Improved standardization of type II-P supernovae: Application to an expanded sample

In the epoch of precise and accurate cosmology, cross-confirmation using a variety of cosmographic methods is paramount to circumvent systematic uncertainties. Owing to progenitor histories and explosion physics differing from those of Type Ia supernovae (SNeIa), Type II-plateau supernovae (SNeII-P) are unlikely to be affected by evolution in the same way. Based on a new analysis of 17 SNeII-P, and on an improved methodology, we find that SNeII-P are good standardizable candles, almost comparable to SNeIa. We derive a tight Hubble diagram with a dispersion of 10% in distance, using the simple correlation between luminosity and photospheric velocity introduced by Hamuy and Pinto. We show that the descendent method of Nugent etal. can be further simplified and that the correction for dust extinction has low statistical impact. We find that our SN sample favors, on average, a very steep dust law with total to selective extinction R_V < 2. Such an extinction law has been recently inferred for many SNeIa. Our results indicate that a distance measurement can be obtained with a single spectrum of a SNII-P during the plateau phase combined with sparse photometric measurements.