Luminosity Indicators in the Ultraviolet Spectra of Type Ia Supernovae

We present a complete sample of International Ultraviolet Explorer and Hubble Space Telescope ultraviolet (UV) spectra of Type Ia supernovae (SNe Ia) through 2004. We measure the equivalent width (EW) and blueshifted velocity of the minimum of the one strong UV feature, Fe II λ3250. We also quantify the slope of the near-UV spectra using a new parameter, the “UV ratio.” We find that the velocity of the Fe II line does not correlate with light-curve shape, while the EW shows distinct behavior for the slow and fast-declining objects. Using precise Cepheid and surface brightness fluctuation distance measurements of 6 objects with UV spectra observed near maximum light (a total of 12 spectra), we determine that the UV ratio at maximum light is highly correlated with SN Ia luminosity. A larger sample of UV spectra is necessary to determine the validity of these luminosity indicators and whether they can be combined with light-curve shape to improve measured SN Ia distances. Subject headings: supernovae — general, individual (SNe 1980N, 1981B, 1982B, 1983G, 1986G, 1989B, 1989M, 1990M, 1990N, 1991T, 1992A, 2001ay, 2001ba, 2001eh, 2001el, 2001ep, 2001ex, 2003bf, 2003bt), cosmology — observations, distance scale