Directly tracing the vertical stratification of molecules in protoplanetary disks

Context. The specific location from where molecules emit in a protoplanetary disk depends on the system properties. Therefore, directly constraining the emitting regions radially, azimuthally, and vertically is key to studying the environment of planet formation. Due to the difficulties and lack of high resolution observations, most of the current observational constraints for the vertical distribution of molecular emission rely on indirect methods.
Aims: We aim to directly trace the vertical location of the emitting surface of multiple molecular tracers in protoplanetary disks. Our sample of disks includes Elias 2-27, WaOph 6, and the sources targeted by the MAPS ALMA Large Program. The set of molecules studied includes CO isotopologues in various transitions, HCN, CN, H₂CO, HCO⁺, C₂H, and c-C₃H₂.
Methods: The vertical emitting region is determined directly from the channel maps by tracing the location of emission maxima along the upper surface. This method has been used in previous studies, but here we implement an accurate masking of the channel emission in order to recover the vertical location of the emission surface even at large radial distances from the star and for low-S/N lines. Parametric models are used to describe the emission surfaces and characterize any structure within the vertical profile.
Results: The vertical location of the emitting layer is obtained for ten different molecules and transitions in HD 163296. In the rest of the sample it is possible to vertically locate between four and seven lines. Brightness temperature profiles are obtained for the entire sample and for all CO isotopologues. IM Lup, HD 163296, and MWC 480 ¹²CO and ¹³CO show vertical modulations, which are characterized and found to be coincident with dust gaps and kinematical perturbations. We also present estimates of the gas pressure scale height in the disks from the MAPS sample. Compared to physical-chemical models, we find good agreement with the vertical location of CO isotopologues. In HD 163296, CN and HCN trace a similar intermediate layer, which is expected from physical-chemical models. For the other disks, we find that UV flux tracers and the vertical profiles of HCN and C₂H are lower than predicted in theoretical models. HCN and H₂CO show a highly structured vertical profile, possibly indicative of different formation pathways in the case of H₂CO.
Conclusions: It is possible to trace the vertical locations of multiple molecular species that in turn trace a wide variety of physical and chemical disk properties. The distribution of CO isotopologues is in agreement with theoretical predictions, and the emission is found at a wide range of vertical heights, z/r = 0.5-0.05. The vertical location of CO may be inversely related to the stellar mass. Other molecular lines are mostly found at z/r ≤ 0.15. The vertical layering of molecules is in agreement with theoretical predictions in some systems, but not in all. Therefore, dedicated physical-chemical models are needed to further study and understand the diversity of the emission surfaces.