Turbulence measured in the outer disk of HL Tauri reveals strong nonthermal motions

Star system: HL Tauri | Distance from the Sun: 185 light years

HL Tauri is a young star surrounded by a protoplanetary disk with rings and gaps, where planets may be forming. Understanding turbulence in such disks is important because it affects how material moves and how planets grow. Until now, the strength of turbulence in the outer parts of these disks was not well known, especially in systems still gathering gas from their surroundings.

Researchers used ALMA telescope data to study the disk around HL Tauri. They measured the width of a specific molecular line, which allowed them to separate the effects of temperature from other motions. After removing the thermal component, they found that the remaining, nonthermal motions are quite strong in the outer disk, with speeds around 0.15 kilometers per second. This turbulence is stronger farther from the star and is likely caused by gravitational instability or gas falling in from the surrounding envelope.

The study also found that the turbulence is not the same throughout the disk. The outer disk is much more turbulent than the inner regions, which are more settled. This suggests that turbulence varies with distance from the star and may be different in different directions within the disk.

Key findings
- Strong nonthermal turbulence detected in HL Tau's outer disk
- Turbulence increases with distance from the star
- Likely caused by gravitational instability or infalling gas
- Outer disk is more turbulent than the inner disk
- Turbulence may vary in different directions within the disk

Scientific study

Nonthermal Velocity Dispersion in the Outer Disk of HL Tau (external link)