Rare Type Icn supernova SN 2024abvb is among the most luminous known

An international team of astronomers has carried out photometric and spectroscopic observations of SN 2024abvb—a recently discovered supernova of a rare Type Icn. The new observational campaign yields important information regarding the properties and nature of this supernova. The study was published February 18 on the arXiv pre-print server.

Supernovae and their extreme subtype

Supernovae (SNe) are powerful and luminous stellar explosions. They are important for the scientific community as they offer essential clues into the evolution of stars and galaxies. In general, SNe are divided into two groups based on their atomic spectra: Type I and Type II. Type I SNe lack hydrogen in their spectra, while those of Type II showcase spectral lines of hydrogen.

Type Icn SNe are an extreme subtype of interacting stripped-envelope supernovae (SESN). They have strong, narrow oxygen and carbon lines but weak or absent hydrogen and helium lines, presenting additional complications to the stripping mechanism. They have narrow emission features indicative of circumstellar interaction.

Newfound rare Type Icn supernova

So far, only six Type Icn SNe have been discovered, and SN 2024abvb is the latest addition to this short list. It was detected on November 22, 2024 by the Asteroid Terrestrial-impact Last Alert System Project (ATLAS) and was found to be associated with the host galaxy PSO J011055.760-054416.73, however highly offset from it—about 73,000 light years from the core of the host.

Recently, a group of astronomers led by Jennifer Shi of the University of Melbourne in Australia, performed follow-up monitoring of SN 2024abvb using various ground-based telescopes, hoping to get more insights into its enigmatic nature.

After explosion

The new observations found that SN 2024abvb reached a peak magnitude of -19.55, which makes it one of the most luminous Type Icn SNe. It declines at a rate of 0.07 mag/day in the r band, therefore consistent with other known supernovae of this subclass.

During the first week after explosion, SN 2024abvb shows evidence of interaction with a hydrogen/helium-poor, carbon-rich circumstellar medium (CSM) in its optical spectra. Moreover, the persistent lack of helium emission at late times of observations indicates a transition from Type Ibn to Icn with low carbon ionization. This makes SN 2024abvb the first known example of such an event.

Host, progenitor and ejecta mass

When it comes to the host of SN 2024abvb, PSO J011055.760-054416.73, the study found that it is an intermediate-mass galaxy with a stellar mass of about 6 billion solar masses. It has an extremely low specific star-formation rate and low metallicity.

According to the astronomers, these results, together with a large off-set of SN 2024abvb, suggest that the progenitor of this supernova was a low-mass star stripped by a compact companion, which then exploded within a dense, hydrogen/helium-poor CSM.

“Based on its spectral features, local and host environment properties, and host-galaxy offset, we favor an 8–10 solar masses star that is stripped by a compact companion as the progenitor that had a sufficient runaway velocity to reach the offset of SN 2024abvb,” the researchers conclude.

Regarding the explosion itself, the results indicate that the ejecta mass of SN 2024abvb was at a level of 2.59 solar masses, interacting with about 0.28 solar masses of CSM material. The astronomers note that these values are generally larger than other SNe Icn but consistent with the rapidly evolving SNe Ibn.

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