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