SN2024cld: Transitional Core-Collapse SN

• The paper details SN2024cld's discovery and rapid classification via early spectroscopy, highlighting prolonged flash-ionisation and CSM interaction.
• Multi-phase light curve and spectral evolution reveal a structured CSM comprising a dense inner envelope, disk/torus, and an extended RSG-like wind.
• Polarimetric and kinematic analyses indicate significant asphericity and binary-triggered mass-loss episodes in the progenitor’s late evolutionary stages.

SN2024cld is a transitional core-collapse supernova discovered in 2024 that exhibits persistent and complex interaction with circumstellar material (CSM), providing a detailed view into the pre-supernova mass-loss history of an evolved supergiant star likely influenced by binary interaction. Straddling the photometric and spectroscopic properties of both SN II and SN IIn, SN2024cld reveals a multi-phase, multi-component CSM environment shaped by distinct episodes of pre-explosion mass loss and offers one of the most comprehensive datasets ever obtained for a "98S-like" transient (Killestein et al., 31 Oct 2025).

1. Discovery, Initial Classification, and Environmental Context

SN2024cld was discovered by the GOTO-FAST high-cadence program on 2024 February 13, only 11.6 hours after its explosion epoch (MJD 60352.74±0.03), constrained further by a preceding non-detection within 23.3 hours. The initial detection was at a luminosity of L=17.97 mag. Prompt follow-up spectroscopy (INT/IDS, 28 min post-discovery) revealed a blue continuum featuring both narrow hydrogen lines and a broad He II/N III/C III blend, characteristic of strong "flash-ionised" CSM interaction. These early observations enabled rapid classification as a Type II supernova.

A medium-resolution ANU-2.3m/WiFeS spectrum taken at +25 d established a precise host galaxy redshift z=0.01252±0.00001 (v=3750 km/s), translating to a distance of D≃39.3 Mpc. The line-of-sight extinction was determined by host Na I D absorption as E(B–V)_host≈0.01 mag, with total E(B–V)=0.1181 mag. The explosion site falls in a region of low host-galaxy extinction.

2. Photometric Evolution and Light Curve Morphology

The light-curve evolution of SN2024cld is dominated by CSM interaction. The multi-band light curves reach peak g-band magnitude m_g=15.39±0.01 (M_g=–17.58±0.09, systematic ±0.64 mag), with a 14.65±0.1 d rise post-explosion, placing it among the "long-riser" SNe II. The explosion epoch and early light curve were fitted using a piecewise quadratic model:

$$f(\phi) = \begin{cases} B, & \phi < 0 \ B + a_0\phi + a_1\phi^2, & \phi \ge 0 \end{cases}$$

where $\phi = t - t_{\mathrm{expl}}$.

After the peak, the SN exhibited a rapid decline to ≃50 d, followed by a first plateau (P1, ≈50–100 d) and then a second, flatter plateau (P2, from ≈110 d, extending beyond 220 d). The r-band decline during P2 is only ~0.2 mag per 100 d (≈0.002 mag d⁻¹), highlighting persistent energy injection from ongoing CSM interaction. Pseudo-bolometric luminosity fits reveal a peak of $$\log\,L_{\mathrm{bol}}=43.02\pm0.09\,\mathrm{erg}\,\mathrm{s}^{-1}$$. The inferred early-time blackbody radius is $R_0 \approx 1.5 \times 10^{14}$ cm ($\simeq2100\,R_\odot$), exceeding typical red supergiant (RSG) radii.

3. Spectroscopic Evolution and Diagnostic Features

The spectroscopic sequence charts a transition from flash-ionised CSM to sustained, complex CSM-ejecta interaction. For 0.5–14 days, spectra are dominated by flash-ionised, electron-scattered Lorentzian hydrogen and He II lines, as well as high-excitation C/C/N features, indicating a confined, dense inner CSM ($R_{\mathrm{CSM}} \approx 10^{15}$ cm). In contrast to typical SNe II, flash-ionised lines persist unusually long, implying elevated pre-shock densities.

Past ≈14 d, the spectrum transitions to a more standard SN II appearance, but the usual broad Hα P Cygni trough is masked by CSM emission. At late times, the Hα profile splits into four Gaussian components:

• Narrow (FWHM ≈ 65 km/s), tracking a pre-shock wind,
• Broad ejecta (~0 km/s),
• Blue shoulder (–5500 km/s) and red shoulder (+6000 km/s), both with substantial FWHM (2000 and 5000 km/s, respectively), emerging after ~90 d and tracing interaction with an aspherical disk/torus CSM.

The velocity of the narrow Hα absorption minimum drops from ~200 km/s at +25 d to ~57±13 km/s at +220 d, at a rate of $d v/dt = 0.22 \pm 0.08$ km s⁻¹ d⁻¹, consistent with the SN ejecta sweeping through a wind with decelerating velocity gradient.

