Study investigates rare Icn-like supernova

An international team of astronomers has made optical and near-infrared observations of a rare Icn-like supernova known as SN 2022ann. The results of the study, published on November 9 on the prepublication server arXivshed more light on the nature of this supernova and its unique properties.

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

The Icn SNe type is an extreme subtype of interacting stripped-envelope supernova (SESN). They have strong and narrow oxygen and carbon lines, but weak or absent hydrogen and helium lines, which presents additional complications for the extraction mechanism. They have narrow emission characteristics indicating circumstellar interaction.

To date, only five Icn SNe types have been discovered, and SN 2022ann is the latest addition to the short list of this SN subtype. SN 2022ann was detected on January 27, 2022 in the faint host galaxy SDSS J101729.72–022535, at a distance of about 710 million light-years.

Shortly after the explosion, a team of astronomers led by Kyle Davis of the University of California, Santa Cruz, began photometric and spectroscopic monitoring of SN 2022ann using various ground facilities.

“We presented optical photometry and optical/NIR spectroscopy of SN 2022ann, the fifth reported SN Icn, and its host galaxy, SDSS J101729.72–022535.6. (…) Our observations of SN 2022ann provide unique insight origins of the rarest SN explosions and undiscovered endpoints of stellar evolution,” the researchers wrote in the paper.

Observations show that the early optical spectra of SN 2022ann are dominated by narrow P-Cygni features of carbon and oxygen with absorption velocities of about 800 km/s. This is slower than other SNe Icn and less than the escape velocity of a compact massive star which is necessary to avoid strong hydrogen emission. Therefore, astronomers suggest that the progenitor star of SN 2022ann may have been “puffed up” by an explosion and out of hydrostatic equilibrium before the explosion.

The study found that SN 2022ann has a constant brightness unique to the early times after the explosion and a relatively rapid late decline of redder bands after this plateau. Additionally, SN 2022ann at the peak has a relatively low luminosity compared to the other four known Icn SNe types. Based on the bolometric light curve of SN 2022ann, researchers estimate that its ejecta mass is at a level of 1.73 solar masses.

According to the researchers, all of the results suggest that a binary star companion is needed to adequately strip the ancestor before the explosion and produce a low-velocity outflow like that of SN 2022ann. They added that the rarity of SNe Icn may indicate that they are created during a brief or uncommon step in binary evolution.

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