2025A&A...698A.115R

Element isotopic ratios are powerful tools for reconstructing the journey of planetary material from the parental molecular cloud to protoplanetary disks, where planets form and accrete their atmosphere. Radial variations in the isotopic ratios in protoplanetary disks reveal local pathways that can critically affect the degree of isotope fractionation of planetary material. We present spatially resolved profiles of the ¹⁴N/¹⁵N, ¹²C/¹³C, and D/H isotopic ratios of the HCN molecule in the PDS 70 disk, which hosts two actively accreting giant planets. ALMA observations of HCN, H¹³CN, HC¹⁵N, and DCN with a high spatial resolution reveal radial variations in the fractionation profiles. We extracted the HCN/HC¹⁵N ratio out to ∼120 au. It shows a decreasing trend outside the inner cavity wall of the PDS 70 disk, which is located at ∼50 au. We suggest that the radial variations observed in the HCN/HC¹⁵N ratio are linked to isotope-selective photodissociation of N₂. We leveraged the spectrally resolved hyperfine component of the HCN line to extract the radial profile of the HCN/H¹³CN ratio between ∼40 and 90 au and obtained a value that is consistent with the ¹²C/¹³C ratio in the interstellar medium. The deuteration profile is also mostly constant throughout the disk, with a DCN/HCN ratio ∼0.02 that is in line with other disk-averaged values and radial profiles in disks around T Tauri stars. The extracted radial profiles of isotopolog ratios show that different fractionation processes dominate at different spatial scales in the planet-hosting disk of PDS 70.