SN 2024cld: unveiling the complex mass-loss histories of evolved supergiant progenitors to core collapse supernovae

Pre-explosion mass loss in supernova (SN) progenitors is a crucial unknown factor in stellar evolution, yet has been illuminated recently by the diverse zoo of interacting transients. We present SN 2024cld, a transitional core-collapse SN at a distance of 39 Mpc, straddling the boundary between SN II and SN IIn, showing persistent interaction with circumstellar material (CSM) similar to H-rich SN 1998S and PTF11iqb. The SN was discovered and classified just 12 h post-explosion via the GOTO-FAST high-cadence program. Optical spectroscopy, photometry, and polarimetry over 220 d chart the complex, long-lived interaction in this transient. Early evolution is dominated by CSM interaction, showing a 14 d rise to a peak absolute magnitude of g = −17.6 mag, with clear flash-ionisation signatures. SN 2024cld also shows a slowly-evolving late time light curve powered by CSM interaction, with high-velocity (6000 kms−1) shoulders on a strong multi-component Hα profile. Dense polarimetric coverage reveals marked evolution in the photospheric geometry – peaking at 2% polarisation 10 days post-explosion, and rotating ≈60○ as the ejecta sweep more distant CSM. We observe a narrow (≈ 60 kms−1) Hα P Cygni feature throughout, associated with pre-shock CSM. SN 2024cld represents among the best-observed 98S-like SNe to date, revealing a multi-component CSM structure: a dense, inner aspherical envelope, CSM disk/torus, and tenuous, extended wind. We propose this SN arose from an evolved supergiant progenitor experiencing multiple mass loss episodes in its terminal years, with binary interaction plausibly generating the CSM disk. SN 2024cld constrains the progenitors and mass-loss paradigms of 98S-like SNe, unveiling the chaotic ends of evolved supergiant stars from afar.