CO line emission surfaces and vertical structure in a sample of protoplanetary disks

Protoplanetary disks are highly stratified in their physical and chemical properties with flared emitting surfaces set by hydrostatic equilibrium. Much of the line emission in disks originates not from the planet-forming midplanes but from these elevated surfaces. Emission surfaces are thus critically important to understand the connection between gas molecular abundances derived from line observations and gas molecular abundances in the midplane. Measurements of line emission heights also provide valuable input to disk thermochemical models and serve as useful diagnostics of planet-disk interactions. We used ALMA archival CO line data at high spatial resolution (~0.1 arcsec) to extract CO line emission surfaces in a sample of 10 mid-inclination (~30-75 deg) protoplanetary disks by exploiting spatially-resolved emission from elevated regions above and below the midplane. We found a wide range of vertical heights among disks, both in terms of absolute line emitting heights from a few tens of au to over 150 au and in flaring z/r values ranging from 0.1 to over 0.5. We fit parametric models to all emission surfaces and identified two tentative trends: sources with higher stellar masses tend to show less elevated CO emission surfaces, while those with larger CO gas disks exhibit more flared surfaces. Overall, these results demonstrate that high spatial resolution observations of CO lines in mid-inclination disks offer a powerful probe of disk vertical structure.