Analysis of the Early-Time Optical Spectra of SN 2011fe in M101
  The nearby Type Ia supernova SN 2011fe in M101 (cz=241 km s^-1) provides a
unique opportunity to study the early evolution of a "normal" Type Ia
supernova, its compositional structure, and its elusive progenitor system. We
present 18 high signal-to-noise spectra of SN 2011fe during its first month
beginning 1.2 days post-explosion and with an average cadence of 1.8 days. This
gives a clear picture of how various line-forming species are distributed
within the outer layers of the ejecta, including that of unburned material
(C+O). We follow the evolution of C II absorption features until they diminish
near maximum light, showing overlapping regions of burned and unburned material
between ejection velocities of 10,000 and 16,000 km s^-1. This supports the
notion that incomplete burning, in addition to progenitor scenarios, is a
relevant source of spectroscopic diversity among SNe Ia. The observed evolution
of the highly Doppler-shifted O I 7774 absorption features detected within five
days post-explosion indicate the presence of O I with expansion velocities from
11,500 to 21,000 km s^-1. The fact that some O I is present above C II suggests
that SN 2011fe may have had an appreciable amount of unburned oxygen within the
outer layers of the ejecta.
