Edge-On Disk Study (EODS): III. Molecular stratification in the Flying Saucer disk

Context. Investigating the vertical distribution of molecular content in protoplanetary disks remains difficult in most disks mildly inclined along the line of sight. In contrast, edge-on disks provide a direct (tomographic) view of the 2D molecular brightness. Aims. We study the radial and vertical molecular distribution as well as the gas temperature and density by observing the Keplerian edge-on disk surrounding the Flying Saucer, a Class II object located in Ophiuchus. Methods. We used new and archival ALMA data to perform a tomography of 12CO, 13CO, C18O, CN, HCN, CS, H2CO, c-C3H2, N2D+, DCN, and 13CS. We analyzed molecular tomographies and modeled data using the radiative transfer code DISKFIT. Results. We directly measured the altitude above the mid-plane for each observed species. For the first time, we unambiguously demonstrate the presence of a common molecular layer and measure its thickness. Most molecules are located at the same altitude versus radius. Beyond CO, as predicted by chemical models, the CN emission traces the upper boundary of the molecular layer, whereas the deuterated species (DCN and N2D+) reside below one scale height. Our best fits from DISKFIT show that most observed transitions in the molecular layer are thermalized because their excitation temperature is the same, around ∼17–20 K. Conclusions. These long-integration observations clearly reveal a molecular layer predominantly located around one to two scale heights at a temperature above the CO freeze-out temperature. The deuterated molecules are closer to the mid-plane, and N2D+ may be a good proxy for the CO snowline. Some molecules, such as CN and H2CO, are likely influenced by the disk environment, at least beyond the millimeter dust disk radius. The direct observation of the molecular stratification opens the door to detailed chemical modeling in a disk that appears representative of T Tauri disks.