SALT2: Using distant supernovae to improve the use of type la supernovae as distance indicators

Aims. We present an empirical model of type la supernovae spectro-photometric evolution with time. Methods. The model is built using a large data set including light-curves and spectra of both nearby and distant supernovae, the latter being observed by the SNLS collaboration. We derive the average spectral sequence of type la supernovae and their main variability components including a color variation law. The model allows us to measure distance moduli in the spectral range 2500-8000 Å with calculable uncertainties, including those arising from variability of spectral features. Results. Thanks to the use of high-redshift SNe to model the rest-frame UV spectral energy distribution, we are able to derive improved distance estimates for SNe la in the redshift range 0.8 < z < 1.1. The model can also be used to improve spectroscopic identification algorithms, and derive photometric redshifts of distant type la supernovae. © ESO 2007