These debris disks often show sharply defined rings or spiral patterns, features that could signal the presence of orbiting planets. Astronomers study the disk features as a way to better understand the physical properties of known planets and possibly uncover new ones.
This chart compares the gas mass for several debris disk systems and shows where the photoelectric instability is most important. Systems like TW Hydrae contain so much gas that the instability is suppressed, but it could arise in relatively gas-free regions near the center of the disk.
Image Credit: NASA's Goddard Space Flight Center
But a new study by NASA scientists sounds a cautionary note in interpreting rings and spiral arms as signposts for new planets. Thanks to interactions between gas and dust, a debris disk may, under the right conditions, produce narrow rings on its own, no planets needed.
"When the mass of gas is roughly equal to the mass of dust, the two interact in a way that leads to clumping in the dust and the formation of patterns," said lead researcher Wladimir Lyra, a Sagan Fellow at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "In essence, the gas shepherds the dust into the kinds of structures we would expect to be see if a planet were present."
A paper describing the findings was published in the July 11 issue of Nature.
The warm dust in debris disks is easy to detect at infrared wavelengths, but estimating the gas content of disks is a much greater challenge. As a result, theoretical studies tend to focus on the role of dust and ice particles, paying relatively little attention to the gas component. Yet icy grains evaporate and collisions produce both gas and dust, so at some level all debris disks must contain some amount of gas.
"All we need to produce narrow rings and other structures in our models of debris disks is a bit of gas, too little for us to detect today in most actual systems," said co-author Marc Kuchner, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Md.
Here's how it works. When high-energy ultraviolet light from the central star strikes a clump of dust and ice grains, it drives electrons off the particles. These high-speed electrons then collide with and heat nearby gas.
The rising gas pressure changes the drag force on the orbiting dust, causing the clump to grow and better heat the gas. This interaction, which the astronomers refer to as the photoelectric instability, continues to cascade. Clumps grow into arcs, rings, and oval features in tens of thousands of years, a relatively short time compared to other forces at work in a young solar system.
A model developed by Lyra and Kuchner shows the process at work.
"We were fascinated to watch this structure form in the simulations," Lyra said. "Some of the rings begin to oscillate, and at any moment they have the offset appearance of dust rings we see around many stars, such as Fomalhaut."
In addition, dense clumps with many times the dust density elsewhere in the disk also form during the simulation. When a clump in a ring grows too dense, the ring breaks into arcs and the arcs gradually shrink until only a single compact clump remains. In actual debris disks, some of these dense clumps could reflect enough light to be directly observable.
"We would detect these clumps as bright moving sources of light, which is just what we're looking for when we search for planets," adds Kuchner.
The researchers conclude that the photoelectric instability provides a simple and plausible explanation for many of the features found in debris disks, making the job of planet-hunting astronomers just a little bit harder.Francis Reddy
Francis Reddy | EurekAlert!
Astronomers find unexpected, dust-obscured star formation in distant galaxy
24.03.2017 | University of Massachusetts at Amherst
Gravitational wave kicks monster black hole out of galactic core
24.03.2017 | NASA/Goddard Space Flight Center
Astronomers from Bonn and Tautenburg in Thuringia (Germany) used the 100-m radio telescope at Effelsberg to observe several galaxy clusters. At the edges of these large accumulations of dark matter, stellar systems (galaxies), hot gas, and charged particles, they found magnetic fields that are exceptionally ordered over distances of many million light years. This makes them the most extended magnetic fields in the universe known so far.
The results will be published on March 22 in the journal „Astronomy & Astrophysics“.
Galaxy clusters are the largest gravitationally bound structures in the universe. With a typical extent of about 10 million light years, i.e. 100 times the...
Researchers at the Goethe University Frankfurt, together with partners from the University of Tübingen in Germany and Queen Mary University as well as Francis Crick Institute from London (UK) have developed a novel technology to decipher the secret ubiquitin code.
Ubiquitin is a small protein that can be linked to other cellular proteins, thereby controlling and modulating their functions. The attachment occurs in many...
In the eternal search for next generation high-efficiency solar cells and LEDs, scientists at Los Alamos National Laboratory and their partners are creating...
Silicon nanosheets are thin, two-dimensional layers with exceptional optoelectronic properties very similar to those of graphene. Albeit, the nanosheets are less stable. Now researchers at the Technical University of Munich (TUM) have, for the first time ever, produced a composite material combining silicon nanosheets and a polymer that is both UV-resistant and easy to process. This brings the scientists a significant step closer to industrial applications like flexible displays and photosensors.
Silicon nanosheets are thin, two-dimensional layers with exceptional optoelectronic properties very similar to those of graphene. Albeit, the nanosheets are...
Enzymes behave differently in a test tube compared with the molecular scrum of a living cell. Chemists from the University of Basel have now been able to simulate these confined natural conditions in artificial vesicles for the first time. As reported in the academic journal Small, the results are offering better insight into the development of nanoreactors and artificial organelles.
Enzymes behave differently in a test tube compared with the molecular scrum of a living cell. Chemists from the University of Basel have now been able to...
20.03.2017 | Event News
14.03.2017 | Event News
07.03.2017 | Event News
24.03.2017 | Materials Sciences
24.03.2017 | Physics and Astronomy
24.03.2017 | Physics and Astronomy