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 overluminous -19.6. The spectra show weak and narrow Si II lines. These profiles do not change in width or depth for 10 days beginning around maximum light, indicating a constant expansion velocity of approximate to 10,600 km s(-1). We analyzed the observations based on detailed radiation dynamical models. Delayed detonation and deflagration models that have been used to explain the majority of SNe Ia do not predict a long velocity plateau in the Si II minimum with an unvarying line profile. Pulsating delayed detonations and merger scenarios form shell-like density structures with properties mostly related to the mass of the shell, M(shell). We discuss how these models may explain the observed Si II line evolution; however, these models are based on spherical calculations and other possibilities may exist. SN 2005hj is consistent with respect to the onset, duration, and velocity of the plateau, the peak luminosity and, within the uncertainties, with the intrinsic colors for models with Mshell 0.2 M(circle dot). Our analysis suggests a distinct class of events hidden within the Branch-normal SNe Ia. The predicted relations between observables are confirmed, they may provide a way to separate these two groups. We discuss the implications for cosmological studies.