Astronomers have gained new insight into the formation of planets around twin stars that orbit each other.
Despite the fact that we are most familiar with planets orbiting a central star – similar to the arrangement of our solar system – over 50% of the stars in the universe exist in a binary system, meaning they have a companion star. These binary systems can also have planets around them that either orbit one of the stars in a “circumstellar orbit” or orbit both stars in a much wider “circumstellar orbit”.
Using the Atacama Large Millimeter/submillimeter Array (ALMA) — made up of a combination of 66 radio telescopes located in northern Chile — and the 10-meter Keck II telescope in Hawaii, astronomers collected data on two twin star systems. What they found could change our understanding of the conditions that can either foster or hinder such planet formation in binary systems.
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The formation of binary stars is not very different from the formation of single stars. These bodies are created when dense clouds of cool interstellar gas form overdense blobs 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 prenatal cocoon of gas and dust until it gains enough mass to trigger the nuclear fusion of hydrogen into helium in its core, the process that defines a main-sequence star. Importantly, some of these interstellar clouds are large 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 those stars formed surrounds them as what astronomers call a “protoplanetary disk.” As the name suggests, planets form from these disks. Like the planets themselves, disks can be circumstellar, surrounding only one star, or circumstellar, surrounding the entire system.
Currently, scientists are not clear about the factors that allow these disks to stick around long enough for planets to be born, nor are they sure what ultimately causes the disks to disperse. As it turns out, circumstellar disks in pre-main-sequence binary systems of protostars may be ideal laboratories for investigating these questions.
The properties of these early disks, such as their sizes, substructures, and even their inclinations (compared to protostellar characteristics such as rotation speed and magnetic field strength) can reveal details of the complex interactions that shape such planet-birth environments.
In addition, the ubiquity of multi-star systems in the Universe means that studying the formation of planets around twin stars is vital to understanding this process at a deeper level.
One of the binary systems the team honed in on with ALMA and Keck II was DF Tau, made of two protostars with masses about 0.6 times that of the sun, located about 150 light-years from Earth in the formation region stars in Taurus.
The two DF Tau stars are separated by a distance equivalent to about 14 times the distance between Earth and the sun; they take about 44 Earth years to complete their highly elongated orbits.
Amazingly, ALMA found that the interstellar cloud responsible for the birth of these stars has split into two circumstellar disks. One is magnetically locked to the central star, DF Tau A, and is 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 have eroded as the young star spins rapidly.
This suggests to the team that there may be a connection between the spin of the young stars as well as the magnetic locking of the disks to them, and thus the early dissipation of those in the disks. In addition to this, it appears that discrepancies between the orbit of DF Tau, its circumstellar discs and the inclinations of its stars may affect the overall evolution of the disc.
The second binary system the team focused on was the very young FO Tau system at 2.8 million years old (for context, recall that the Solar System is 4.6 billion years).
This system is also approximately 450 light years away. Its stars, FO Tau A and B, are in a more circular orbit than those of DF Tau. They are also further apart, 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 FO Tau’s disks are aligned with the orbit of this binary system. Both stars show rotation rates on the slower side, and both circumstellar disks remain magnetically locked to their protostars. This suggests that systems like FO Tau, with slower stars and more circular orbits, may be better suited to forming planetary bodies around their two stellar components than fast systems with elongated orbits.
ALMA observations of other single and binary stellar disks have revealed complex substructures in 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, determining the larger-scale properties in these two close binary systems has greatly improved our understanding of the planet’s formation environment.
The team’s results were revealed at the 244th meeting of the American Astronomical Society (AAS).
