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

TL;DR

This study presents detailed early-time optical spectra of SN 2011fe, revealing the distribution of unburned and burned material in the ejecta, and providing insights into the supernova's explosion mechanism and progenitor system.

Contribution

It offers the first extensive high-cadence spectral dataset of SN 2011fe's early evolution, highlighting the coexistence of unburned oxygen and burned material in the outer layers.

Findings

Detection of C II features diminishing near maximum light.

Presence of unburned oxygen at high velocities in the ejecta.

Support for incomplete burning as a source of spectral diversity.

Abstract

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.