by Astronomers have gained new insight into the formation of planets around twin stars that orbit each other.
Despite the fact that we know most about the planets orbiting a single central star – similar to the layout of our solar system – over 50% of the stars in the cosmos are in a binary system, meaning that they have a companion star . These binary systems can also have planets around them that circle one of the stars in a “circumstantial orbit” or bend both stars in a much wider “curtain orbit”.
Using the Atacama Large Millimeter/submillimeter Array (ALMA) – made of a combination of 66 radio telescopes located in northern Chile – and the 10-meter Keck II telescope in Hawaii, astronomers collected data of two twin star systems. What they found could transform our understanding of the conditions that can foster or prevent the formation of such planets in binary systems.
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Binary star formation is not very different from single star formation. These bodies are formed when dense clouds of cool interstellar gas form dense patches that gather more mass, eventually collapsing under their own gravity to give birth to a baby star called a “protostar.”
This protostar continues to gather material from its natal cocoon of gas and dust until it has enough mass to trigger nuclear fusion of hydrogen in its core, the process that defines a main sequence star. Most importantly, some of these interstellar clouds are extensive enough to allow two or even three main-sequence stars to form within them.
Then, whatever material is left over from that cloud of gas and dust after these stars formed around them is what astronomers call a “protoplanetary disk”. As the name suggests, planets form from these disks. Like the planets themselves, the disks can be circumferential, around a single star, or curtain, around the entire system.
Scientists currently do not know the factors that allow these discs to stick around long enough for natal planets, and they are not sure what causes the discs to eventually disperse. As it turns out, circumstellar disks in binary pre-sequence-protostar systems could be ideal laboratories for investigating these questions.
The properties of these early disks, such as their sizes, substructures and even their inclination (compared to protostar characteristics such as rotation speed and magnetic field strength) can reveal details about the complex interactions that shape the birth environments of such planets.
Furthermore, the ubiquity of multi-star systems in the universe means that studying the formation of planets around multiple stars is essential to understanding this process at a deeper level.
One of the binary systems observed by the ALMA and Keck II team was DF Tau, made up of two protostars with masses about 0.6 times that of the Sun located about 150 light-years from Earth in the star-forming region of Taurus.
The two stars of DF Tau are separated by a distance equal to about 14 times the distance between Earth and the sun; they take about 44 Earth years to complete their very long orbits.
Amazingly, ALMA detected that the interstellar cloud responsible for the birth of these stars had split into two curtain disks. One is magnetically locked to the central star, DF Tau A, and actively feeds material to facilitate its growth. The other appears to have detached from the other star, DF Tau B. The central region of the disc appears to be eroding as the young star rapidly spins.
This gave the team the impression that there could be a link between the rotation of the young stars as well as the magnetic locking of the disks to them, and, therefore, the early dissipation of disks. In addition, it appears that misalignments between the orbit of DF Tau, its curtain discs, and the inclinations of the stars may affect the general evolution of the disc.
The second binary system the team focused on was the very young FO Tau system, 2.8 million years old (for context, remember that the solar system is 4.6 billion years old).
This system is also located about 450 light years away. Their stars, FO Tau A and B, are in more circular orbits than those of DF Tau. They are also more widely separated, with FO Tau B orbiting FO Tau A at a distance equal to about 22 times the distance between Earth and the sun.
Using ALMA, the astronomers discovered that FO Tau’s disks are aligned with the orbit of this binary. Both stars exhibit slowest rotational speeds, and both curtain disks remain magnetically locked to their protuberances. This suggests that systems like FO Tau, with slower stars and more circular orbits, may be better suited to forming planetary bodies around the two stellar components than fast systems with elongated orbits.
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ALMA observations of individual star disks and other binaries have revealed complex substructures within the disks, including features such as spiral patterns, gaps, and ring formations.
Although these structures are not currently visible for DF Tau and FO Tau, determining the larger, large-scale properties of these two close binary systems has greatly advanced our understanding of planet-forming environments.
The team’s findings were revealed at the 244th meeting of the American Astronomical Society (AAS).
