2023ApJ...948...60L

High-spatial-resolution observations of CO isotopologue line emission in protoplanetary disks at mid-inclinations (≈30°-75°) allow us to characterize the gas structure in detail, including radial and vertical substructures, emission surface heights and their dependencies on source characteristics, and disk temperature profiles. By combining observations of a suite of CO isotopologues, we can map the two-dimensional (r, z) disk structure from the disk upper atmosphere, as traced by CO, to near the midplane, as probed by less abundant isotopologues. Here, we present high-angular-resolution (≲0.″1 to ≈0.″2; ≈15-30 au) observations of CO, ¹³CO, and C¹⁸O in either or both J = 2-1 and J = 3-2 lines in the transition disks around DM Tau, Sz 91, LkCa 15, and HD 34282. We derived line emission surfaces in CO for all disks and in ¹³CO for the DM Tau and LkCa 15 disks. With these observations, we do not resolve the vertical structure of C¹⁸O in any disk, which is instead consistent with C¹⁸O emission originating from the midplane. Both the J = 2-1 and J = 3-2 lines show similar heights. Using the derived emission surfaces, we computed radial and vertical gas temperature distributions for each disk, including empirical temperature models for the DM Tau and LkCa 15 disks. After combining our sample with literature sources, we find that ¹³CO line emitting heights are also tentatively linked with source characteristics, e.g., stellar host mass, gas temperature, disk size, and show steeper trends than seen in CO emission surfaces.