Peak 'Nebular' Emission and Early Flux Excesses in Ca-strong Supernovae

Based on the paper, here is a detailed summary:

Calcium-strong supernovae (Ca-strong SNe) are a class of thermonuclear transient characterised by rapid photometric evolution (rise times of ≤ 16 d) and nebular-phase spectra dominated by strong [Ca II] λλ7291,7324 emission, in comparison to the strength of [O I] λλ6300,6364 emission. Despite their unique spectroscopic signatures and association with remote environments at large physical offsets from their host galaxy, their origins remain uncertain. Using new discoveries from surveys such as the Zwicky Transient Facility, supplemented by archival data, we compile a sample of 46 Ca-strong SNe, including 8 newly-classified objects – the largest sample presented to date. We present a comprehensive optical photometric and spectroscopic study of Ca-strong SNe, aimed at better constraining their diversity, progenitor systems, and explosion mechanisms. This includes Gaussian decomposition of [Ca II] and [O I] nebular emission lines to measure bulk velocities and line profile evolution from peak brightness through to deep nebular-phase epochs. Of the 35 Ca-strong SNe with spectra near peak light, we find that 32 objects exhibit significant [Ca II] emission within 10 days of peak. Forbidden [Ca II] emission is typically expected to emerge after the ejecta have expanded, cooled, and reached a sufficiently low density for forbidden transitions to occur. The prevalence of these early features in Ca-strong SNe, however, remains unexplained by current explosion models of these objects. In several objects, we identify complex, multi-component line profiles that suggest a transition between two distinct velocity regimes. These transitional features, combined with the common presence of early forbidden emission, lead us to favour a scenario in which extended material surrounds the progenitor and pre-dates the SN itself. Our results suggest that the diverse spectroscopic behaviours of Ca-strong SNe are consistent with white dwarfs exploding in extended environments polluted by prior activity of the progenitor system.

The paper details that the core of the investigation was focussed on the spectroscopic evolution of the sample, specifically, the evolution of forbidden [Ca II] and [O I] emission lines. Historically, these features have been associated with the nebular phase of these objects, their key diagnostic being a high [Ca II]/[O I] ratio in the nebular-phase. To observe these forbidden transitions we typically assume the photosphere to have receded into the inner layers of the SN, and for the ejecta to have expanded, cooled, and become optically thin, reaching a low enough density for these forbidden transition emission lines to occur. This therefore often requires the SN to be deep in its nebular phase for these features to be observed. Across the sample of Ca-strong SNe, however, these forbidden emission lines are detected from as early as peak light – far earlier than this typical understanding predicts.

To explain the early presence of these features, the authors investigated the behaviour of the forbidden lines extensively, drawing the following conclusions:

– The early presence of [Ca II] and [O I] emission in each object are consistent with the presence of external low-density Ca and O regions that pre-date the SN.

– The broad, non-zero, distribution of velocities in the photospheric phase suggests an extended aspherical external system. Such a system would reproduce a broad range of measured velocities dependant upon the observed angle of the SN, in line with the observations.

– The narrower, zero-centred, velocity distribution of these forbidden lines in the truly nebular-phase spectra of each object may originate from the SN core.

– Investigating higher-resolution spectra observed during transitional epochs revealed a complex line evolution: an initial high-velocity component which is gradually dominated by a zero-velocity component at later times. This transition is consistent with a shift in the dominant emission region from the external aspherical material at early times, to the inner core at late times as the ejecta expand, become optically thin, and the photosphere recedes entirely.

– Analysis focussed on the early flux excesses observed before peak light in the majority of the sample of Ca-strong SNe indicates that these objects occur in environments that are polluted by pre-existing, extended material.

The results are consistent with the contribution from two distinct emission regions in the structure of Ca-strong SNe: an external, aspherical structure responsible for early-time forbidden line emission – interaction with which may be responsible for the observed early photometric behaviour of Ca-strong SNe – and a central, core component that dominates in the nebular phase. The authors therefore hypothesise that Ca-strong SNe result from WD explosions in polluted environments. They suggest this pollution may originate from precursor activity of the progenitor itself, e.g. through prior He-novae in AM CVn-based systems. Following previous work in CGTP25, they suggest that the observed behaviours across the sample of Ca-strong SNe are consistent with the proposed system of a Ca-strong SN progenitor embedded in an environment polluted by an AM CVn recurrent He-nova. In such a scenario, the polar outflow lobes produced by prior nova activity are analogous to the external aspherical structure concluded to be responsible for the photospheric [Ca II] and [O I] emission. The observed emergence of nebular emission would instead be directly produced from the core of the SN itself.

Improvements to AI Systems:

1. Early-Time Forbidden-Line Prediction Model

• Improvement: Train a neural network on the 46-object sample to predict the presence and strength of [Ca II] and [O I] emission within 10 days of peak brightness, using photometric light-curve shapes and early flux excesses as input features.

• Capability: The AI can automatically classify newly discovered transients as Ca-strong SNe candidates within days of discovery, enabling rapid follow-up observations and reducing reliance on nebular-phase spectra.

2. Two-Component Spectral Decomposition Engine

• Improvement: Implement a Bayesian or deep-learning-based spectral fitting tool that decomposes [Ca II] and [O I] line profiles into a high-velocity, angle-dependent external component and a zero-centered core component, accounting for time evolution.

• Capability: The AI can disentangle emission from extended circumstellar material versus the SN core, providing real-time estimates of progenitor environment geometry (e.g., asphericity, outflow orientation) from single-epoch spectra.

3. Progenitor Environment Inference System

• Improvement: Build a generative model (e.g., a variational autoencoder) trained on the observed diversity of Ca-strong SNe (light curves, velocities, line ratios) to map them to physical parameters of the proposed AM CVn He-nova scenario, such as mass-loss rate, outflow density, and inclination angle.

• Capability: The AI can infer the likely progenitor system and pre-explosion mass-loss history for any new Ca-strong SN, enabling population-level tests of the pollution hypothesis and distinguishing it from alternative explosion models.

4. Transition-Epoch Line-Profile Classifier

• Improvement: Train a convolutional neural network on the multi-component line profiles observed during the transition between photospheric and nebular phases to automatically identify the velocity-regime shift and quantify the timescale of core emergence.

• Capability: The AI can flag objects where external material dominates early emission, allowing automated prioritization for high-resolution spectroscopic follow-up to study the interaction physics and constrain the external structure's density profile.

5. Predictive Early Flux Excess Detector

• Improvement: Develop a time-series anomaly detection model using the early photometric data (pre-peak) from the 46-object sample, trained to identify the characteristic flux excess indicative of extended material interaction.

• Capability: The AI can issue early warnings for Ca-strong SNe based solely on rising light curves, enabling coordinated multi-wavelength campaigns (e.g., UV/X-ray) to capture the interaction signature before peak, which is critical for testing the external pollution scenario.