Study examines a rare Icn-type supernova

Study examines a rare Icn-type supernova

Search maps of SN 2022ann (right) and its host galaxy SDSS J101729.72–022535.6 (middle and left). Source: Davis et al., 2022.

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

Supernovae (SNe) are powerful and luminous stellar explosions. They are important to the scientific community because they provide essential clues about the evolution of stars and galaxies. In general, SNs are divided into two groups based on their atomic spectra: Type I and Type II. Type I SNs lack hydrogen in their spectra, while those of Type II show 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 that indicate circumstellar interaction.

Only five Icn-type SNe have been discovered to date, and SN 2022ann is the latest addition to the short list of this SN subtype. SN 2022ann was discovered 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-based 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 a unique insight into the origins of the rarest SN explosions and undiscovered endpoints in stellar evolution,” the researchers write in the paper.

The observations show that early optical spectra of SN 2022ann are dominated by narrow carbon and oxygen P-Cygni structures with absorption velocities around 800 km/s. This is slower than other SNe-Icn and below the escape velocity for a compact massive star required to avoid strong hydrogen emission. Therefore, the astronomers suggest that the progenitor star of SN 2022ann may have been “inflated” by an outburst and out of hydrostatic equilibrium before the explosion.

The study found that SN 2022ann exhibits a uniquely constant brightness early in time after the explosion and, after this plateau, a relatively rapid decline in the redder bands in the late period. In addition, SN 2022ann has a relatively low peak luminosity compared to the other four known Icn-type SNes. Based on SN 2022ann’s bolometric light curve, researchers estimate its ejecta mass to be at a level of 1.73 solar masses.

According to the researchers, all of the results suggest that a binary companion is required to adequately remove the progenitor before the explosion and create a low-velocity outflow like that in SN 2022ann. They added that the rarity of SNe Icn might indicate that they arose during a brief or unusual phase in binary evolution.

More information:
KW Davis et al, SN 2022ann: An Icn-type supernova from a dwarf galaxy revealing helium in its circumstellar environment, arXiv (2022). DOI: 10.48550/arxiv.2211.05134

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