4. Polarimetric Signatures and Photospheric Geometry

Time-resolved linear polarimetry (B, V bands) between +3.5 and +115 d displays marked evolution. The continuum polarization rises from $\lesssim0.5\%$ at +3.5 d to a peak $p_{\mathrm{peak}} \simeq2\%$ at +10 d, before decreasing to ~1% by +115 d. After correcting for the Milky Way interstellar polarization ($\sim0.24\%$), the polarization angle shows an approximately 60° rotation over 60 d. This evolution indicates a shift from near-spherical to highly aspherical photospheric geometry, matching the epoch at which fast ejecta begin interacting with the dense, flattened CSM. Polarimetric behavior is analogous to previous 98S-like SNe such as SN 1998S and PTF11iqb.

5. Circumstellar Structure and Multi-Episode Mass Loss

Interpretation of the photometric, spectroscopic, and polarimetric data indicates a distinctly multi-component CSM:

• A dense, inflated inner envelope ($\phi = t - t_{\mathrm{expl}}$0–$\phi = t - t_{\mathrm{expl}}$1 cm), driving the long rise and flash-ionized signature,
• A disk/torus CSM at $\phi = t - t_{\mathrm{expl}}$2–$\phi = t - t_{\mathrm{expl}}$3 cm, responsible for high-velocity emission shoulders once impacted by ejecta,
• An extended, low-density RSG-like wind beyond $\phi = t - t_{\mathrm{expl}}$4 cm, evidenced by persistent narrow Hα P Cygni absorption (terminal wind velocity $\phi = t - t_{\mathrm{expl}}$5 km/s).

The CSM sweep is reconstructed chronologically through both spectral evolution and plateau durations. The second plateau ($\phi = t - t_{\mathrm{expl}}$6–220 d, shock radius $\phi = t - t_{\mathrm{expl}}$7 cm) requires that the disk-wind CSM was expelled approximately $\phi = t - t_{\mathrm{expl}}$8 yr before the SN. Integrating $\phi = t - t_{\mathrm{expl}}$9 over the most recent $$\log\,L_{\mathrm{bol}}=43.02\pm0.09\,\mathrm{erg}\,\mathrm{s}^{-1}$$0–$$\log\,L_{\mathrm{bol}}=43.02\pm0.09\,\mathrm{erg}\,\mathrm{s}^{-1}$$1 years yields $$\log\,L_{\mathrm{bol}}=43.02\pm0.09\,\mathrm{erg}\,\mathrm{s}^{-1}$$2.

Inner envelope ejection, inferred from the $$\log\,L_{\mathrm{bol}}=43.02\pm0.09\,\mathrm{erg}\,\mathrm{s}^{-1}$$3 d flash-ionized phase ($$\log\,L_{\mathrm{bol}}=43.02\pm0.09\,\mathrm{erg}\,\mathrm{s}^{-1}$$4 cm, $$\log\,L_{\mathrm{bol}}=43.02\pm0.09\,\mathrm{erg}\,\mathrm{s}^{-1}$$5–500 km/s), must have occurred only $$\log\,L_{\mathrm{bol}}=43.02\pm0.09\,\mathrm{erg}\,\mathrm{s}^{-1}$$6–3 yr before explosion.

Mass-loss rates, derived under a shock-powered luminosity formalism ($$\log\,L_{\mathrm{bol}}=43.02\pm0.09\,\mathrm{erg}\,\mathrm{s}^{-1}$$7 with $$\log\,L_{\mathrm{bol}}=43.02\pm0.09\,\mathrm{erg}\,\mathrm{s}^{-1}$$8–6000 km/s, $$\log\,L_{\mathrm{bol}}=43.02\pm0.09\,\mathrm{erg}\,\mathrm{s}^{-1}$$9 km/s), require $R_0 \approx 1.5 \times 10^{14}$0 (±0.3 dex) for the disk.

6. Progenitor Typing, Evolution, and Broader Implications

The requirement for a large pre-explosion stellar radius ($R_0 \approx 1.5 \times 10^{14}$1 cm $R_0 \approx 1.5 \times 10^{14}$2) and the multi-component, highly aspherical CSM are best explained by an evolved supergiant progenitor—potentially a red supergiant (RSG) or yellow supergiant (YSG). Zero-age main-sequence mass is plausibly $R_0 \approx 1.5 \times 10^{14}$3–$R_0 \approx 1.5 \times 10^{14}$4, consistent with analogs such as SN 1998S.

The sharply defined CSM disk or torus, and the brief timescale between envelope ejection and collapse, implicate binary interaction as the dominant channel (rather than steady single-star wind drivers). Observational analogs include WOH G64 and Hen 3-1379, both displaying multi-shell aspherical CSM geometries. A plausible implication is that the late evolutionary stages of such massive stars may be characterized by repeated, multi-modal mass-loss episodes, possibly triggered or enhanced by binary interaction.

SN2024cld demonstrates that the spectral and photometric behavior of type II SNe can be profoundly altered by structured CSM, and provides critical constraints on the terminal mass-loss regimes and pre-supernova envelope dynamics of evolved supergiants. The case further highlights the need for rapid, multi-modal follow-up of transients to disentangle progenitor histories (Killestein et al., 31 Oct 2025).