SN 2005hj: Evidence for Two Classes of Normal‐Bright SNe Ia and Implications for Cosmology

HET Optical spectra covering the evolution from about 6 days before to about 5 weeks after maximum light and the ROTSE-IIIb unfiltered light curve of the “Branch-normal ” Type Ia Supernova SN 2005hj are presented. The host galaxy shows H II region lines at redshift of z = 0.0574, which puts the peak unfiltered absolute magnitude at a somewhat over-luminous −19.6. The spectra show weak and narrow Si II lines, and for a period of at least 10 days beginning around maximum light these profiles do not change in width or depth and they indicate a constant expansion velocity of ≈ 10, 600 km s −1. Our observations indicate that Si is confined to a relatively narrow velocity region. We analyzed the observations based on detailed radiation dynamical models in the literature. The models considered include delayed detonations, deflagrations, pulsating delayed detonations, and tamped detonation/merger scenarios. Whereas the first two classes of models have been used to explain the majority of SNe Ia, they do not predict a long velocity plateau in the Si II minimum with an unvarying