Radial variations in nitrogen, carbon, and hydrogen fractionation in the PDS 70 planet-hosting disk

Abstract

Element isotopic ratios are powerful tools to reconstruct the journey of planetary material, from the parental molecular cloud to protoplanetary disks, where planets form and accrete their atmosphere. Radial variations in isotopic ratios in protoplanetary disks reveal local pathways which can critically affect the degree of isotope fractionation of planetary material. In this work 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 high spatial resolution observations of HCN, H13CN, HC15N, and DCN reveal radial variations of fractionation profiles. We extract the HCN/HC15N ratio out to ~120 au, which shows a decreasing trend outside the inner cavity wall of the PDS 70 disk 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 leverage the spectrally resolved hyperfine component of the HCN line to extract the radial profile of the HCN/H13CN ratio between ~40 and 90 au, obtaining a value consistent with the ISM 12C/13C ratio. The deuteration profile is also mostly constant throughout the disk extent, with a DCN/HCN ratio ~0.02, in line with other disk-averaged values and radial profiles in disks around T Tauri stars. The extracted radial profiles of isotopologue ratios show how different fractionation processes dominate at different spatial scales in the planet-hosting disk of PDS 70.

18 pages, 10 figures, 3 tables

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