Type II-P Supernovae as Standard Candles: The SDSS-II Sample Revisited

TL;DR

This study revisits the velocity correlation in Type II-P supernovae, demonstrating its potential as a precise standard candle with an intrinsic scatter of about 11%, and discusses improvements and limitations in current samples.

Contribution

It refines the velocity correlation model for SNe II-P and assesses its effectiveness as a distance indicator using a revised analysis of existing data.

Findings

Velocity correlation modeled with 300 km/s dispersion

Hubble diagram with 11% intrinsic scatter

Measuring Hbeta velocities reduces systematic errors

Abstract

We revisit the observed correlation between Hbeta and FeII velocities for Type II-P supernovae (SNe~II-P) using 28 optical spectra of 13 SNe II-P and demonstrate that it is well modeled by a linear relation with a dispersion of about 300 km/s. Using this correlation, we reanalyze the publicly available sample of SNe II-P compiled by D'Andrea et al. and find a Hubble diagram with an intrinsic scatter of 11% in distance, which is nearly as tight as that measured before their sample is added to the existing set. The larger scatter reported in their work is found to be systematic, and most of it can be alleviated by measuring Hbeta rather than FeII velocities, due to the low signal-to-noise ratios and early epochs at which many of the optical spectra were obtained. Their sample, while supporting the mounting evidence that SNe II-P are good cosmic rulers, is biased toward intrinsically brighter objects and is not a suitable set to improve upon SN II-P correlation parameters. This will await a dedicated survey.