This composite image combines X-ray data from the Chandra X-ray Observatory and optical data from the Hubble Space Telescope to create this unique view of the galaxy Messier 51, home to the transient AT 2019abn. [X-ray: NASA/CXC/Wesleyan Univ./R.Kilgard, et al; Optical: NASA/STScI]
What Caused AT 2019abn?
Across the universe, cosmic mysteries are afoot. Collisions, explosions, flares, and other phenomena are sending transient signals into space, and new surveys are detecting more of these fleeting events than ever before. Many of these transients are easily identified, their brightness, color, and evolution neatly matching expectations for known types of events.
Others are not so readily classified, like intermediate-luminosity red transients, or ILRTs. ILRTs are red in color and lie between novae and supernovae in peak brightness. While ILRTs span a broad category of events that might have many different causes, today’s article focuses on the small but growing group of ILRTs that contains AT 2019abn, which was spotted in the galaxy Messier 51 in January 2019.
Spectrum of AT 2019abn (black) compared to the spectra of several classes of polycyclic aromatic hydrocarbons (PAHs) as well as other transient events. NGC 300 2008-OT is a prototypical red transient belonging to the same group of ILRTs as AT 2019abn. Click to enlarge. [Rose et al. 2025]
Assembling the Clues
To learn more about the origins of this class of transients, a team led by Sam Rose (California Institute of Technology) analyzed JWST spectra of AT 2019abn taken nearly four years after the transient was discovered. The spectra show features that likely come from a class of large molecules called polycyclic aromatic hydrocarbons (PAHs). These features most closely resemble those from class C PAHs, which are relatively rare and appear only in carbon-rich environments.
Rose’s team also tracked AT 2019abn’s brightness in the years after it hit its peak. At the time of the JWST observations reported in this article, the source was still brighter than it was before its 2019 outburst. However, more recent observations show that the source has now faded below the brightness of its progenitor, indicating that the star may have been destroyed in the outburst.
Possible Identification
Luminosity of AT 2019abn and two other similar ILRTs. The horizontal lines show the pre-outburst luminosity of each system. Click to enlarge. [Rose et al. 2025]
These low-luminosity supernovae are thought to arise from super-asymptotic giant branch stars that are less massive than the progenitors of core-collapse supernovae. After these stars cease their core nuclear fusion — ending with an oxygen-neon-magnesium core rather than an iron core like a core-collapse supernova progenitor — their cores are fortified by electron degeneracy pressure. When atomic nuclei in the core capture some of these electrons, the core collapses, triggering a supernova that destroys the star. This premise is compatible with AT 2019abn’s progenitor mass and could create the carbon-rich environment necessary to craft class C PAHs.
The stellar merger and stellar eruption hypotheses both run into trouble in terms of AT 2019abn’s progenitor mass and the presence of class C PAHs. However, there’s enough uncertainty in the details of these scenarios that they can’t be ruled out fully. The final necessary clue to this mystery is confirmation of the fate of the progenitor star. If future long-wavelength observations detect a surviving dust-shrouded progenitor, that would rule in favor of the stellar merger or stellar eruption scenarios, leaving astronomers puzzling over how to create a cohesive picture out of the mismatched clues. If no surviving progenitor is found, the evidence paints a clear portrait of an electron-capture supernova.
Citation
“Investigating the Electron-Capture Supernova Candidate AT 2019abn with JWST Spectroscopy,” Sam Rose et al 2025 ApJL 980 L14. doi:10.3847/2041-8213/adad61