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

TL;DR

This paper investigates how early-time ejecta-CSM interaction in Type II supernovae influences both photometric and spectroscopic observables. By assembling 68 SNe II with ATLAS photometry and ePESSTO+ spectroscopy, the authors quantify rise times, peak magnitudes, and decline rates, and measure Halpha velocities and the a/e ratio, while classifying events by early high-ionization signatures. They find that SNe II with early spectroscopic signs of CSM interaction decline faster and are brighter, and show systematically lower Halpha a/e ratios, though rise times show no clear separation; ledge features complicate the interpretation and may trace a distinct interaction mode. The work highlights the importance of large, uniform datasets in disentangling ejecta-CSM effects from intrinsic SN II diversity and points to future high-cadence surveys with rapid spectroscopy to refine progenitor mass-loss histories and explosion physics.

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.

Figures (14)

• Figure 1: Example of LC of a SN II from our sample. The weighted average detection epoch and last non-detection are marked as green and blue stars, respectively. Dashed horizontal lines indicate the flux levels of the last non-detection plus one sigma uncertainty (blue line) and of the first detection minus one sigma uncertainty (green line). If the difference between the two dashed lines is > 0, we consider the non detection to be deep enough. The orange dashed line corresponds to the Bazin fit performed on the $o$-band LC (see Section \ref{['sec:bazin']}), and the dashed vertical gray line shows the time of maximum light based on the fit.
• Figure 2: Example of spectral sequence for SN 2022mm. The phase in days from first light is indicated in red next to each spectrum.
• Figure 3: Spectra of the sample taken within 6 days past first light. Marked with vertical dashed lines are the main emission lines in the early phase. Lines in gray mark typical flash emission features that we do not observe in our spectra. Non-flash SNe are marked with black, flash SNe with blue, and ledgers with green (see Section \ref{['sec:class']}). The colour scheme is kept for the rest of the paper.
• Figure 4: Example of the fits and measurements of the minimum and the bluest wavelength for $\mathrm{H\alpha}$ (top) and $H_{\beta}$ (bottom). Marked in blue dashed lines are the fits to the pseudo-continuum to the blue side of $\mathrm{H\alpha}$ (top), and on the feature next to $H_{\beta}$ (bottom). The fits to the absorption minimum are marked in red for both panels, while the green line represents the value of the flux for which we consider the bluest wavelength of the feature.
• Figure 5: An example of a flash (blue), non-flash (black), and ledge (green) SN. Vertical lines mark the main emission lines during the early phase.