Checking the speed of spirals
by Susanna Kohler for AAS News
Washington DC (SPX) Nov 04, 2020
It's an old debate: gravity or companions? The spectacular spiral arms we see in some protoplanetary disks are likely caused by one or the other - and a recent study has taken a speedy approach to figuring out which.
What's Driving Patterns?
The formation of these arms remains an open question. Are they caused by a gravitational instability that drives a spiral pileup of disk material? Or are they induced by the presence of a hidden planetary companion orbiting within the disk?
Either way, the answer will reveal valuable information about the system. If the spirals are driven by a gravitational instability, then we can constrain the mass of the disk. If they're caused by the interaction of the disk with a planetary companion, we can infer the mass and location of the planet.
Past explorations of spiral arms have focused on examining static observations of disks. But a team of scientists led by Bin Ren (California Institute of Technology) has now taken a different approach, instead looking at how the arms of one disk move over time.
A Need for Speed ... Measurements
The authors were looking for one of two potential outcomes linked to the origin of the arms:
1/ If driven by a gravitational instability, the material of the arms will move at the local Keplerian speed, set by the mass of the central star. This means the inner parts of the arms will move quickly and the outer parts more slowly, winding the arms up ever more over time.
2/ If driven by a planetary companion, the arms will move as a solid structure that matches the speed of the companion.
Time for Planet Hunting
Based on imaging of the disk, the authors set upper limits of about 5 Jupiter masses for the hidden planetary companion that's driving the arms. The MWC 758 system makes an ideal target for future observations with current and upcoming telescopes to try to spot the planet driving its arms - especially now that we know where to look!
Susanna Kohler is the editor of AAS Nova.
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