An Intermediate Type Ia Supernova Between Normal And Super-Chandrasekhar

TL;DR

This paper reports on a peculiar Type Ia supernova, iPTF13asv, exhibiting features intermediate between normal and super-Chandrasekhar supernovae, with stratified ejecta and implications for its progenitor system.

Contribution

It provides detailed observations and analysis of iPTF13asv, revealing its unique intermediate properties and suggesting a double-degenerate origin for this supernova.

Findings

Stratified ejecta with confined iron in low-velocity regions.

Large ejecta mass (~1.59 M_sun) indicating a possible double-degenerate origin.

Persistent carbon absorption and constant Si II velocities.

Abstract

In this paper, we report observations of a peculiar Type Ia supernova iPTF13asv (a.k.a., SN2013cv) from the onset of the explosion to months after its peak. The early-phase spectra of iPTF13asv show absence of iron absorption, indicating that synthesized iron elements are confined to low-velocity regions of the ejecta, which, in turn, implies a stratified ejecta structure along the line of sight. Our analysis of iPTF13asv's light curves and spectra shows that it is an intermediate case between normal and super-Chandrasekhar events. On the one hand, its light curve shape (B-band $\Delta m_{15}=1.03\pm0.01$) and overall spectral features resemble those of normal Type Ia supernovae. On the other hand, similar to super-Chandrasekhar events, it shows large peak optical and UV luminosity ($M_B=-19.84\,\rm{mag}$, $M_{uvm2}=-15.5\,\rm{mag}$) a relatively low but almost constant \ion{Si}{2} velocities of about $10,000\,\rm{km}\,\rm{s}^{-1}$, and persistent carbon absorption in the spectra. We estimate a $^{56}$Ni mass of $0.81^{+0.10}_{-0.18}M_\odot$ and a total ejecta mass of $1.59^{+0.45}_{-0.12}M_\odot$. The large ejecta mass of iPTF13asv and its stratified ejecta structure together seemingly favor a double-degenerate origin.