NPF researchers characterize the production of dust grains in a young debris disk

This disk surrounds the star HR4796A. The observations were made with SPHERE/ZIMPOL at the VLT, located in Chile.

The formation of a debris disk is the natural consequence of the star and planet formation processes. When a star is formed, gas and very small dust grains are distributed in a disk around it, and planets, like in our solar system, will form within this gas-rich disk, but the gas will be removed within the first millions of years. On the other hand, the dust grains, that were once very small, can become larger bodies, the size of Pluto, or even larger, what astronomers call “planetesimals”.

After 10 million years, the disk becomes less massive, but the planetesimals are still there and collide with each other. In these collisions, smaller and smaller dust grains are produced, and that is why these discs are called “debris discs” (the dust is referred to as “second generation” dust). Hundreds of debris discs are known, but this process of planetesimal collisions and crucial details thereof remains largely unrestricted from the point of view of observation.

Therefore, an international group of astronomers, led by Johan Olofsson, associate researcher of the NPF and leader of the Max Planck MPIA-UV Tandem group, studied the debris disk around the HR4796A star, to understand how the observed small dust grains are produced in the debris disk that surrounds the young star. The research was published in the prestigious journal Astronomy & Astrophysics and the study also included Amelia Bayo, director of the NPF, and postdoctoral researchers Juan Carlos Beamín and Matias Montesinos, graduate students Daniela Iglesias and Catalina Zamora, and the assistant director of the center Matthias Schreiber.

“It has been known for decades that the disk around HR4796A is slightly eccentric -not perfectly circular- which means that one side is closer to the star. We found that dust is preferably produced near the pericenter (point of the orbit closest to the star), while some simulations show that it should rather be produced near the apocenter (furthest point of the orbit), due to the difference in time that the particles pass in those two regions. On the apocenter side, any bodies, dust grains or planetesimals, orbits at smaller speeds and, therefore, should spend more time there. The natural expectation is that small dust grains are preferably produced at the furthest point of the star ”, says Johan Olofsson.

The astronomer adds that there are several possibilities to explain the results. “The first is that at the pericenter the orbital velocities are larger since this is the region closest to the star. Therefore, any collision between two planetesimals can be more destructive and, therefore, release more dust grains. An alternative explanation is that a catastrophic collision has occurred, thousands of years ago, between two massive planetesimals. All fragments released during that collision would have to go through the same point in each orbit, and this would increase the chances of having more collisions between those fragments later”, he says.

For the investigation, the SPHERE / ZIMPOL instrument was used, installed in the Very Large Telescope (VLT), on Cerro Paranal, Chile. The observations were made in the optical, detecting the light coming from the star, scattered by the very small dust grains in the disk. “The typical size of those dust grains is thinner than a human hair, and it is very interesting because such small dust grains are sensitive to the radiation pressure of the star. The size of their orbits depends on two things: where they were produced in the disk and their own size. The orbit of a grain of 1 micron in size will extend more than the orbit of a grain of 5 microns”, explains Olofsson, who is also a researcher at the Institute of Astronomy and Astrophysics at the University of Valparaíso.

Therefore, says the scientist, a model of the disk can be made, taking into account all these effects, to try to characterize how dust grains are produced in it.

“We present a new model for the disk around HR4796A, using state-of-the-art instruments. Our model can consistently reproduce the observations of the SPHERE instrument, but also that of most other available observations, such as those of ALMA, at millimeter wavelengths”, explains Olofsson. “This new model and our results provide new insights about what may have happened in the early stages of the solar system evolution”, he concludes.

You can read the research here. Also, we invite you to see the outreach video, produced by Johan Olofsson.

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