We analyze the mean rest-frame ultraviolet (UV) spectrum of Type Ia Supernovae(SNe) and its dispersion using high signal-to-noise Keck-I/LRIS-B spectroscopyfor a sample of 36 events at intermediate redshift (z=0.5) discoveredby the Canada-France-Hawaii Telescope Supernova Legacy Survey (SNLS). Weintroduce a new method for removing host galaxy contamination in our spectra,exploiting the comprehensive photometric coverage of the SNLS SNe and theirhost galaxies, thereby providing the first quantitative view of the UV spectralproperties of a large sample of distant SNe Ia. Although the mean SN Ia spectrumhas not evolved significantly over the past 40 percent of cosmic history, preciseevolutionary constraints are limited by the absence of a comparable sample ofhigh quality local spectra. The mean UV spectrum of our z 0.5 SNe Ia and itsdispersion is tabulated for use in future applications. Within the high-redshiftsample, we discover significant UV spectral variations and exclude dust extinctionas the primary cause by examining trends with the optical SN color. Although progenitor metallicity may drive some of these trends, the variations we see aremuch larger than predicted in recent models and do not follow expected patterns.An interesting new result is a variation seen in the wavelength of selected UVfeatures with phase. We also demonstrate systematic differences in the SN Iaspectral features with SN lightcurve width in both the UV and the optical. Weshow that these intrinsic variations could represent a statistical limitation in thefuture use of high-redshift SNe Ia for precision cosmology. We conclude thatfurther detailed studies are needed, both locally and at moderate redshift wherethe rest-frame UV can be studied precisely, in order that future missions canconfidently be planned to fully exploit SNe Ia as cosmological probes.

We present an extensive new time series of spectroscopic data of the peculiar SN 1999aa in NGC 2595. Our data set includes 25 optical spectra between -11 and +58 days with respect to B-band maximum light, providing an unusually complete time history. The early spectra resemble those of an SN 1991T-like object but with a relatively strong Ca H and K absorption feature. The first clear sign of Si II lambda6355, characteristic of Type Ia supernovae, is found at day -7, and its velocity remains constant up to at least the first month after B-band maximum light. The transition to normal-looking spectra is found to occur earlier than in SN 1991T, suggesting SN 1999aa as a possible link between SN 1991T-like and Branch-normal supernovae. Comparing the observations with synthetic spectra, doubly ionized Fe, Si, and Ni are identified at early epochs. These are characteristic of SN 1991T-like objects. Furthermore, in the day -11 spectrum, evidence is found for an absorption feature that could be identified as high velocity C II lambda6580 or H alpha. At the same epoch C III lambda4648.8 at photospheric velocity is probably responsible for the absorption feature at 4500 8. High-velocity Ca is found around maximum light together with Si II and Fe II confined in a narrow velocity window. Implied constraints on supernovae progenitor systems and explosion hydrodynamic models are briefly discussed.