Chinese, Australian astronomers detect key process of binary evolution

Photo taken on May 4, 2022 shows a view of the starry night sky as seen from the Mount Qomolangma base camp. (Xinhua/Jiang Fan)

Photo taken on May 4, 2022 shows a view of the starry night sky as seen from the Mount Qomolangma base camp. (Xinhua/Jiang Fan)

A joint research team of Chinese and Australian astronomers has detected a binary star system ejecting a common envelope, a key process of the binary star evolution, which could be of great importance to studies of the expansion of the universe and dark energy.

This is the first time that scientists have observed direct evidence of the key process of the evolution of the common envelope of binary stars. The study was published online in the Monthly Notices of the Royal Astronomical Society Thursday (Beijing Time).

Li Jiangdan, the first author of the article from the Yunnan Observatories of the Chinese Academy of Sciences (CAS), said a sun-like star in a binary system would evolve into a red giant star, and would eventually become a hot subdwarf star and then a white dwarf. The research team has found a binary system consisting of a hot subdwarf and a white dwarf, coded as J1920, about 23,000 lightyears from Earth.

In the system, the hot dwarf transfers its mass to the white dwarf via an accretion disk. The two stars get closer and spin with each other faster and faster. About 10,000 years ago, they ejected their common envelope, made up of gas, which is expanding and leaving the binary system at a speed of 200 km per second, Li said.

Han Zhanwen, the leader of the Chinese research team and an academician of the CAS, said that this binary system is like a double-yolk egg that is ejecting its egg whites.

"More than 50 percent of the stars in our universe are binaries. Therefore, understanding binaries is an important content of astrophysics," Han said.

Chen Xuefei, deputy director of the Yunnan Observatories, said binaries consisting of a hot subdwarf star and an accreting white dwarf are sources of gravitational wave radiation at low frequencies and possible progenitors of type Ia supernovae if the white dwarf mass is massive enough. Type Ia supernova is regarded by astronomers as the standard candle in measuring distances in the universe.

Brian Schmidt, the winner of the 2011 Nobel Prize in Physics, said, "Nearly 25 years ago, we measured cosmic distances with type Ia supernovae and discovered the accelerating expansion of the universe, which implies the existence of dark energy. Across the world, we are now refining our supernova measurements to better understand the expansion so as to glean the nature of dark energy. But to reach such a goal, we need a better understanding of type Ia supernovae. These exploding stars are from binary evolution, where the common envelope phase is crucial for their understanding."

"Common envelopes were first postulated in 1976, and have been widely used as the explanation for double black holes, double neutron stars, double white dwarfs and many other compact binaries. However, until this day, a common envelope has never been seen yet. This new detection provides a way to deepen our understanding of common envelope evolution," said Schmidt.

It is great to see this collaboration between Australian and Chinese astronomers yield such an exciting result, he added.

Zhao Gang, a researcher at the National Astronomical Observatories of the CAS, said that this discovery not only fills the gap in the study of binary evolution but also opens up a new research direction, making it possible for astronomers to study the early physical properties and states of dense celestial binary formation and evolution in the near future.