Astrochemistry

Radial Variations in Nitrogen, Carbon, and Hydrogen Fractionation in the PDS 70 Planet-hosting Disk

By Keith Cowing

Status Report

astro-ph.SR

April 11, 2025

Filed under astrochemistry, astrogeology, atmosphere, exoplanet, PDS 70, Protoplanetary Disk

Radial Variations in Nitrogen, Carbon, and Hydrogen Fractionation in the PDS 70 Planet-hosting Disk — Integrated intensity maps of HCN isotopologues in the PDS 70 disk. Brightness temperatures were obtained under the assumption of Rayleigh-Jeans approximation. The HCN (4-3) map in the bottom right panel is obtained from SB observations only. The white contours show the bright ring in the sub-mm continuum emission (Isella et al. 2019), the white dots mark the position of the two forming planets (2020 astrometry data from Wang et al. 2021), and the gray ellipse on the bottom left of each panel shows the synthesized beam. — astro-ph.EP

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.

L. Rampinelli, S. Facchini, M. Leemker, P. Curone, M. Benisty, K. I. Öberg, R. Teague, S. Andrews, J. Bae, C. J. Law, B. Portilla-Revelo

Comments: 18 pages, 10 figures, 3 tables
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2504.03833 [astro-ph.EP] (or arXiv:2504.03833v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2504.03833
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Submission history
From: Luna Rampinelli
[v1] Fri, 4 Apr 2025 18:00:01 UTC (8,806 KB)
https://arxiv.org/abs/2504.03833
Astrobiology,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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