta of SN 1992A as in model DD4. However, further investigation of the extent of Ni-Co m aterial in SN 1992A and other Type Ia supernovae is needed; this inves tigation could greatly help constrain Type Ia models. An analysis of t he O I lambda7773 line in SN 1992A and other Type Ia supernovae implie s that the oxygen-rich layer in typical Type Ia's extends over a veloc ity range of at least approximately 11,000-19,000 km s-1. None of the explosion models we considered, including model DD4, has an oxygen-ric h layer that completely extends over this range. Model DD4, however, i s promising, and further investigation of delayed-detonation models of its kind is merited. Spectropolarimetric observations taken by Spyrom ilio & Bailey of SN 1992A are consistent with the SN 1992A being spher ically symmetric. We discuss these observations and the importance of taking spectropolarimetry of other Type Ia supernovae.