Analysis of the Early-time Optical Spectra of SN 2011fe in M101


The nearby Type Ia supernova (SN Ia) SN 2011fe in M101 (cz = 241 km s$^-1$) provides a unique opportunity to study the early evolution of a ``normal’’ SN Ia, 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 łambda7774 absorption features detected within 5 days post-explosion indicates 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.

Astrophysical Journal Letters