Hubble Space Telescope spectra of the type Ia supernova SN2011fe: a tail
  of low-density, high-velocity material with Z<Zsolar
  Hubble Space Telescope spectroscopic observations of the nearby type Ia
supernova (SN Ia) SN2011fe, taken on 10 epochs from -13.1 to +40.8 days
relative to B-band maximum light, and spanning the far-ultraviolet (UV) to the
near-infrared (IR) are presented. This spectroscopic coverage makes SN2011fe
the best-studied local SN Ia to date. SN2011fe is a typical moderately-luminous
SN Ia with no evidence for dust extinction. Its near-UV spectral properties are
representative of a larger sample of local events (Maguire et al. 2012). The
near-UV to optical spectra of SN2011fe are modelled with a Monte Carlo
radiative transfer code using the technique of 'abundance tomography',
constraining the density structure and the abundance stratification in the SN
ejecta. SN2011fe was a relatively weak explosion, with moderate Fe-group
yields. The density structures of the classical model W7 and of a delayed
detonation model were tested. Both have shortcomings. An ad-hoc density
distribution was developed which yields improved fits and is characterised by a
high-velocity tail, which is absent in W7. However, this tail contains less
mass than delayed detonation models. This improved model has a lower energy
than one-dimensional explosion models matching typical SNe Ia (e.g. W7, WDD1).
The derived Fe abundance in the outermost layer is consistent with the
metallicity at the SN explosion site in M101 (~0.5 Zsolar). The spectroscopic
rise time (~19 days) is significantly longer than that measured from the early
optical light curve, implying a 'dark phase' of ~1 day. A longer rise time has
significant implications when deducing the properties of the white dwarf and
binary system from the early photometric behaviour.
