2018/02/12 by Ruud Visser, Simon Bruderer, Paolo Cazzoletti +3 · 1 citation
Chemistry · Physics and Astronomy · #Abundance (ecology) #Astro and Planetary Science #Astrochemistry #Astrophysics and Star Formation Studies #Fractionation #Fullerene Chemistry and Applications #Isotope #Isotope fractionation #Natural abundance #Photodissociation #Protoplanetary disk #Stable isotope ratio #T Tauri star #astro-ph.GA #astro-ph.SR
paper · pdf · doi:10.1051/0004-6361/201731898
published as A&A 615, A75 (2018) · Accepted by A&A; 17 pages, 17 figures; note added in proof
openalex publication_date 2018/02/12 · openalex created_date 2018/02/23 · arxiv created 2018/05/15 · arxiv updated 2018/07/18 · openalex updated_date 2026/08/05
Aims. The two stable isotopes of nitrogen, 14 N and 15 N, exhibit a range of abundance ratios both inside and outside the solar system. The elemental ratio in the solar neighborhood is 440. Recent ALMA observations showed HCN/HC 15 N ratios from 83 to 156 in six T Tauri and Herbig disks and a CN/C 15 N ratio of 323 ± 30 in one T Tauri star. We aim to determine the dominant mechanism responsible for these enhancements of 15 N: low-temperature exchange reactions or isotope-selective photodissociation of N 2 . Methods. Using the thermochemical code DALI, we model the nitrogen isotope chemistry in circumstellar disks with a 2D axisymmetric geometry. Our chemical network is the first to include both fractionation mechanisms for nitrogen. The model produces abundance profiles and isotope ratios for several key N-bearing species. We study how these isotope ratios depend on various disk parameters. Results. The formation of CN and HCN is closely coupled to the vibrational excitation of H 2 in the UV-irradiated surface layers of the disk. Isotope fractionation is completely dominated by isotope-selective photodissociation of N 2 . The column density ratio of HCN over HC 15 N in the disk’s inner 100 au does not depend strongly on the disk mass, the flaring angle or the stellar spectrum, but it is sensitive to the grain size distribution. For larger grains, self-shielding of N 2 becomes more important relative to dust extinction, leading to stronger isotope fractionation. Between disk radii of ~50 and 200 au, the models predict HCN/HC 15 N and CN/C 15 N abundance ratios consistent with observations of disks and comets. The HCN/HC 15 N and CN/C 15 N column density ratios in the models are a factor of 2–3 higher than those inferred from the ALMA observations.