Investigating the Relative Gas and Small Dust Grain Surface Heights in Protoplanetary Disks

Dust evolution in protoplanetary disks from small dust grains to pebbles is key to the planet formation process. The gas in protoplanetary disks should influence the vertical distribution of small dust grains (∼1 μm) in the disk. Utilizing archival near-infrared polarized light and millimeter observations, we can measure the scale height and flare parameter β of the small dust grain scattering surface and ¹²CO gas emission surface for three protoplanetary disks: IM Lup, HD 163296, and HD 97048 (CU Cha). For two systems, IM Lup and HD 163296, the ¹²CO gas and small dust grains at small radii from the star have similar heights, but at larger radii (>100 au), the dust grain scattering surface height is lower than the ¹²CO gas emission surface height. In the case of HD 97048, the small dust grain scattering surface has similar heights to the ¹²CO gas emission surface at all radii. We ran a protoplanetary disk radiative transfer model of a generic protoplanetary disk with TORUS and showed that there is no difference between the observed scattering surface and ¹²CO emission surface. We also performed analytical modeling of the system and found that gas-to-dust ratios larger than 100 could explain the observed difference in IM Lup and HD 163296. This is the first direct comparison of observations of gas and small dust grain height distribution in protoplanetary disks. Future observations of gas emission and near-infrared scattered-light instruments are needed to look for similar trends in other protoplanetary disks.