Light-Curve and Spectral Properties of Type II Supernovae from the ATLAS survey

TL;DR

This study analyzes 68 Type II supernovae from the ATLAS survey, revealing that early spectral signs of circumstellar interaction correlate with faster decline rates, higher brightness, and lower a/e ratios, emphasizing the importance of large, uniform datasets.

Contribution

The paper provides a comprehensive analysis of early-time photometric and spectroscopic data of SNe II, linking spectral interaction features with light curve properties for the first time at this scale.

Findings

SNe II with early CSM interaction decline faster and are brighter.

Spectroscopic signs of interaction correlate with lower a/e ratios.

No difference in rise times between interacting and non-interacting SNe II.

Abstract

Type II supernovae (SNe II) are the most common terminal stellar explosions in the Universe. With SNe now being detected within days after explosion, there is growing evidence that the majority of Type II SNe show signs of interaction with a confined, dense cirumstellar material (CSM) in the first few days post explosion. In this work we aim to bridge the gap between single SN studies showing early-time interaction in their spectra, and the statistical studies of early-time SN light curves, which imply the existence of CSM. We present a sample of 68 Type II SNe with both early photometric data, obtained with the ATLAS survey, and spectroscopic data, obtained with 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 CSM. We characterise the photometric and spectroscopic properties of the sample by measuring rise times to maximum light, peak magnitudes, decline rates and line velocities. Additionally, we measure the ratio of absorption to emission (a/e) of the H alpha P-Cygni profile. Our analysis reveals that SNe II showing early spectroscopic signs of interaction with CSM decline faster and are brighter than those without. However no difference is found in rise times between the two groups. A clear separation is observed in the a/e ratio: 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.