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.
Categories: astro-ph.SR astro-ph.CO astro-ph.GA astro-ph.HE