Hubble Space Telescope spectra of the type Ia supernova SN2011fe: a tail of low-density, high-velocity material with Z<Zsolar

TL;DR

This paper presents extensive Hubble Space Telescope spectra of SN2011fe, revealing detailed ejecta structure, a high-velocity tail, and insights into explosion energy, metallicity, and rise time, advancing understanding of Type Ia supernovae.

Contribution

It introduces a new density model with a high-velocity tail that better fits observations, providing improved insights into SN2011fe's explosion characteristics and ejecta composition.

Findings

SN2011fe has a longer spectroscopic rise time (~19 days) than optical light curve measurements.

A high-velocity tail in the density structure improves spectral fits compared to classical models.

The outer Fe abundance aligns with local metallicity, supporting progenitor environment studies.

Abstract

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.