We present photometric and spectroscopic observations of SN 2007if , an overluminous ( M _ { V } = -20.4 ) , red ( B - V = 0.16 at B -band maximum ) , slow-rising ( t _ { \mathrm { rise } } = 24 days ) type Ia supernova ( SN Ia ) in a very faint ( M _ { g } = -14.10 ) host galaxy .
A spectrum at 5 days past B -band maximum light is a direct match to the super-Chandrasekhar-mass candidate SN Ia 2003fg , showing Si ii and C ii at \sim 9000 km s ^ { -1 } .
A high signal-to-noise co-addition of the SN spectral time series reveals no Na i D absorption , suggesting negligible reddening in the host galaxy , and the late-time color evolution has the same slope as the Lira relation for normal SNe Ia .
The ejecta appear to be well mixed , with no strong maximum in I -band and a diversity of iron-peak lines appearing in near-maximum-light spectra .
SN 2007if also displays a plateau in the Si ii velocity extending as late as +10 days , which we interpret as evidence for an overdense shell in the SN ejecta .
We calculate the bolometric light curve of the SN and use it and the Si ii velocity evolution to constrain the mass of the shell and the underlying SN ejecta , and demonstrate that SN 2007if is strongly inconsistent with a Chandrasekhar-mass scenario .
Within the context of a “ tamped detonation ” model appropriate for double-degenerate mergers , and assuming no host extinction , we estimate the total mass of the system to be 2.4 \pm 0.2 M _ { \odot } , with 1.6 \pm 0.1 M _ { \odot } of ^ { 56 } \mathrm { Ni } and with 0.3–0.5 M _ { \odot } in the form of an envelope of unburned carbon/oxygen .
Our modeling demonstrates that the kinematics of shell entrainment provide a more efficient mechanism than incomplete nuclear burning for producing the low velocities typical of super-Chandrasekhar-mass SNe Ia .