Cyanide photochemistry and fractionation in the MWC 480 disk

TL;DR

This study uses ALMA observations to analyze the photochemistry and nitrogen fractionation of HCN and CN in protoplanetary disks, revealing differences in chemical profiles and providing the first nitrogen isotopic ratio measurement in a disk.

Contribution

It presents the first detection of HC15N in a disk and compares photochemical structures between Herbig Ae and T Tauri disks, advancing understanding of disk chemistry.

Findings

CN extends beyond HCN in MWC 480, indicating photodissociation effects.

Different radial abundance profiles for CN and HCN in MWC 480.

Measured 14N/15N ratio of 200+-100 in MWC 480, similar to interstellar values.

Abstract

HCN is a commonly observed molecule in Solar System bodies and in interstellar environments. Its abundance with respect to CN is a proposed tracer of UV exposure. HCN is also frequently used to probe the thermal history of objects, by measuring its degree of nitrogen fractionation. To address the utility of HCN as a probe of disks, we present ALMA observations of CN, HCN, H13CN and HC15N toward the protoplanetary disk around Herbig Ae star MWC480, and of CN and HCN toward the disk around T Tauri star DM Tau. Emission from all molecules is clearly detected and spatially resolved, including the first detection of HC15N in a disk. Toward MWC 480, CN emission extends radially more than 1" exterior to the observed cut-off of HCN emission. Quantitative modeling further reveals very different radial abundance profiles for CN and HCN, with best-fit outer cut-off radii of >300 AU and 110+-10 AU, respectively. This result is in agreement with model predictions of efficient HCN photodissociation into CN in the outer-part of the disk where the vertical gas and dust column densities are low. No such difference in CN and HCN emission profiles are observed toward DM Tau, suggestive of different photochemical structures in Herbig Ae and T Tauri disks. We use the HCN isotopologue data toward the MWC 480 disk to provide the first measurement of the 14N/15N ratio in a disk. We find a low disk averaged 14N/15N ratio of 200+-100, comparable to what is observed in cloud cores and comets, demonstrating interstellar inheritance and/or efficient nitrogen fractionation in this disk.