JWST Probes Rare Calcium-Strong Supernova SN 2024uj, Revealing Helium and Molecular Surprises
Featured image: JWST NIRSpec spectral data visualization showing emission lines from SN 2024uj; credit: NASA/ESA/CSA
The James Webb Space Telescope has turned its infrared gaze on one of the rarest and most enigmatic classes of stellar explosions, revealing unexpected complexity in the calcium-strong transient SN 2024uj. The observations, published as Paper I in a new series on arXiv, mark the first time JWST has studied a calcium-strong transient (CaST) and have already overturned key assumptions about the nature of these events.
SN 2024uj was discovered in the galaxy NGC 3566 at a distance of approximately 60 megaparsecs (about 196 million light-years), located 22.7 arcseconds from its host center, a remote environment roughly 6.6 kiloparsecs from the nearest substantial star formation. This isolation is a hallmark of CaSTs and already hinted at an unusual progenitor.
What Makes a Calcium-Strong Transient
Calcium-strong transients are a rare subclass of stellar explosions defined by anomalously strong forbidden calcium emission relative to oxygen during the nebular phase, when the expanding debris becomes optically thin. They are faint, rapidly evolving, and occupy a luminosity gap between novae and normal supernovae. Only about 15 to 20 such events have been classified to date.
Their rarity is matched by their scientific importance. CaSTs do not fit cleanly into either the thermonuclear or core-collapse supernova paradigm, making them a powerful probe of exotic stellar death channels. They also produce significant amounts of calcium, the element essential for life as we know it.
JWST’s Key Discoveries
JWST’s NIRSpec instrument observed SN 2024uj at 150 days after explosion across a wavelength range of 0.96 to 5.1 micrometers, revealing several first-of-their-kind detections.
The helium emission lines at 1.083 and 2.058 micrometers are highly asymmetric with multicomponent structure, extending to velocities greater than 5,000 kilometers per second with a strong, narrow peak at 1,500 kilometers per second. This indicates that helium is distributed throughout the ejecta but concentrated off-center, suggesting a degree of chemical mixing that would be very difficult to produce in a core-collapse explosion of a massive star.
The paper also reports the first detection of molecular carbon monoxide emission in any CaST, observed in the fundamental band between 4.5 and 5.1 micrometers. A rising continuum beyond 2.5 micrometers combined with a detection at 10 micrometers from MIRI provides evidence of dust formation within the ejecta.
A Thermonuclear Origin
The researchers compared the observational data against two competing models: a massive stripped helium star undergoing core collapse, and a thermonuclear explosion of a white dwarf. The results were decisive.
Even artificially enhancing the calcium abundance in the core-collapse model to 1 percent of the helium mass produced calcium emission far too weak to match observations at 17 days after explosion. In contrast, thermonuclear white dwarf models naturally produced strong calcium emission at those early times. The narrow helium peak at 1,500 kilometers per second may trace material stripped from a companion star in a binary white dwarf system, a signature seen in some Type Ia supernovae.
The evidence points strongly to a thermonuclear explosion involving at least one low-mass, partially helium-rich white dwarf. The paper’s conclusion: SN 2024uj is most likely the result of such a detonation, with low ejecta mass (0.61 solar masses), low nickel-56 mass (0.0136 solar masses), and low kinetic energy.
Broader Implications
The findings suggest that calcium-strong transients are a heterogeneous population, with some events arising from core collapse and others — like SN 2024uj — from thermonuclear white dwarf mergers. JWST’s mid-infrared spectroscopy has opened new diagnostic windows: helium line profiles, molecular CO, and dust emission are now available as tools for understanding these exotic explosions.
The detection of dust formation in CaST ejecta suggests these rare events may contribute to the cosmic dust inventory, while the molecular CO detection opens the door to studying molecule formation in environments that bridge the gap between novae and supernovae.
The paper is available on arXiv (ID: 2607.00111) as the first in a planned series of studies on calcium-strong transients with JWST.
Source
Khan, R. et al. “JWST Observations of the Calcium-Strong Transient SN 2024uj.” arXiv:2607.00111 (2026).