Edge-On Disk Study (EODS)

II. HCO⁺ and CO vertical stratification in the disk surrounding SSTTau042021

¹ Laboratoire d’Astrophysique de Bordeaux, Université de Bordeaux, CNRS, B18N, Allée Geoffroy Saint-Hilaire, 33615 Pessac, France
² IRAM, 300 Rue de la Piscine, 38046 Saint Martin d’Hères, France
³ Departamento de Física, Facultad de Ciencias, Universidad de Chile, Av. Las Palmeras 3425, Ñuñoa, Santiago, Chile
⁴ Institut des Sciences Moléculaires d’Orsay, CNRS, Univ. Paris-Saclay, Orsay, France
⁵ Dipartimento di Fisica e Astronomia, Università di Bologna, Via Gobetti 93/2, 40122 Bologna, Italy
⁶ Carl Sagan Center, SETI Institute, Mountain View, CA, USA
⁷ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁸ Konkoly Observatory, HUN-REN Research Centre for Astronomy and Earth Sciences, MTA Centre of Excellence, Konkoly-Thege Miklós út 15-17, 1121 Budapest, Hungary
⁹ Institute of Physics and Astronomy, ELTE Eötvös Loránd University, Pázmány Péter sétány 1/A, 1117 Budapest, Hungary
¹⁰ LUX, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, 92190 Meudon, France
¹¹ Exoplanets and Planetary Formation Group, School of Earth and Planetary Sciences, National Institute of Science Education and Research, Jatni 752050, Odisha, India
¹² Homi Bhabha National Institute, Training School Complex, Anushaktinagar, Mumbai 400094, India
¹³ Astronomy Unit, School of Physics and Astronomy, Queen Mary University of London, London E1 4NS, UK
¹⁴ Department of Astrophysics, Vietnam National Space Center, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam
¹⁵ Zentrum für Astronomie der Universität Heidelberg, Institut für Theoretische Astrophysik, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
¹⁶ Academia Sinica Institute of Astronomy and Astrophysics, 11F of AS/NTU Astronomy-Mathematics Building, No.1, Sec. 4, Roosevelt Rd, Taipei 106319, Taiwan, ROC
¹⁷ Institut für Theoretische Physik und Astrophysik, Christian-Albrechts-Universität zu Kiel, Leibnizstrasse 15, 24118 Kiel, Germany

^★ Corresponding authors: coralie.foucher@u-bordeaux.fr

Received: 27 March 2025
Accepted: 1 October 2025

Abstract

Context. Edge-on disks offer a unique opportunity to directly examine their vertical structure, which provides valuable insights into planet formation processes. We investigated the dust properties as well as the CO and HCO⁺ gas properties in the edge-on disk surrounding the T Tauri star 2MASS J04202144+281349 (SSTTau042021).

Aims. We estimated the radial and vertical temperature and density profile for the gas and the dust.

Methods. We used ALMA archival data of CO isotopologs and continuum emission at 2, 1.3, and 0.9 mm together with new NOEMA HCO⁺ 3-2 observations. We retrieved the gas and dust disk properties using the tomographic method and the DISKFIT model.

Results. The vertical CO emission appears to be very extended and partly traces the H₂ wind observed by JWST. The C¹⁸O, ¹³CO, and HCO⁺ emission characterize the bulk of the molecular layer. The dust and gas have midplane temperatures of ∼7-11 K. The temperature of the molecular layer (derived from ¹³CO and HCO⁺) is on the order of 16 K. Because HCO⁺ 3-2 is thermalized, we derived a lower limit for the H₂ volume density of ∼3 × 10⁶ cm⁻³ at a radius of 100-200 au between one and two scale heights. The atmosphere temperature of the CO gas is ∼31 K at a radius of 100 au. We directly observed CO and HCO⁺ gas in the midplane beyond the dust outer radius (≥300 au). The gas plus dust disk mass estimated up to a radius of 300 au is on the order of 4.6 × 10⁻² M_⊙.

Conclusions. The favorable disk inclination allows us to present the first quantitative evidence for vertical molecular stratification with a direct observation of CO and HCO⁺ gas in the midplane. We estimate a temperature profile with a temperature of 7-11 K near the midplane and of 15-20 K in the dense part of the molecular layer up to ∼35 K above the midplane.