Hubble Space Telescope spectra of the Type Ia supernova SN 2011fe: a tail of low-density, high-velocity material with Z < Z⊙

Abstract

Hubble Space Telescope spectroscopic observations of the nearby type Ia supernova (SN Ia) SN 2011fe, 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 SN 2011fe the best-studied local SN Ia to date. SN 2011fe 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 SN 2011fe 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. SN 2011fe was a relatively weak explosion, with moderate Fegroup 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, Iwamoto et al. 1999). The derived Fe abundance in the outermost layer is consistent with the metallicity at the SN explosion site in M101 (∼ 0.5Z ⊙ ). 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.

Spectrum formation in superluminous supernovae (Type I)

“…This has been described in several papers (Mazzali & Lucy 1993;Mazzali 2000) and has been used to model SNe of different spectral types (Mazzali et al 2013(Mazzali et al , 2014b. The code assumes that the SN luminosity is emitted at a lower boundary as a black body and transported through an expanding SN 'atmosphere' of variable density and composition.…”

“…The O II lines are no longer very strong, and lines of lower ionization species become visible. No rest-frame UV information is available, which limits our ability to constrain the metal content in the regions where the spectra form (see, e.g., Mazzali et al 2014b). …”

“…is in line with our estimates in SLSNe. So our models may be used to estimate the metallicity of the progenitor, as in the case of other SNe (Mazzali et al 2014b). …”

Constraining the progenitor of the Type Ia Supernova SN 2012cg

“…Here, instead of estimating the gas temperature by modelling the spectral energy distribution from the photometry and spectrum of SN 2012cg, we simply adopt a gas temperature of 15 000 K (or 20 000 K) in our calculation. By mod- elling the near-UV to optical spectra of SN 2011fe with a Monte Carlo radiative transfer code, Mazzali et al (2014) obtained that the photospheric temperature of the photospheric blackbody at -16 days is about 10 800 K. In addition, adding a blackbody emission with a temperature of about 10 000 K to the spectra of SN 2011fe at about -16 days, Marion et al (2016) derived that the resulting continua can fit that of SN 2012cg at a comparable phase (see their figure 9). Therefore, adopting a gas temperature of 15 000 K-20 000 K should be safe for our calculations.…”

Abundance stratification in Type Ia supernovae – IV. The luminous, peculiar SN 1991T

“…This is used to rescale the density profile to the epoch of each spectrum and has a major influence on the computed synthetic spectra, especially at the earliest times (e.g. Mazzali et al 2014). Explosion time is referred to the time of the B maximum for which we use the date found by Lira et al (1998), JD 2448375.7 ± 0.5.…”