An Intermediate Type Ia Supernova Between Normal And Super-Chandrasekhar

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 $\hat{I}”m_{15}=1.03\pm 0.01$) 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 ($M_B=-19.84\mathrm{mag}$, $M_{uvm2}=-15.5\mathrm{mag}$) a relatively low but almost constant \ion{Si}{2} velocities of about $10,000\mathrm{km}{\mathrm{s}}^{-1}$, and persistent carbon absorption in the spectra. We estimate a ${}^{56}$Ni mass of $0.81_{-0.18}^{+0.10}{M}_{\odot }$ and a total ejecta mass of $1.59_{-0.12}^{+0.45}{M}_{\odot }$. The large ejecta mass of iPTF13asv and its stratified ejecta structure together seemingly favor a double-degenerate origin.

13 pages, 16 figures, ApJ in press