A team of astronomers led by Professor HAN Zhanwen of the Yunnan Observatories (YAO) of the Chinese Academy of Sciences and Professor Chris Wolf of the Australian National University have jointly discovered a binary system ejecting a common envelope at a speed of about 200 kilometers per second.
This is the first time that scientists have observed direct evidence for the evolution of the common envelope, which is a key process in the evolution of binary stars. This important discovery provides a way to accurately characterize the evolution of the common envelope of binary stars through observation.
The study was published in Royal Astronomical Society Monthly Notices July 7.
Most luminous objects in the Universe are stars. More than half of all stars are in binary systems. Two stars in a binary system orbit each other due to their gravitational attraction.
Binary evolution plays an important role in determining the fate of stellar objects. It has been widely used as an explanation of most of the mysteries of astronomy and astrophysics such as the formation of exotic stellar objects including Type Ia supernovae, double black holes and double neutron stars, etc. .
Common envelope evolution is one of the key processes in binary evolution. In this process, the donor star of a binary system expands significantly due to loss of mass, causing the two stars to spiral towards each other and form a common envelope. This process determines the further evolution of the binary system. A binary system with a shorter orbital period should form if the common envelope is successfully ejected. Otherwise, the two stars inside the common envelope would merge into a single object.
Common envelopes were first postulated by B. Paczynski in 1976. However, a common envelope had never been seen before. As a result, scientists didn’t know exactly what was happening during the common envelope phase of binary star evolution.
Based on observations from the Australian National University’s 2.3-meter wide-field telescope and the Kepler telescope, Chinese and Australian scientists jointly found a binary system consisting of a hot sub-dwarf and a hot dwarf blanche, named J 1920. In this binary system, the two stars orbit each other with an orbital period of about 3.5 hours and get closer and closer.
Moreover, scientists have seen that this binary system is surrounded by an expanding shell moving at a speed of about 200 kilometers per second. This expanding shell is further confirmed to be a common shell that was ejected from the binary system around 10,000 years ago. The continuous orbital contraction observed in the J 1920 binary system indicates that the friction caused by the orbital motion of the two stars in the envelope can severely dissipate the orbital angular momentum. This is a new angular momentum loss mechanism in addition to magnetic breakdown and gravitational radiation mechanisms.
The significance of this important discovery is that it turns a theoretical idea into reality. Scientists not only saw the first evidence of common envelope evolution, but were also able to accurately characterize the evolution of the common envelope of binary stars through observation.
Royal Astronomical Society Monthly Notices
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A hot sub-dwarf filling the lobes of Roche and a white dwarf binary: possible detection of an ejected common envelope?
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