CYANIDE PHOTOCHEMISTRY AND NITROGEN FRACTIONATION IN THE MWC 480 DISK
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 Atacama Large (sub-) Millimeter Array observations of CN, HCN, H{sup 13}CN, and HC{sup 15}N toward the protoplanetary disk around Herbig Ae star MWC 480, 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 HC{sup 15}N 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 {sup 14}N/{sup 15}N ratio in a disk. We find a low disk averaged {sup 14}N/{sup 15}N 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.
- OSTI ID:
- 22521887
- Journal Information:
- Astrophysical Journal, Vol. 814, Issue 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
Similar Records
CHEMISTRY IN DISKS. III. PHOTOCHEMISTRY AND X-RAY DRIVEN CHEMISTRY PROBED BY THE ETHYNYL RADICAL (CCH) IN DM Tau, LkCa 15, AND MWC 480
CHEMISTRY IN DISKS. VII. FIRST DETECTION OF HC{sub 3}N IN PROTOPLANETARY DISKS