An international team of astronomers has conducted optical and near-infrared observations of a rare Icn-type supernova known as SN 2022ann. Study results posted November 9 on a preprint server arXivsheds more light on the nature of this supernova and its unique properties.
Supernovae (SNe) are powerful, luminous stellar explosions. They are important to the scientific community because they provide essential evidence for 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 lacks hydrogen in its spectra, while type II displays hydrogen spectral lines.
Type Icn SNe is an extreme subtype of interacting supernovae (SESN). They have strong, narrow oxygen and carbon lines but weak or absent hydrogen and helium lines, which presents an additional complication of the stripping mechanism. They have narrow emitting features indicative of an envelope reaction.
To date, only five Icn SNe have been discovered, 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 optical and spectroscopic observations of SN 2022ann using various ground-based facilities.
“We have provided photophotometry and optical/NIR spectroscopy of SN 2022ann, the fifth reported SN Icn, and host galaxy SDSS J101729.72–022535.6. (…) Our observations of SN 2022 provide unique insight into the origins of the rarest SN bursts, 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 carbon and oxygen P-Cygni 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 necessary to avoid strong hydrogen emission. Therefore, astronomers suggest that SN 2022ann’s progenitor star may have been “bloated” by an explosion and out of hydrostatic equilibrium prior to the explosion.
The study finds that SN 2022ann has a uniquely stable brightness in the early times after the explosion and a relatively rapid late dip in the red bands after this plateau. Moreover, SN 2022ann peak has relatively low luminance compared to the other four known Icn SNe species. Based on the bolometric light curve of SN 2022ann, the researchers estimate its ejecta mass to be on the order of 1.73 solar masses.
According to the researchers, all results indicate that a binary star companion is required to sufficiently strip the progenitor before exploding and produce a low-velocity outflow like that found in SN 2022ann. They added that the rarity of SNe Icn may indicate that it was created during a short or uncommon stage in binary evolution.
KW Davis et al, SN 2022ann: an Icn-type supernova from a dwarf galaxy revealing helium in its surrounding environment, arXiv (2022). doi: 10.48550/arxiv.2211.05134
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