Astronomers report the discovery of a new supernova in the galaxy UGC 3855. The supernova was discovered using the HMT telescope at the Xingming Observatory in China. The discovery is detailed in a paper published October 7 on the preprint server arXiv. Supernovae (SNe) are powerful stellar explosions that can help us better understand the evolution of stars and galaxies. Astronomers divide supernovae into two groups based on their spectra: type I and type II. The spectrum of type I SNE lacks hydrogen, while type II SNE exhibits hydrogen spectral lines. Type Ib supernovae (SNe Ib) are a subclass of SNe that has lost its outer core. They form when a massive star, lacking an outer hydrogen shell, collapses under the influence of its own gravity. Additionally, astronomers distinguish ultrashort-shell SNe (USSNe), which exhibits similar spectral features but is weaker than SNe Ib/Ic. In these rare cases, the star’s crust has been severely damaged before the explosion.
Now, a team of astronomers led by Shenyu Yang of Tsinghua University in Beijing, China, reports the discovery of an unusual type Ib USSNe. The supernova was first identified by HMT on December 5, 2021, and was designated SN 2021agco. SN 2021agco was discovered about 130 million light-years away, in the relatively old intermediate spiral galaxy UGC 3855. The supernova is located about 15,600 light-years from the center of the galaxy. SN 2021agco grew very quickly, reaching a peak of -16.06 mag just 2.4 days after the explosion. The study found that the mass of SN 2021agco is approximately 0.26 solar masses and the kinetic energy of the supernova is estimated to be 95.7 million erg. The researchers calculate that the progenitor star SN 2021agco had an envelope with a radius of approximately 78.4 solar radii, a mass of 0.1 solar masses, and an ejected energy of 89.3 million billion ergs.
According to the paper’s authors, the results indicate that the progenitor star SN 2021agco lost a lot of mass and that much of its outer layer was stripped away before the explosion. The observations also revealed information about the properties of UGC 3855. Astronomers found that the galaxy is about 10.6 billion years old and has a mass of about 2.6 billion solar masses. This galaxy has a relatively low star formation rate, only 0.2 solar masses per year.