NEARBY SUPERNOVA FACTORY OBSERVATIONS OF SN 2007if: FIRST TOTAL MASS MEASUREMENT OF A SUPER-CHANDRASEKHAR-MASS PROGENITOR

Abstract

We present photometric and spectroscopic observations of SN 2007if, an\noverluminous (M_V = -20.4), red (B-V = 0.16 at B-band maximum), slow-rising\n(t_rise = 24 days) type Ia supernova in a very faint (M_g = -14.10) host\ngalaxy. A spectrum at 5 days past B-band maximum light is a direct match to the\nsuper-Chandrasekhar-mass candidate SN Ia 2003fg, showing Si II and C II at\n~9000 km/s. A high signal-to-noise co-addition of the SN spectral time series\nreveals no Na I D absorption, suggesting negligible reddening in the host\ngalaxy, and the late-time color evolution has the same slope as the Lira\nrelation for normal SNe Ia. The ejecta appear to be well mixed, with no strong\nmaximum in I-band and a diversity of iron-peak lines appearing in\nnear-maximum-light spectra. SN2007 if also displays a plateau in the Si II\nvelocity extending as late as +10 days, which we interpret as evidence for an\noverdense shell in the SN ejecta. We calculate the bolometric light curve of\nthe SN and use it and the \\ion{Si}{2} velocity evolution to constrain the mass\nof the shell and the underlying SN ejecta, and demonstrate that SN2007 if is\nstrongly inconsistent with a Chandrasekhar-mass scenario. Within the context of\na "tamped detonation" model appropriate for double-degenerate mergers, and\nassuming no host extinction, we estimate the total mass of the system to be 2.4\n+/- 0.2 solar masses, with 1.6 +/- 0.1 solar masses of nickel-56 and with\n0.3-0.5 solar masses in the form of an envelope of unburned carbon/oxygen. Our\nmodeling demonstrates that the kinematics of shell entrainment provide a more\nefficient mechanism than incomplete nuclear burning for producing the low\nvelocities typical of super-Chandrasekhar-mass SNeIa.\n