A team of astronomers including researchers from binary population synthesis group led by HAN Zhanwen at the Yunnan Observatories of Chinese Academy of sciences and researchers from the SkyMapper team led by Chris Wolf of the Australian National University has 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 the direct evidence of common envelope evolution, which is a key process of binary star evolution. This important discovery provides a way to accurately characterize the common envelope evolution of binary stars through observations. The work has been published online on July 7, 2022 in Monthly Notices of the Royal Astronomical Society.

Most of the luminous objects in the Universe are stars. More than half of the stars are found to be in binary systems. Two stars in a binary system orbit around each other due to their gravitational attraction. The binary evolution plays an important role in determining the fate of stellar objects. It has been widely used as the explanation for most mysteries in Astronomy and Astrophysics such as the formation of exotic stellar objects including Type Ia supernovae, double black holes and double neutron stars, and so on.

The common envelope evolution is one of the key processes of binary evolution, in which the donor star of a binary system expands dramatically due to the mass loss, leading to the in-spiral of the two stars towards each other and the formation of a common envelope (Fig. 1). The common envelope evolution determines the subsequent evolution of the binary system. A binary system with a shorter orbital period would expect to form if the common envelope is ejected successfully. Otherwise, two stars within the common envelope would merge into a single object. Common envelopes were first postulated by B. Paczynski in 1976. However, until this day, a common envelope has never been seen yet. As a consequence, it is quite unclear to scientists what exactly happens during the common envelope phase in binary star evolution.

Based on the observations from the Australian National University’s 2.3-meter wide-field telescope and the Kepler telescope, Chinese and Australian scientists have jointly found a binary system consisting of a hot subdwarf and a white dwarf, which is named J 1920. In this binary system, two stars orbit around each other with an orbital period of about 3.5 hours, and they are getting closer and closer (Fig. 2a). In addition, scientists have seen that this binary system is surrounded by an expanding shell moving at a speed of about 200 kilometers per second (Fig. 2b). This expanding shell is further confirmed to be a common envelope that was ejected from the binary system about 10,000 years ago (Fig. 2c). A continuous orbit contraction observed in binary system of J1920 indicates that the friction caused by the orbital motion of two stars in the envelope can severely dissipate orbital angular momentum, which is a new angular momentum loss mechanism in addition to the magnetic breaking and gravitational radiation.

The significance of this important discovery is that it turns a theoretical idea into reality. Scientists have not only seen the first observational evidence of common envelope evolution, but they have also been able to accurately characterize the common envelope evolution of binary stars through observations. This is a milestone discovery in the field of binary evolution, where scientists have previously been able to describe the common-envelope process only in broad terms, without precise numerical modeling and without direct observational evidence.

This work is partly supported by National Natural Science Foundation of China (Grant Nos. 12090040/3, 12125303).

Contact:
LI Jiangdan
Yunnan Observatories, CAS
Email: lijiangdan@ynao.ac.cn

Fig. 1 An illustration of common envelope evolution in binary star evolution.

 Fig. 2 (a): Orbital period of the binary system (i.e., J1920) as a function of time. (b) Ca II H&K lines in two selected spectra of J1920，which have been used to measure the speed of expanding envelope. (c) Evolutionary track for a binary system. (d) Schematic diagram of J1920, in which a binary system is ejecting a common envelope.