摘要：Context.Observations of type Ia supernovae (SNe Ia) can be used to derive accurate cosmological distances through empirical standardization techniques. Despite this success neither the progenitors of SNe Ia nor the explosion process are fully understood. TheU-band region has been less well observed for nearby SNe, due to technical challenges, but is the most readily accessible band for high-redshift SNe.Aims.Using spectrophotometry from the Nearby Supernova Factory, we study the origin and extent ofU-band spectroscopic variations in SNe Ia and explore consequences for their standardization and the potential for providing new insights into the explosion process.Methods.We divide theU-band spectrum into four wavelength regionsλ(uNi),λ(uTi),λ(uSi) andλ(uCa). Two of these span the CaH&Kλλ 3934, 3969 complex. We employ spectral synthesis usingSYNAPPSto associate the two bluer regions with Ni/Co and Ti.Results.The flux of the uTi feature is an extremely sensitive temperature/luminosity indicator, standardizing the SN peak luminosity to 0.116 ± 0.011 mag root mean square (RMS). A traditionalSALT2.4fit on the same sample yields a 0.135 mag RMS. Standardization using uTi also reduces the difference in corrected magnitude between SNe originating from different host galaxy environments. EarlyU-band spectra can be used to probe the Ni+Co distribution in the ejecta, thus offering a rare window into the source of light curve power. The uCa flux further improves standardization, yielding a 0.086 ± 0.010 mag RMS without the need to include an additional intrinsic dispersion to reachχ2∕dof ~ 1. This reduction in RMS is partially driven by an improved standardization of Shallow Silicon and 91T-like SNe.
关键词：Key wordsensupernovae: generalcosmology: observationsdark energy
