Puffed-up Inner Rings and Razor-thin Outer Rings in Structured Protoplanetary Disks

The vertical distribution of pebbles in protoplanetary disks is a fundamental property influencing planet formation, from dust aggregation to the assembly of planetary cores. In the outer region of protoplanetary disks, the intensity of the optically thin but geometrically thick dust ring decreases along the minor axis due to reduced line-of-sight optical depth. Multi-ring disks thus provide an excellent opportunity to study the radial variation of the vertical properties of dust. We investigate the vertical dust distribution in six protoplanetary disks with resolved double rings, using high-resolution Atacama Large Millimeter/submillimeter Array Band 6 continuum observations. By modeling the azimuthal intensity variations in these rings, we constrain the dust scale heights for each ring. Our results reveal a dichotomy: inner rings exhibit puffed-up dust layers with heights comparable to the gas scale height, while outer rings are significantly more settled, with dust scale heights less than 20% of the gas scale height. This suggests a radial dependence in dust settling efficiency within the disks, potentially driven by localized planetary interactions or the global radial dependency of the vertical shear instability. We discuss the implications of these findings for dust trapping, planet formation, and protoplanetary disk evolution. Our work highlights the importance of vertical dust distribution in understanding the early stages of planet formation and suggests that outer (>80 au), settled rings are preferred sites for planet formation over inner (<80 au), turbulent rings. © 2025. The Author(s).