Type II supernova spectral diversity II: Spectroscopic and photometric correlations

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 ${}^{56}$Ni masses. Using a larger sample than previous studies, we argue that $Pd$' - 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 ($OPTd$: explosion epoch to end of plateau). This argument is supported by the fact that $Pd$ also correlates with s${}_3$, the light-curve decline rate at late times: lower $Pd$ values correlate with larger s${}_3$ decline rates. Large s${}_3$ decline rates are likely related to lower envelope masses that enables gamma-ray escape. We also find a significant anticorrelation between $Pd$ and s${}_2$ (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 $Pd$ values.