Type II-P Supernovae from the SDSS-II Supernova Survey and the Standardized Candle Method

TL;DR

This study applies the Standardized Candle Method to 15 Type II-P supernovae from SDSS-II, demonstrating low intrinsic scatter and highlighting the need for improved spectral velocity measurements for cosmological applications.

Contribution

It provides the first large sample of SNe II-P in the Hubble flow for SCM calibration and analyzes systematic effects impacting its cosmological utility.

Findings

SDSS-II SNe II-P have a small intrinsic I-band dispersion of 0.22 mag

Combining SDSS and literature data increases dispersion to 0.29 mag

Current velocity measurement methods need improvement for better standardization

Abstract

We apply the Standardized Candle Method (SCM) for Type II Plateau supernovae (SNe II-P), which relates the velocity of the ejecta of a SN to its luminosity during the plateau, to 15 SNe II-P discovered over the three season run of the Sloan Digital Sky Survey - II Supernova Survey. The redshifts of these SNe - 0.027 < z < 0.144 - cover a range hitherto sparsely sampled in the literature; in particular, our SNe II-P sample contains nearly as many SNe in the Hubble flow (z > 0.01) as all of the current literature on the SCM combined. We find that the SDSS SNe have a very small intrinsic I-band dispersion (0.22 mag), which can be attributed to selection effects. When the SCM is applied to the combined SDSS-plus-literature set of SNe II-P, the dispersion increases to 0.29 mag, larger than the scatter for either set of SNe separately. We show that the standardization cannot be further improved by eliminating SNe with positive plateau decline rates, as proposed in Poznanski et al. (2009). We thoroughly examine all potential systematic effects and conclude that for the SCM to be useful for cosmology, the methods currently used to determine the Fe II velocity at day 50 must be improved, and spectral templates able to encompass the intrinsic variations of Type II-P SNe will be needed.