Abstract
Context.
Type II supernovae (SNe) are the most common terminal stellar explosions in the Universe. With SNe now being detected within days after first light, there is growing evidence that most Type II SNe (SNe II) show signs of interaction with a dense, confined circumstellar material (CSM) in the first few days after first light.
Aims.
We aim to bridge the gap between single SN studies showing early-time interaction in their spectra and statistical studies of early-time SN light curves, which imply the existence of CSM.
Methods.
We present a sample of 68 Type II SNe with both early photometric data obtained by the ATLAS survey and spectroscopic data obtained by the ePESSTO+ collaboration. A subset of the sample is classified based on the presence or absence of narrow spectral features with electron-scattered broadened wings in the early spectra, indicative of interaction with the CSM. We characterised the photometric and spectroscopic properties of the sample by measuring rise times to maximum light, peak magnitudes, decline rates, and line velocities. Additionally, we measured the ratio of absorption to emission (a/e) of the H
α
P-Cygni profile.
Results.
Our analysis reveals that SNe II showing early spectroscopic signs of interaction with the CSM decline faster and are brighter than those without. However no difference was found in rise times between the two groups. A clear separation can be observed in the a/e ratio, namely, SNe with signs of interaction exhibit lower a/e ratios at all epochs compared to those without. Our results highlight that understanding SN II ejecta-CSM interaction requires large uniform samples of photometric and spectroscopic data, such as the one presented in this work.