Protoplanetary disks - the birthplace of planets and planetary systems - are accretion disks: gas drifts toward the central star. The latest computer models show that this accretion is induced by large-scale magnetic fields. Sometimes, the disk can exhibit a strong inwards radial flow at its midplane, as shown in the above figure, and develop "pressure bumps". In planet formation theory, pressure bumps are often invoked as sites where planetesimals, the building blocks of planets, can form efficiently. But is this true when the gas is accreting?
In this collaboration with my research assistant Chun-Yen Hsu, we looked into this problem by developing models of the "streaming instability" (SI) in accreting disks. The SI is probably the most popular candidate for planetesimal formation but in conventional theory, it requires a pressure gradient and so would not occur at a pressure bump. However, we found was that if the gas is undergoing accretion, then the SI can develop!
The above figure shows SI growth rates outside a pressure bump (left) and at the bump (right). If the gas wasn't accreting, then the right panel would simply be blank. However, a different version of the SI still develops, where it is powered by the difference in the azimuthal motions of the dust and gas; whereas for the classic SI, it is the relative radial motion.
Our finding shows that pressure bumps are indeed viable sites of planetesimal formation, but the SI takes on a different form than the classic version. Chun-Yen is currently developing simulations to study the nonlinear evolution of this new SI to see if it can indeed produce particle clumps dense enough for gravitational collapse.