2025A&A...704A..32F

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.