SN 2021agco Discovered As the Closest Ultrastripped Supernova on Earth, Chinese Observatory Studies Its Host Galaxy UGC 3855

A team of scientists has discovered a new ultrastripped supernova, which was detected using the Half Meter Telescope (HMT) at the Xingming Observatory in China.

Types of Supernovae

Supernovae (SNe)refer to dramatic explosions during the final stages of the death of a supermassive star. During this stage, the star may reach a maximum intrinsic luminosity millions of times its normal level. The powerful and luminous stellar explosions can help scientists gain better insights about the evolutions of stars and galaxies.

Astronomers divide supernovae into two categories 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.

Supernovae are classified as Type I if their light curves show sharp maxima and then gradually die away. Their maxima can be about 10 billion solar luminosities. Meanwhile, Type II supernovae have less sharp peaks with maxima at around one billion solar luminosities. They also die away more sharply than the Type I supernovae.

There is a subclass of stripped-envelope core-collapse SNe called Type Ib supernovae (SNe Ib). These are formed when a massive star, stripped of its outer hydrogen envelope, collapses under its gravity. Experts also distinguish ultra stripped-envelope SNe (USSNe), which reveal similar features resembling those of SNe Ib/Ic but are relatively faint. In these rare SNe, the progenitor explosion has been extremely stripped before exploding.

Discovery of SN 2021agco

A group of astronomers led by Shengyu Yan of the Tsinghua University in Beijing, China, has discovered a peculiar Type Ib USSNe. The ultrastipped supernova was first observed with HMT on December 5, 2021, and was named SN 2021agco.

The rare supernova was found at a distance of 130 million light years. It is about 15,600 light years from the center of a relatively old intermediate spiral galaxy called UGGC 3855. SN 2021agco is characterized by a swift evolution, which reaches a peak of -16.06 mag within only 2.4 days after exploding.

Further analysis revealed that the ejecta mass of this supernova was about 0.26 solar masses while its kinetic energy was at a level of 95.7 quintillion erg. The scientists estimated that the progenitor of SN 2021agco had an envelope with a radius of almost 78.4 solar radii, injection energy of 89.3 quintillion erg, and a mass of 0.1 solar masses.

These figures suggest that the progenitor of the ultra-stripped supernova suffered violent mass loss and that most of its outer layer was stripped before exploding. The results show that SN 2021agco is considered the closest object to Earth under the SN Ib subtype.

The assessments done on the rare supernova also reveal some information about the properties of its host. Researchers discovered that SN 2021agco is approximately 10.6 billion years old, possesses 2.6 billion solar masses, and has a relatively low star-formation rate of 0.2 yearly solar masses.

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