The Impact of Gas on Dust Grains in Debris Disks

An international group of astrophysicists, led by NPF researcher Johan Olofsson, concluded that the presence of gas in these disks affects the dynamics of most grains, especially the smaller ones.

The main components of the debris disks, young analogs to the Kuiper belt in the Solar System, are kilometer-sized bodies known as planetesimals. These objects collide with each other and release a large amount of small dust grains. From research, we also know that some of these disks contain some gas, whose origin is not always clear. 

These debris disks are not perfectly flat, and have a non-negligible vertical height. Recently, an international team of astrophysicists led by Johan Olofsson, a research associate at the Nucleus Millennium of Planetary Formation (NPF), studied the vertical structure of these debris disks to investigate how it is modified when gas is present in the system.

The study, published in the prestigious scientific journal Monthly Notices of the Royal Astronomical Society, also involved Amelia Bayo, director of the NPF; Nicolás Godoy, PhD student and member of the center; Karina Mauco, postdoctoral researcher; and Matias Montesinos, adjunct researcher. 

The research consisted of two parts. First, observations of eight disks oriented sideways with respect to an observer on Earth were analyzed to constrain their vertical thickness and use models to try to reproduce the observed geometry. In the second part, numerical simulations were used to quantify the impact that gas can have on dust particles. Once the simulation was completed, images of the debris disks were made to simulate observations with the SPHERE instrument of the Very Large Telescope at Paranal or with the ALMA interferometer in the Atacama Desert.

The main results of this research are that the presence of gas in a debris disk affects the dynamics of most grains, but especially those of small and intermediate size, between ~1 and 20 microns  (smaller than the width of a hair). “These grains will move far away from the central star because they are pushed both by radiation pressure  from the central star and by the additional force exerted by the gas on the small particles.. In addition, their altitude in the vertical direction will also decrease due to the effect of the gas,  leading to a thinner disk” says Olofsson, who also leads the Max Planck MPIA-UV Tandem group and is a member of the Institute of Physics and Astronomy at Valparaiso University.

The astrophysicist adds that, in the case of larger grains, with sizes of 100 to 500 microns, the effect of both, radiation pressure and gas drag, are smaller.  “The effect of the interaction with the gas takes longer to become significant, so those grains will not migrate very far and will be destroyed by collisions before their altitude can decrease significantly. However, in the simulations with a large gas mass gas mass, all the grains moved outward more efficiently, because the force exerted by the gas is stronger. Being far away from the place where they were released, they can survive longer and their altitude can decrease,” he explains.

For Olofsson, this is an interesting result because it could help to better understand the origin of this gas. “We are not entirely sure whether the gas in the debris disk is of primordial origin or of secondary origin. In the former, it would be gas that was present in the younger disk and was not removed from the system. Such a gaseous disk would be expected to be quite massive. In the second scenario, most of the gas in the protoplanetary disk has dissipated, and a new generation of gas can be created by collisions between planetesimals that are covered with ices (e.g., comets). In this second scenario, we would expect a much smaller amount of gas,” says Olofsson.

Future work

Unfortunately, the scientist points out, it is currently very difficult to precisely constrain the vertical structure of debris disks from observations. “We have fantastic instruments like SPHERE and ALMA, but we don’t reach an angular resolution that allows us to say whether a disk is flatter than expected. This can be done, to some extent, for disks that are close to the Earth, but, unfortunately, those disks have no gas, so we cannot yet confirm our predictions from the numerical simulations,” says Johan Olofsson. 

However, the researcher explains that with SPHERE, for example, there is a technique to transform the telescope into a small interferometer, equivalent to dividing a large telescope into several smaller ones. This technique makes it possible to increase the angular separation. If the disk geometry is favorable, then the vertical thickness can be better measured, thanks to the better resolution of the observations.

In the case of ALMA observations, Olofsson continues, in most cases the full capabilities of the facility are not used. “It is possible to increase the angular resolution by a factor 2 or 3, but you would have to observe the disks for much longer. And since debris disks are usually quite faint, this is quite a challenge, although there are several promising targets that we could try to observe in the near future,” he concludes.

Scientific publication 

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