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

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.

💡 Research Summary

This paper presents a comprehensive study of 68 Type II supernovae (SNe II) observed by the Asteroid Terrestrial-impact Last Alert System (ATLAS) and spectroscopically followed up by the ePESSTO+ collaboration. The authors aim to bridge the gap between detailed spectroscopic analyses of individual events that show early‑time “flash” features—narrow, high‑ionisation emission lines with electron‑scattered wings indicative of interaction with dense, confined circumstellar material (CSM)—and statistical investigations of early light‑curve (LC) behaviour that suggest the presence of CSM in the majority of SNe II.

Sample construction
From 2019 to 2023 the team selected all ATLAS transients classified as SNe II that had a constraining non‑detection no more than four days before the first detection, were brighter than 18 mag at peak, and were observable from La Silla. Starting from 77 candidates, they removed Type IIn, IIb, and objects with ambiguous classifications, ending with 68 SNe II (54 of which have sufficient spectroscopic coverage). The authors re‑derived first‑light epochs using forced‑photometry fluxes, requiring that the last non‑detection be consistent with zero flux within 1σ and that its error bars not overlap those of the first detection.

Classification by flash features
Early spectra (≤ 3 days after first light) were examined for the presence of flash features. Thirty‑one SNe displayed the characteristic narrow, high‑ionisation lines with broad electron‑scattered wings; these were labelled the “CSM‑interacting” group. The remaining 37 SNe, lacking such signatures, formed the “non‑interacting” group.

Photometric analysis
ATLAS provides two broad filters, c (≈ 4200–6500 Å) and o (≈ 5600–8200 Å). The authors fitted each LC with a Bazin function to determine the time of maximum light, rise time (time from first light to peak), absolute peak magnitude (M_max), and post‑peak decline rates measured over 30 days (s_30) and 50 days (s_50) after maximum. The two groups show indistinguishable rise times (≈ 7.2 d vs 7.5 d, p > 0.2). However, CSM‑interacting SNe are on average 0.4 mag brighter (M_max ≈ –17.8 mag) and decline faster (s_30 ≈ 0.84 × 10⁻² mag d⁻¹, s_50 ≈ 0.56 × 10⁻² mag d⁻¹) than their counterparts (s_30 ≈ 0.71 × 10⁻² mag d⁻¹, s_50 ≈ 0.45 × 10⁻² mag d⁻¹). Kolmogorov‑Smirnov tests give p < 0.01 for both peak‑magnitude and decline‑rate differences.

Spectroscopic measurements
For each spectrum the authors measured expansion velocities from the minima of H α, H β, and Fe II λ5169, as well as the “bluest” velocity (the most blue‑shifted absorption detectable). Initial H α velocities are similar (~10,000 km s⁻¹) in both groups, but the CSM‑interacting SNe show a more rapid velocity decline. The key diagnostic is the absorption‑to‑emission ratio (a/e) of the H α P‑Cygni profile. Across all epochs, the CSM‑interacting group maintains a/e ≈ 0.3–0.5, whereas the non‑interacting group stays at a/e ≈ 0.7–0.9, a separation that is statistically robust (p < 0.001). This indicates that CSM interaction suppresses the absorption component while enhancing emission, consistent with electron‑scattering in a dense shell.

Interpretation and context
The findings confirm that early flash signatures are not rare outliers but occur in roughly 45 % of SNe II, and that their presence correlates with brighter peaks and faster post‑peak fading. The lack of a rise‑time difference suggests that CSM does not significantly alter the diffusion time of the initial shock‑cooling emission, or that the CSM is confined to radii small enough that the early light curve is still dominated by the progenitor’s radius and explosion energy. The authors discuss how their results compare with previous works: Bruch et al. (2023) argued that flash SNe are not systematically brighter, whereas Jacobson‑Galán et al. (2024, 2025) reported the opposite. The present study reconciles these discrepancies by using a uniform, well‑defined sample and by separating objects based on spectroscopic flash detection rather than solely on photometric behaviour.

Implications for progenitor mass loss
Modelling studies (Moriya et al. 2011; Dessart et al. 2017) often require high mass‑loss rates (Ṁ ∼ 10⁻³–10⁻² M_⊙ yr⁻¹) to reproduce the observed early luminosities, far above typical red‑supergiant winds (Ṁ ∼ 10⁻⁶ M_⊙ yr⁻¹). The authors suggest that even modest mass‑loss rates can produce a dense, compact CSM if wind acceleration is taken into account, leading to flash signatures without dramatically boosting the light‑curve luminosity. Their measured a/e ratios provide an observational handle on the optical depth of this CSM shell.

Limitations and future work
The study is limited by ATLAS’s two‑band photometry, which hampers precise temperature and colour evolution analysis. Some SNe lack spectra within the first few days, leaving their classification ambiguous. The authors propose that future high‑cadence, multi‑band surveys (e.g., Vera C. Rubin Observatory) combined with rapid spectroscopic follow‑up will enable tighter constraints on CSM density profiles, composition, and geometry. Incorporating radio and X‑ray observations could also probe the longer‑term interaction phase.

Conclusions
By coupling early‑time ATLAS light curves with systematic ePESSTO+ spectroscopy, the paper demonstrates that early CSM interaction—identified via flash spectral features—has a measurable impact on the photometric and spectroscopic evolution of Type II supernovae. Flash‑positive SNe are brighter, decline faster, and exhibit lower H α a/e ratios, confirming that CSM plays a non‑negligible role in a substantial fraction of the SN II population. These results underscore the necessity of incorporating CSM physics into SN II explosion models and provide a benchmark dataset for future theoretical and observational studies.