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

Protoplanetary disks exhibit a rich chemical structure over their heights with molecular line emission originating from elevated surface layers. A detailed understanding of this vertical structure is required to interpret kinematic signals in CO emission and the effects of embedded protoplanets on the density distribution, temperature, and pressure of gas in disks. Moreover, it is critical to determine how well connected molecular gas abundances derived from line observations are to their abundances in planet-forming midplanes. To this end, the Molecules with ALMA at Planet-forming Scales (MAPS) ALMA Large Program provided a unique opportunity to study the vertical distribution of gas, chemistry, and temperature in the protoplanetary disks around IM Lup, GM Aur, AS 209, HD 163296, and MWC 480. By using the asymmetry of molecular line emission relative to the disk major axis, we infer the emission height above the midplane as a function of radius. Using this method, we measure emitting surfaces for a suite of CO isotopologues. We find that 12CO emission traces the most elevated regions with z/r > 0.3, while emission from the less abundant 13CO and C18O probes deeper into the disk at altitudes of z/r < 0.2. We determine peak brightness temperatures of the optically thick CO isotopologues and use these to trace 2D disk temperature structures. Several CO temperature profiles and emission surfaces show dips in temperature or vertical height, some of which are associated with gaps and rings in line and/or continuum emission. These substructures may be due to local changes in CO column density, gas surface density, or gas temperatures, and detailed models are necessary to better constrain their origins. Overall, we show that emission surfaces in a set of lines with varying optical depths, such as CO isotopologues, provide direct observational constraints on the overall 2D disk structure and establish a powerful new method of probing the vertical distribution of gas in disks.