July 13, 2013
Disk Patterns Not Always Formed In Presence Of Planets
[WATCH VIDEO: It Doesn't Take A Planet To Make A Ring]
April Flowers for redOrbit.com - Your Universe Online
Young stars known to host planets generally also have disks containing dust and icy grains, particles produced by collisions among asteroids and comets also orbiting the star. Often, these debris disks have sharply-defined rings or spiral patterns, which could signal the presence of orbiting planets. As a way to better understand the physical properties of known planets and possibly uncover new ones, astronomers study these disk features.
A new NASA study, however, introduces a cautionary note in the interpretation of rings and spiral arms as signposts for new planets. The team discovered interactions between gas and dust may cause a debris disk, under the right circumstances, to produce narrow rings on its own without a planet present. The findings of this study were published in the journal Nature.
"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 Wladimir Lyra, a Sagan Fellow at NASA's Jet Propulsion Laboratory. "In essence, the gas shepherds the dust into the kinds of structures we would expect to be see if a planet were present."
At infrared wavelengths, it is easy for scientists to detect the warm dust in the debris disks. Estimating the gas content of disks, however, is a much greater challenge, which results in theoretical studies focusing on the role of dust and ice particle while paying relatively little attention to the gas component. Yet scientists know that 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 are a bit of gas, too little for us to detect today in most actual systems," said Marc Kuchner, an astrophysicist at NASA's Goddard Space Flight Center.
As high energy ultraviolet light from the central star strikes a clump of dust and ice grains the energy drives electrons off the particles, which then collide with and heat the nearby gas.
The drag force on the orbiting dust is changed by rising gas pressure. This causes the clump to grow and better heat the gas. This interaction is called the photoelectric instability, which continues to cascade causing clubs to grow into arcs, rings and oval features over tens of thousands of years -- a relatively short time compared to other forces at work in a young solar system.
Lyra and Kuchner created a model that shows this 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."
Dense clumps with many times the dust density found elsewhere in the disk also form during the simulation. If a clump in the simulated disk grows too dense, the ring breaks into arcs that gradually shrink until only a single compact clump remains. Some of these dense clumps in actual debris disks 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.
According to the study, 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.