A Search for New Candidate Super-Chandrasekhar-Mass Type Ia Supernovae
  in the Nearby Supernova Factory Dataset
  We present optical photometry and spectroscopy of five type Ia supernovae
discovered by the Nearby Supernova Factory selected to be spectroscopic
analogues of the candidate super-Chandrasekhar-mass events SN 2003fg and SN
2007if. Their spectra are characterized by hot, highly ionized photospheres
near maximum light, for which SN 1991T supplies the best phase coverage among
available close spectral templates. Like SN 2007if, these supernovae are
overluminous (-19.5 < M_V < -20) and the velocity of the Si II 6355 absorption
minimum is consistent with being constant in time from phases as early as a
week before, and up to two weeks after, $B$-band maximum light. We interpret
the velocity plateaus as evidence for a reverse-shock shell in the ejecta
formed by interaction at early times with a compact envelope of surrounding
material, as might be expected for SNe resulting from the mergers of two white
dwarfs. We use the bolometric light curves and line velocity evolution of these
SNe to estimate important parameters of the progenitor systems, including
nickel-56 mass, total progenitor mass, and masses of shells and surrounding
carbon/oxygen envelopes. We find that the reconstructed total progenitor mass
distribution of the events (including SN 2007if) is bounded from below by the
Chandrasekhar mass, with SN 2007if being the most massive. We discuss the
relationship of these events to the emerging class of super-Chandrasekhar-mass
SNe Ia, estimate the relative rates, compare the mass distribution to that
expected for double-degenerate SN Ia progenitors from population synthesis, and
consider implications for future cosmological Hubble diagrams.
