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HET Optical spectra covering the evolution from about 6 days before to about5 weeks after maximum light and the ROTSE-IIIb unfiltered light curve of the"Branch-normal" Type Ia Supernova SN 2005hj are presented. The host galaxyshows HII region lines at redshift of z=0.0574, which puts the peak unfilteredabsolute magnitude at a somewhat over-luminous -19.6. The spectra show weak andnarrow SiII lines, and for a period of at least 10 days beginning aroundmaximum light these profiles do not change in width or depth and they indicatea constant expansion velocity of ~10,600 km/s. We analyzed the observationsbased on detailed radiation dynamical models in the literature. Whereas delayeddetonation and deflagration models have been used to explain the majority ofSNe Ia, they do not predict a long velocity plateau in the SiII minimum with anunvarying line profile. Pulsating delayed detonations and merger scenarios formshell-like density structures with properties mostly related to the mass of theshell, M_shell, and we discuss how these models may explain the observed SiIIline evolution; however, these models are based on spherical calculations andother possibilities may exist. SN 2005hj is consistent with respect to theonset, duration, and velocity of the plateau, the peak luminosity and, withinthe uncertainties, with the intrinsic colors for models with M_shell=0.2 M_sun.Our analysis suggests a distinct class of events hidden within theBranch-normal SNe Ia. If the predicted relations between observables areconfirmed, they may provide a way to separate these two groups. We discuss theimplications of two distinct progenitor classes on cosmological studiesemploying SNe Ia, including possible differences in the peak luminosity tolight curve width relation.

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