Ringed Disks Don’t Always Mean Planets

Ringed Disks Don’t Always Mean Planets

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Astronomers presenting at the winter American Astronomical Society meeting caution that the features often pointed to as evidence of growing natal planets could form on their own.

protoplanetary disk around HL Tauri

This striking image from the Atacama Large Millimeter/submillimeter Array reveals several gaps in the dusty disk around the star HL Tau. It wowed astronomers when released in 2014 as a quintessential example of planet formation at work, but since then others have cautioned that the gaps might not be from baby planets.
ALMA (ESO / NAOJ / NRAO)

As astronomers peer into the galaxy, they’re increasingly discovering young stars with ringed, dusty disks around them. The most iconic image is perhaps ALMA’s of HL Tau, a star only about a million years old surrounded by a large disk with several clear gaps. These gaps appear to be from forming planets, which sweep up the dust as they grow, cleaning out tracks.

But what if these disks aren’t what they seem?

New simulations by Alexander Richert (Penn State and NASA Goddard Space Flight Center), Wladimir Lyra (California State University Northridge and Caltech), and Marc Kuchner (NASA Goddard) suggest that interactions between starlight, dust, and gas might form these patterns all on their own, no planet required.

The team used a NASA supercomputer cluster to simulate what happens when a young star’s ultraviolet light interacts with its disk. The ultraviolet photons strip electrons from the dust, which then heat the surrounding gas. But hotter gas has a higher pressure. If the gas is dense enough, the change in pressure can create a “pressure trap” in the disk that drags the dust into rings, spirals, and arcs.

“The dust heats the gas, the gas pushes the dust,” Kuchner sums up. The effect, called the photoelectric instability, could explain odd things such as the “moving blobs” of the AU Mic disk and the rings and arcs around HD 141569A.

“I stand before you today a scientist torn by conflict,” he joked to journalists during a press conference January 11th at the American Astronomical Society meeting in Washington, D.C. For years he and his colleagues have struggled to understand what the features seen in young stellar disks tell us about the planets putatively forming there. But now, he warns that astronomers shouldn’t assume planets create these patterns.

NASA's Goddard Space Flight Center

Let It Snow

Earlier the same day, Joan Najita (NOAO) suggested that in some cases, ices might be to blame for these disk features. Water, carbon dioxide, and other “volatile” compounds are generally in gas form closer to stars, but they freeze out farther away. Each volatile freezes at a different distance from the star, distances called snow lines.

Najita points to work by Katrin Jos and Anders Johansen (Lund University, Sweden) that shows that, when vapor from within the snow line diffuses back out past the line, it can carry small dust grains with it. When it condenses farther out, the volatile attaches to particles already there, encouraging dust (and planetary building blocks called planetesimals) to clump up. In this scenario, rings could form at the snow-line positions, which vary from system to system depending on the star’s brightness and accretion rate. (Similar logic comes into play with stars’ habitable zones.)

If a disk’s gaps all fell on the expected snow-line positions for that system, then that might indicate ice is to blame, she says. And in fact, HL Tau’s rings correspond to the locations of different ice lines in the system — perhaps explaining why astronomers have had trouble finding planets in the gaps. But it might not be a simple either-or: if the gas chemistry encourages planetesimal growth in disks, then planet formation is involved, albeit indirectly.

What is increasingly clear is that a ringed disk does not necessarily mean a planet lurks within.

References:

Alexander J. W. Richert et al. “The Interplay Between Radiation Pressure and the Photoelectric Instability in Optically Thin Disks of Gas And Dust.” Posted to arXiv.org September 23, 2017.

Marc Kuchner. “Rings and Spirals Forming by Themselves: the Photoelectric Instability in Debris Disks and Transitional Disks, Now with Radiation Pressure.” 231st AAS meeting. Abstract 428.01.

Katrin Jos and Anders Johansen. “Ice Condensation as a Planet Formation Mechanism.” Astronomy & Astrophysics. April 2013.

Joan Najita. “Water in Planet-Forming Disks.” 231st AAS meeting. Abstract 301.01.