Striking images of protoplanetary disks with unique arced and spiral
features, captured by the SPHERE instrument on ESO’s Very Large
Telescope. [ESO]
Some of the most spectacular images to come out of observatories like
the Atacama Large Millimeter/submillimeter Array (ALMA) or the Very
Large Telescope (VLT) are detailed views of protoplanetary disks. These
disks of gas and dust around young stars aren’t just smooth and
featureless; instead, they exhibit arcs, rings, gaps, and spirals. What
causes this impressive array of structure?
Scientists have primarily focused on two explanations:
- The structures are caused by the perturbations of massive baby planets interacting with the disk as they orbit.
- The structures are generated by various instabilities within the disk that cause the gas and dust to clump.
A new study has now put forward an alternative explanation: the
structures are the result of catastrophic, destructive collisions of
planetesimals within the disk. Scientists Tatiana Demidova (Crimean
Astrophysical Observatory) and Vladimir Grinin (Pulkovo Observatory of
the Russian Academy of Sciences; St. Petersburg University, Russia) lay
out their scenario of destruction in a recent publication.
This ALMA image of the protoplanetary disk surrounding the star HL Tauri reveals the detailed substructure of the disk. [ALMA (ESO/NAOJ/NRAO)]
Outcome of a Crash
Collisions of large bodies — planetesimals and planetary embryos — are likely common during the formation of planetary systems around young stars. Some gentle collisions may help build up the mass of these bodies as they grow into planets. But objects that smash together at high enough velocities will be completely destroyed in the process, generating an expanding cloud of many smaller bodies and particles.
This cloud won’t remain stationary, however; instead, it will
continue to orbit within the protoplanetary disk. Due to the different
speeds of the various particles, the initial debris clump should be
sheared out into arced structures that might persist for multiple disk
orbits.
Could this process faithfully reproduce the disk structures that
we’ve observed with ALMA or the VLT? Demidova and Grinin conduct
simulations to find out.
Simulated 1.3-mm observations of the evolution of an expanding debris cloud over 40 orbital periods of the cloud center. Time steps advance from top left to bottom right panel. Click to enlarge. [Demidova & Grinin 2019]
Dragging Debris in a Disk
By modeling an expanding debris cloud within a disk that starts at a
distance of 30 AU from its solar-mass star, the authors show how the
dust and gas will evolve over several disk orbits. They then produce
simulated observations of the results at a wavelength of 1.3 mm.
The result? Demidova and Grinin find that as the dust cloud stretches, it
successively reproduces all three structures we’ve seen in
protoplanetary disks — first, it shapes into an arc, then a tightly
wound spiral, and eventually into a ring. The simulated observations at
1.3 mm look very similar to various disk images we’ve captured.
There are still many open questions about the structure of the disks
around young stars, but this work shows that there are also many
potential answers. As planetary systems form, collisions may both grow
planetary embryos and destroy them, possibly causing some of the disk
features that we’ve observed. One thing is for certain: the environment
around young stars is certainly dramatic!
Citation
“Catastrophic Events in Protoplanetary Disks and Their Observational
Manifestations,” Tatiana V. Demidova and Vladimir P. Grinin 2019 ApJL 887 L15. doi:10.3847/2041-8213/ab59e0
