Are Small Dust Grains Actually Coupled to the Gas in Protoplanetary Disks?

Dust evolution in protoplanetary disks from small dust grains to pebbles is key to the planet formation process. When modeling and interpreting these protoplanetary disks, a common assumption is that small dust grains (~ 1 micron) are strongly coupled to the gas. We can now independently measure the gas's height and the hight of small dust grains in protoplanetary disks and test the dust gas coupling assumption. Utilizing near-IR polarized light and sub-mm ALMA observations, we measured the scale height and flare angle of the small dust grains and ¹²CO gas for three protoplanetary disks. We found that in two of the three protoplanetary disks we studied, the gas and the small dust grains have different flare angles, contrary to what is commonly assumed. We computed Monte Carlo Radiative Transfer modeling of a simple protoplanetary disk to show that the flare angle discrepancy is not an observational effect. Finally, simple analytical modeling of protoplanetary disks indicates several disk mechanisms could be responsible, including a difference in gas to dust ratios. This study is the first observational investigation of heights of gas and small dust grains in protoplanetary disks. Future observations with ALMA and near-IR scattered light instruments are needed to see if this trend continues in a larger sample of protoplanetary disks.