Astronomers spotted unusual stellar explosion rich in oxygen and magnesium

Supernova explosions are produced at the deaths of massive stars. The elements seen in a supernova reflect the composition of the dying star at the time of explosion.

"Stars are glowing balls of gas of mostly hydrogen, the lightest element in nature. They shine by fusing atomic nuclei together to create heavier elements and energy," explains Academy of Finland Research Fellow Hanindyo Kuncarayakti from the Department of Physics and Astronomy at the University of Turku, Finland.

Massive stars, which have around 8 times the mass of the Sun or more, contain structures similar to an onion, with layers of different elements inside them. As we go deeper inside a star, we encounter layers of subsequently heavier elements than hydrogen, such as helium, then carbon, oxygen, and so on.

"During its lifetime, a star may lose some, or even most, of its mass. The most common way is through ejecting streams of particles, a process known as stellar winds, which occur also in the Sun. Some stars lose their mass very vigorously, and may completely strip all of their hydrogen envelope. As a result, the inner layers may become exposed. The mass lost by the star may remain in the vicinity of the star, creating circumstellar matter," says Kuncarayakti.

Astronomers have previously identified supernovae with circumstellar matter rich in hydrogen, as well as those rich in helium. Very recently, only in 2021, researchers have discovered supernovae with carbon-oxygen circumstellar matter. These different kinds of objects represent a sequence of stellar envelope stripping and the accumulation of stripped matter around the star, starting from the lightest and outermost element -- hydrogen.

A team led by Academy Research Fellow Kuncarayakti has discovered a supernova that possibly extends our understanding of this sequence where massive stars lose their mass. Supernova (SN) 2021ocs was observed in a survey using the 8.2-m European Southern Observatory (ESO) Very Large Telescope (VLT) in Chile.

"The spectrum looked like nothing we have seen before. It had strong features of oxygen and magnesium, and the object was unusually long-lasting and blue," Kuncarayakti describes.

These observations suggest that the oxygen-magnesium-rich expanding gas from the explosion of SN 2021ocs could be crashing into circumstellar matter. Such circumstellar matter could have been formed by the precursor star via mass loss only around 1,000 days prior to the supernova explosion. As such, the observations act like a time machine, probing the dying star's activities shortly before the final explosion.

"By observing new types of supernovae, we gain valuable information about the later stages of life of massive stars. This, on the other hand, creates new challenges for our theories on stars' evolution," says Professor of Astronomy Seppo Mattila from the University of Turku who also participated in the study.

In addition to Kuncarayakti and Mattila, researchers Takashi Nagao, Claudia Gutierrez and Rubina Kotak from the University of Turku contributed to the study.