Astronomers have obtained new information about the formation of planets around twin stars orbiting each other.
Although we are familiar with planets orbiting a single central star (similar to the arrangement of our solar system), more than 50% of the stars in the universe are found in binary systems, meaning they have a companion star. Planets may also be found around these binary systems, either orbiting one of the stars in a “peristellar orbit” or looping both stars in a much larger “peripheral orbit.”
Astronomers collected data on two twin star systems using the Atacama Large Millimeter/submillimeter Array (ALMA), a combination of 66 radio telescopes in northern Chile, and the 10-meter Keck II telescope in Hawaii. What they find could change our understanding of the conditions that foster or hinder such planet formation in binary systems.
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Binary star formation is not much different from single star formation. These objects form when dense clouds of cold, interstellar gas form extremely dense patches that accumulate more mass and eventually collapse under their own gravity, giving birth to a starlet called a “protostar.”
This protostar continues to accumulate material from its prenatal cocoon of gas and dust until it reaches sufficient mass to trigger nuclear fusion of hydrogen into helium in its core; This process is what defines a main sequence star. More importantly, some of these interstellar clouds are large enough to allow two or three main sequence stars to form within them.
Then, after the formation of these stars, the material left over from this cloud of gas and dust surrounds them in what astronomers call a “protoplanetary disk.” As the name suggests, planets consist of these disks. Like the planets themselves, the disks may orbit the star, surround just one star, or be binary surrounding the entire system.
Scientists are currently unaware of the factors that allow these disks to persist long enough for planets to form, and they are not sure what ultimately causes the disks to break up. It appears that circumstellar disks in pre-main-sequence protostar binary systems may be ideal laboratories for investigating these questions.
Properties of these proto-discs, such as their size, substructure, and even inclination (compared to protostar properties such as rotation speed and magnetic field strength), can reveal details of the complex interactions that shape such planet-birth environments.
Additionally, the ubiquity of multiple star systems in the universe means that studying planet formation around twin stars is vital to understanding this process at a deeper level.
One of the binary systems the team focused on with ALMA and Keck II was DF Tau, a pair of protostars with a mass about 0.6 times that of the Sun, located about 150 light-years from Earth in the Taurus star-forming region.
DF Tau’s two stars are separated by a distance equivalent to about 14 times the distance between the Earth and the sun; It takes them approximately 44 Earth years to complete their extremely long orbit.
Fascinatingly, ALMA detected that the interstellar cloud responsible for the birth of these stars had split into two circumstellar disks. One of these magnetically locks onto the central star DF Tau A, actively feeding it material to facilitate its growth. The other appears to have separated from the other star, DF Tau B. The central region of the disk appears to be eroded as the young star rotates rapidly.
This suggested to the team that there might be a connection between the rotation of young stars and the magnetic locking of disks onto them, and thus the early disintegration of disks. In addition, misalignments between DF Tau’s orbit, circumstellar disks, and the inclinations of its stars appear to affect the overall evolution of the disk.
The second binary system the team focused on was the very young, 2.8-million-year-old FO Tau system (for context, remember that the solar system is 4.6 million years old). billion years old).
This system is also located approximately 450 light-years away from us. Its stars, FO Tau A and B, are in a more circular orbit than that of DF Tau. They are also more widely separated from each other, with FO Tau B orbiting FO Tau A at a distance equivalent to about 22 times the distance between Earth and the sun.
Using ALMA, astronomers found that the disks of FO Tau align with the orbit of this binary. Both stars exhibit rotation rates on the slower side, and the two circumstellar disks remain magnetically locked to their protostar. This suggests that systems such as FO Tau with slower stars and more circular orbits may be better suited to the formation of planetary bodies around both stellar components than fast systems with elongated orbits.
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ALMA observations of other single and binary star disks have revealed complex substructures within the disks, including features such as spiral patterns, voids, and ring formations.
Although these structures are not currently visible for DF Tau and FO Tau, identifying larger-scale features in these two close binary systems has significantly improved our understanding of planet-forming environments.
The team’s results were announced at the 244th meeting of the American Astronomical Society (AAS).