The nebular spectra of the transitional Type Ia Supernovae 2007on and 2011iv: broad, multiple components indicate aspherical explosion cores

Abstract

The nebular-epoch spectrum of the rapidly declining, "transitional" type Ia supernova (SN) 2007on showed double emission peaks, which have been interpreted as indicating that the SN was the result of the direct collision of two white dwarfs. The spectrum can be reproduced using two distinct emission components, one red-shifted and one blue-shifted. These components are similar in mass but have slightly different degrees of ionization. They recede from one another at a line-of-sight speed larger than the sum of the combined expansion velocities of their emitting cores, thereby acting as two independent nebulae. While this configuration appears to be consistent with the scenario of two white dwarfs colliding, it may also indicate an off-centre delayed detonation explosion of a near Chandrasekhar-mass white dwarf. In either case, broad emission line widths and a rapidly evolving light curve can be expected for the bolometric luminosity of the SN. This is the case for both SNe 2007on and 2011iv, also a transitional SN Ia which exploded in the same elliptical galaxy, NGC 1404. Although SN 2011iv does not show double-peaked emission line profiles, the width of its emission lines is such that a two-component model yields somewhat better results than a single-component model. Most of the mass ejected is in one component, however, which suggests that SN 2011iv was the result of the off-centre ignition of a Chandrasekhar-mass white dwarf.

Late-Onset Circumstellar Medium Interactions are Rare: An Unbiased GALEX View of Type Ia Supernovae

“…None of the four near-peak SNe Ia are candidates for CSM interaction. SN 2007on has been identified as a "transitional" SN Ia, in between the SN 1991bg and normal classes of SNe Ia (Gall et al 2018), and it shows no signatures of CSM in its nebular spectrum (Mazzali et al 2018;. We also report seven NUV non-detections for SN 2007on between t disc + 724 days and t disc + 753 days, where we constrain the NUV luminosity to < 1.28 × 10 36 erg s −1 A −1 .…”

Understanding nebular spectra of Type Ia supernovae

“…The authors claim that SN2011fe requires the innermost ejecta to be dominated by stable iron, which aids in cooling instead of heating (via radioactive decay) and rules out a low mass (∼1.02 M ) WD progenitor. Mazzali et al (2018) also used abundance tomography to model the fast declining SNe Ia, SN2007bo and SN2011iv. By analyzing emission components of many [Fe II] and [Fe III] features, the authors reproduce the spectra by using a two component emission model (one blueshifted and one redshifted), which acts like two distinct nebulae.…”

Evidence for a Chandrasekhar-mass explosion in the Ca-strong 1991bg-like type Ia supernova 2016hnk

“…Within these progenitor scenarios four classes of explosion scenarios are most favored: (1) the dynamical merging of two C-O WDs in a binary system after losing angular momentum via gravitational radiation, in which the thermonuclear explosion is triggered by the heat of the merging process (García-Berro & Lorén-Aguilar 2017); (2) the explosion of a C-O WD with a mass close to M Ch triggered by compressional heating near the WD center, caused by accretion from the donor star, which may be a red giant, a main sequence star of mass less than 7−8 M , a He star, or the tidally disrupted WD from a DD system (Piersanti et al 2004); (3) the explosion of a sub-M Ch C-O WD triggered by detonating a thin surface He layer on the WD (double detonations; Kromer et al 2010;Shen & Moore 2014); and (4) direct collision of two WDs in tertiary systems, where the third star induces oscillations in the eccentricity of the other two increasing gravitational-wave losses and finishing in a head-on collision (e.g., Thompson 2011;Mazzali et al 2018). The debate about dynamical mergers, M Ch explosions, and double detonations is ongoing, but all channels may contribute to the SNe Ia population.…”