Type II Supernova Spectral Diversity. II. Spectroscopic and Photometric Correlations

Abstract

We present an analysis of observed trends and correlations between a large range of spectral and photometric parameters of more than 100 type II supernovae (SNe II), during the photospheric phase. We define a common epoch for all SNe of 50 days post-explosion, where the majority of the sample is likely to be under similar physical conditions. Several correlation matrices are produced to search for interesting trends between more than 30 distinct light-curve and spectral properties that characterize the diversity of SNe II. Overall, SNe with higher expansion velocities are brighter, have more rapidly declining light curves, shorter plateau durations, and higher ⁵⁶Ni masses. Using a larger sample than previous studies, we argue that "<i>Pd</i>" - the plateau duration from the transition of the initial to "plateau" decline rates to the end of the "plateau" - is a better indicator of the hydrogen envelope mass than the traditionally used optically thick phase duration (<i>OPTd</i>: explosion epoch to end of plateau). This argument is supported by the fact that <i>Pd</i> also correlates with s₃, the light-curve decline rate at late times: lower <i>Pd</i> values correlate with larger s₃ decline rates. Large s₃ decline rates are likely related to lower envelope masses, which enables gamma-ray escape. We also find a significant anticorrelation between <i>Pd</i> and s₂ (the plateau decline rate), confirming the long standing hypothesis that faster declining SNe II (SNe IIL) are the result of explosions with lower hydrogen envelope masses and therefore have shorter <i>Pd</i> values.