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

¹ Dipartimento di Fisica, Università degli Studi di Milano, Via Celoria 16, 20133 Milano, Italy
² Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
³ Max-Planck Institute for Astronomy (MPIA), Königstuhl 17, 69117 Heidelberg, Germany
⁴ Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
⁵ Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
⁶ Department of Astronomy, University of Florida, Gainesville, FL 32611, USA
⁷ Department of Astronomy, University of Virginia, Charlottesville, VA 22904, USA
⁸ Center for Exoplanets and Habitable Worlds, Penn State University, 525 Davey Laboratory, 251 Pollock Road, University Park, PA 16802, USA
⁹ Department of Astronomy & Astrophysics, The Pennsylvania State University, 525 Davey Laboratory, University Park, PA 16802, USA

^★ Corresponding author: luna.rampinelli@unimi.it

Received: 18 February 2025
Accepted: 1 April 2025

Abstract

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.