Observed Type II supernova colours from the Carnegie Supernova Project-I

We present a study of observed Type II supernova (SN~II) colours using optical/near-infrared photometric data from the \textit{Carnegie Supernovae Project-I}. We analyse four colours ($B-V$, $u-g$, $g-r$, and $g-Y$) and find that SN~II colour curves can be described by two linear regimes during the photospheric phase. The first ($s_{\rm 1,colour}$) is steeper and has a median duration of $\sim 40$ days. The second, shallower slope ($s_{\rm 2,colour}$) lasts until the end of the "plateau" ($\sim 80$ days). The two slopes correlate in the sense that steeper initial colour curves also imply steeper colour curves at later phases. As suggested by recent studies, SNe~II form a continuous population of objects from the colour point of view as well. We investigate correlations between the observed colours and a range of photometric and spectroscopic parameters including the absolute magnitude, the $V$-band light-curve slopes, and metal-line strengths. We find that less luminous SNe~II appear redder, a trend that we argue is not driven by uncorrected host-galaxy reddening. While there is significant dispersion, we find evidence that redder SNe~II (mainly at early epochs) display stronger metal-line equivalent widths. Host-galaxy reddening does not appear to be a dominant parameter, neither driving observed trends nor dominating the dispersion in observed colours. Intrinsic SN~II colours are most probably dominated by photospheric temperature differences, with progenitor metallicity possibly playing a minor role. Such temperature differences could be related to differences in progenitor radius, together with the presence or absence of circumstellar material close to the progenitor stars.