Tracing the Vertical Structure of Protoplanetary Disks with High Spatial Resolution CO Line Emission Observations

Charles J Law, Karin I Oberg, Richard Teague

Observations of molecular lines in protoplanetary disks provide the best probes of characteristics relevant to planet formation, such as surface density, ionization, temperature, and C/N/O ratios. However, line emission originates from elevated surface layers and thus, a detailed knowledge of this vertical structure is critical in interpreting these observations. Planet-forming disks at mid-inclinations (30?75°) offer an opportunity to directly measure the height of bright molecular lines, as the high angular resolution of ALMA allows us to spatially resolve elevated emission above and below the midplane. Using this approach, we mapped this vertical structure, from midplane to disk atmosphere, by extracting emitting heights of several CO isotopologues (12CO, 13CO, C18O) in five disks as part of the Molecules with ALMA at Planet-forming Scales (MAPS) Large Program. These surfaces were then used to derive 2D gas temperatures in each disk, which served as critical inputs to thermochemical models. We developed an automated code to facilitate the extraction of emitting surfaces, which was then applied to a large sample of disks with ALMA archival data. Using this extended sample, we find that disks do not possess a single characteristic 12CO emitting height but span a wide range of altitudes (z/r ? 0.1 to ?0.5) and that emitting heights are strongly correlated with the overall gas disk size. Since the vertical gas distribution in disks influences the chemical environments in which planets assemble, these findings imply substantial diversity in local planet-forming environments across different disks. Overall, we show that CO line emission surfaces provide direct observational constraints on the 2D disk structure and note that such a method can naturally be extended to other important molecular tracers of disk chemistry and structure.