Debris discs in binary stars

About 20% of the stars have what the astronomers call a “debris disk” orbiting around them. Very much like the Kuiper belt or the asteroid belt around our sun, that are, respectively, the “home” of comets and asteroids. Those disks are composed of large planetesimals or bodies with sizes of ~1,000 km, which collide with each other. When there are such collisions, the planetesimals are destroyed, producing smaller and smaller bodies in the disk. Very small dust grains, down to the size of a micron -the width of a hair-, are released in the debris disks, and this is what observers are detecting using different kinds of telescopes and instruments. Imagine the dust that is lifted in the air when a building is demolished for example. What we are seeing around other stars is somewhat similar dust where the origin is planetesimals being destroyed instead of buildings being demolished.

However, about half of the stars in the universe are part of multiple systems -two or more stars that evolve in time and space together-, so, the same why that we know there are “tatooine” like exoplanetary systems, it is perfectly possible that each star of a multiple system may have its very own debris disk.

Recently, an international team of astrophysicists led by Philippe Thebault from the LESIA (Observatoire de Paris) with Quentin Kral (also from LESIA), and including Johan Olofsson, associated researcher from the Millennium Nucleus of Planetary Formation, studied the effect of the second star on the small dust grains of the debris disk from the first star. This week, this article was published in the prestigious scientific magazine Astronomy & Astrophysics.

“The main force that is at play in those systems is the gravitational force: the small dust grains feel the gravity from the primary star, but they also feel the gravity from the secondary star, which can have a significant impact”, explains Olofsson, who also leads the Max Planck MPIA-UV Tandem group. One of the main findings of the article, the scientist says, is that the secondary star quickly steals a fraction of the other star’s disc of debris. 

This work was done on the basis of simulations. “Even though the secondary star started the simulation without a debris disk, after several orbits around the primary star, it has gathered a significant fraction of small dust grains. In the simulations, we can also see that some structures are appearing, such as a spiral arm that seems to connect the main disk to the secondary disk, and there is also an extended halo all around the system”, Olofsson notes.

Although the halo and the spiral arm are probably way too faint to be detected with current instruments, the astrophysicist explains, it is proposed that the secondary disk (“stolen” by the second star) could be detectable with an instrument such as SPHERE, if the star is relatively close to Earth. However, the main challenge is that there are not as many stars that can be observed to test this hypothesis. 

“It would be great to actually observe such systems, but at the moment it is probably too early and we would probably need facilities such as James-Webb Space Telescopes (JWST) or the Extremely Large Telescopes (ELT) to have the sensitivity to see such signatures”, finishes Olofsson.

Scientific text
Image credit:  Philippe Thebault

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