Jeffrey M. Silverman, Mohan Ganeshalingam, Weidong Li, Alexei V. Filippenko
In this third paper in a series we compare spectral feature measurements to
photometric properties of 108 low-redshift (z < 0.1) Type Ia supernovae (SNe
Ia) with optical spectra within 5 d of maximum brightness. We find no
significant relationship between expansion velocity at or near maximum
brightness and SN colour. Furthermore, the pseudo-equivalent width (pEW) of the
Si II 4000 line is found to be a good indicator of light-curve width, and the
pEWs of the Mg II and Fe II complexes are relatively good proxies for SN
colour. We also employ a combination of light-curve parameters (specifically
the SALT2 stretch and colour parameters x_1 and c, respectively) and spectral
measurements to calculate distance moduli. The residuals from these models are
then compared to the standard model which uses only light-curve stretch and
colour. Our investigations show that a distance model that uses x_1, c, and the
velocity of the Si II 6355 feature does not lead to a decrease in the Hubble
residuals. We also find that distance models with flux ratios alone or in
conjunction with light-curve information rarely perform better than the
standard (x_1,c) model. However, when adopting a distance model which combines
the ratio of fluxes near ~3750 Ang. and ~4550 Ang. with both x_1 and c, the
Hubble residuals are decreased by ~10 per cent, which is found to be
significant at about the 2-sigma level. The weighted root-mean-square of the
residuals using this model is 0.130 +/- 0.017 mag (as compared with 0.144 +/-
0.019 mag when using the same sample with the standard model). This Hubble
diagram fit has one of the smallest scatters ever published and at the highest
significance ever seen in such a study. Finally, these results are discussed
with regard to how they can improve the cosmological accuracy of future,
large-scale SN Ia surveys. [Abridged]
View original:
http://arxiv.org/abs/1202.2130
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