The reddening law of Type Ia Supernovae: separating intrinsic
  variability from dust using equivalent widths
  We employ 76 type Ia supernovae with optical spectrophotometry within 2.5
days of B-band maximum light obtained by the Nearby Supernova Factory to derive
the impact of Si and Ca features on supernovae intrinsic luminosity and
determine a dust reddening law. We use the equivalent width of Si II
{\lambda}4131 in place of light curve stretch to account for first-order
intrinsic luminosity variability. The resultant empirical spectral reddening
law exhibits strong features associated with Ca II and Si II {\lambda}6355.
After applying a correction based on the Ca II H&K equivalent width we find a
reddening law consistent with a Cardelli extinction law. Using the same input
data, we compare this result to synthetic rest-frame UBVRI-like photometry in
order to mimic literature observations. After corrections for signatures
correlated with Si II {\lambda}4131 and Ca II H&K equivalent widths, and
introducing an empirical correlation between colors, we determine the dust
component in each band. We find a value of the total-to-selective extinction
ratio, RV = 2.8 \pm 0.3. This agrees with the Milky Way value, in contrast to
the low RV values found in most previous analyses. This result suggests that
the long-standing controversy in interpreting SN Ia colors and their
compatibility with a classical extinction law, critical to their use as
cosmological probes, can be explained by the treatment of the dispersion in
colors, and by the variability of features apparent in SN Ia spectra.
