An improved standardization framework for Type II-P supernovae using a hierarchical Bayesian approach

Abstract

We use Type II-P supernovae (SNe II-P) observed as part of the Nearby Supernova Factory (SNfactory) experiment as standardizable candles to develop an improved standardization framework. The data consist of accurately flux-calibrated, host-subtracted spectrophotometry of low-redshift SNe II-P in the wavelength range $3200<\lambda <10000$ ${\unicode{xC5}}$. We develop an improved method to determine the expansion velocities of SNe II-P based on Gaussian Process regression. We extend the conventional standardized candle method (SCM) by an additional correction term containing the H${}_{\alpha }$ absorption-to-emission ratio and a hierarchical Bayesian framework. We find that the additional correction improves the quality of the standardization, particularly around $30$ to $40$ days after the explosion. We are able to reduce the scatter of the Hubble-flow SNe II-P from $0.28$ mag down to $0.18$ mag compared to previous work. As an example application of this methodology we measure $H_0$ using a pre-existing calibrator sample of SNe~II having Cepheid- or TRGB-based distance moduli. We find a value of $H_0=70.6_{-4.3}^{+4.5}$ km s${}^{-1}$ Mpc${}^{-1}$, including only statistical uncertainty, corresponding to a precision of $6.2$%. We find variations of $1.4$ km s${}^{-1}$ Mpc${}^{-1}$ between hierarchical model variations --- well within our quoted statistical uncertainty. Without the additional correction term, we obtain $H_0=73.0_{-6.1}^{+6.9}$ km s${}^{-1}$ Mpc${}^{-1}$ ($8.9$% precision) highlighting the improved precision of our new methodology. We identify and discuss inconsistencies between the measured parent populations of the calibrator and Hubble-flow standardization characteristics --- present in this and literature samples --- that are not explained by a simple selection on magnitude and which will need to be improved upon as SN II-P samples grow.

54 pages, 67 figures, accepted for publication in A&A. Appendices contain additional figures compared to the A&A version