Edge-On Disk Study (EODS)

III. Molecular stratification in the Flying Saucer disk

¹ Univ. Bordeaux, CNRS, Laboratoire d’Astrophysique de Bordeaux, UMR 5804, 33600 Pessac, France
² Departamento de Física, Facultad de Ciencias, Universidad de Chile, Av. Las Palmeras 3425, Ñuñoa, Santiago, Chile
³ Dipartimento di Fisica e Astronomia, Università di Bologna, via Gobetti 93/2, 40190 Bologna, Italy
⁴ zAh/ITA, Heidelberg University, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
⁵ Max-Planck-Institut für Astronomie (MPIA), Königstuhl 17, 69117 Heidelberg, Germany
⁶ IRAM, 300 Rue de la Piscine, 38046 Saint Martin d’Hères, France
⁷ Institut des Sciences Moléculaires d’Orsay, CNRS, Univ. Paris-Saclay, Orsay, France
⁸ RIKEN Cluster for Pioneering Research, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan
⁹ Carl Sagan Center, SETI Institute, Mountain View, CA, USA
¹⁰ Aix-Marseille Univ, CNRS, CNES, LAM, Marseille, France
¹¹ Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Konkoly-Thege M. út 15-17, 1121 Budapest, Hungary
¹² Institute of Physics and Astronomy, ELTE Eötvös Loránd University, Pázmány Péter sétány 1/A, 1117 Budapest, Hungary
¹³ LUX, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, 92190 Meudon, France
¹⁴ National Institute of Science Education and Research, Jatni 752050, Odisha, India
¹⁵ Homi Bhabha National Institute, Training School Complex, Anushaktinagar, Mumbai 400094, India
¹⁶ Astronomy Unit, School of Physics and Astronomy, Queen Mary University of London, London E1 4NS, UK
¹⁷ National Astronomical Observatory of Japan, Division of Science, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Kanto, Japan
¹⁸ Viet. Nat. Space Center, Viet. Academy of Science and Technology, 18, Hoang Quoc Viet, Nghia Do, Cau Giay, Ha Noi, Vietnam
¹⁹ Academia Sinica Institute of Astronomy and Astrophysics, No. 1, Sec. 4, Roosevelt Rd, Taipei 106319, Taiwan, ROC
²⁰ Institut für Theoretische Physik und Astrophysik, Christian-Albrechts-Universität zu Kiel, Leibnizstraße 15, 24118 Kiel, Germany

^★ Corresponding author: anne.dutrey@u-bordeaux.fr

Received: 23 May 2025
Accepted: 29 September 2025

Abstract

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 ~17-20K.

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.