Do spectra improve distance measurements of Type Ia supernovae?

We investigate the use of a wide variety of spectroscopic measurements to determine distances to low-redshift Type Ia supernovae (SN Ia) in the Hubble flow observed through the CfA Supernova Program. We consider linear models for predicting distances to SN Ia using light-curve width and color parameters (determined using the SALT2 light-curve fitter) and a spectroscopic indicator, and evaluate the resulting Hubble diagram scatter using a cross-validation procedure. We confirm the ability of spectral flux ratios alone at maximum light to reduce the scatter of Hubble residuals by ~10% [weighted rms, or WRMS = 0.189 ± 0.026 mag for the flux ratio ℛ(6630/4400)] with respect to the standard combination of light-curve width and color, for which WRMS = 0.204 ± 0.029 mag. When used in combination with the SALT2 color parameter, the color-corrected flux ratio ℛc(6420/5290) at maximum light leads to an even lower scatter (WRMS = 0.175 ± 0.025 mag), although the improvement has low statistical significance (