Early panchromatic spectra reveal helium in the thermonuclear supernova 2024kce

Abstract

Faint Type I supernovae blur the boundaries between spectroscopic classes, obscuring their progenitors and explosion mechanisms. One proposed origin is a double detonation, in which ignition of an accreted helium shell triggers the explosion of a carbon oxygen white dwarf. Surviving helium is a direct prediction of this mechanism, but its strongest near-infrared feature overlaps with carbon and is ambiguous in isolation. Here we present SN 2024kce, observed with simultaneous optical and near-infrared spectroscopy eleven days before maximum light. We identify He I absorption at three wavelengths, all consistent with a velocity of 16,400 km/s, well above the photosphere. This is the first detection of helium before maximum light in a thermonuclear supernova. Helium at these velocities requires nearby radioactive material for non-thermal excitation, pointing to ash from a detonated surface helium shell. A double detonation model reproduces the optical spectral evolution, suppressed blue flux, and He I 1.083 micron absorption feature. SN 2024kce shares properties with both subluminous Type Ia supernovae and Ia-like calcium-rich transients, providing evidence that the same explosion mechanism operates across these classes. From early optical and near-infrared spectra to Rubin's First Look images, SN 2024kce showcases the full reach of modern observatories in exposing the physics of rare explosions.