Hubble Space Telescope Studies of Nearby Type Ia Supernovae: The Mean
  Maximum Light Ultraviolet Spectrum and its Dispersion
  We present the first results of an ongoing campaign using the STIS
spectrograph on-board the Hubble Space Telescope (HST) whose primary goal is
the study of near ultraviolet (UV) spectra of local Type Ia supernovae (SNe
Ia). Using events identified by the Palomar Transient Factory and subsequently
verified by ground-based spectroscopy, we demonstrate the ability to locate and
classify SNe Ia as early as 16 days prior to maximum light. This enables us to
trigger HST in a non-disruptive mode to obtain near UV spectra within a few
days of maximum light for comparison with earlier equivalent ground-based
spectroscopic campaigns conducted at intermediate redshifts, z ~ 0.5. We
analyze the spectra of 12 Type Ia supernovae located in the Hubble flow with
0.01 < z < 0.08. Although a fraction of our eventual sample, these data,
together with archival data, already provide a substantial advance over that
previously available. Restricting samples to those of similar phase and
stretch, the mean UV spectrum agrees reasonably closely with that at
intermediate redshift, although some differences are found in the metallic
absorption features. A larger sample will determine whether these differences
reflect possible sample biases or are a genuine evolutionary effect.
Significantly, the wavelength-dependent dispersion, which is larger in the UV,
follows similar trends to that observed at intermediate redshift and is driven,
in part, by differences in the various metallic features. While the origin of
the UV dispersion remains uncertain, our comparison suggests that it may
reflect compositional variations amongst our sample rather than being
predominantly an evolutionary effect.
