http://arxiv.org/PS_cache/astro-ph/p.../0609616v1.pdf
The type Ia supernova SNLS-03D3bb from a super-
Chandrasekhar-mass white dwarf star
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Originally Posted by Howell et al
SNLS-03D3bb (SN 2003fg) was discovered on April 24, 2003 (UT) as part of the Supernova Legacy Survey (SNLS). Its redshift is z = 0.2440 ± 0.0003, determined from host galaxy [OII], [OIII], H_, and H_ emission lines...
SNLS-03D3bb also does not follow the lightcurve width luminosity Relationship for SNe Ia that allows them to be calibrated as standard candles — it is too bright for its lightcurve width (“stretch”, s=1.13) by 0.61 ± 0.14 mag (4.4_)...
SNLS-03D3bb also has an unusually low ejecta velocity, as shown in the Keck spectrum taken 2 days after maximum light (Fig. 3). With a SiII velocity of 8000±500 km s−1, it falls well outside the range of velocities seen for this feature at maximum light (Fig. 4). This is hard to understand in the Chandrasekhar mass model, which predicts higher velocities for more luminous SNe Ia, in contrast to the unusually low velocities in SNLS-03D3bb...
A super-Chandrasekhar mass reproduces the low velocities seen in SNLS-03D3bb. Since Chandrasekhar models with more Ni produce higher velocities, the low velocities of SNLS- 03D3bb imply an increased progenitor binding energy and thus a larger total mass. As a caveat, we note that this simple calculation is only intended to illustrate general trends. Future theoretical studies will have to assess such complications as using different ions, different white dwarf density structures, and a wider range of binding energies.
SNe such as SNLS-03D3bb will have to be screened out in cosmological studies. Since younger stellar environments produce more luminous SNe, as the mean stellar age decreases with redshift the mean properties of SNe Ia will change5. This can be calibrated if all SNe obey the same stretch-luminosity relationship, but SNLS-03D3bb does not. Its peculiarity was so obvious that it was excluded from the SNLS cosmological result, but less extreme objects could lurk in SN samples. Future cosmology studies will have to carefully scrutinise SNe Ia from young populations to see if they obey the same lightcurve shape-luminosity relationship as other SNe Ia.
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Frankly, this non-Standard Candle event blows-out the cosmological claims made using earlier supernova samples that did not anticipate superluminous events. We have no idea what percentage of the high redshift supernova observed are queer. What we do know is selection effects favor finding the brightest events at the greatest distances; meaning the high redshift sample may be absolutely saturated with over-brilliant supernovae.
In this later study (released today):
http://arxiv.org/PS_cache/arxiv/pdf/...709.0859v1.pdf
Some of the spectra are similar to the arch-type overluminous supernovae 1991T; yet the authors conclude they are all normal type Ia. None of the supernova spectra observed in this high redshift study are classified as underluminous or overluminous events; even though using current estimates of extinction, there should be a few subluminous events observed, and many overluminous events like the few we see in the local sample. Evolution is one possibility. Grossly underestimated extinction rates is another.