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 Atacama Large (sub-) Millimeter Array observations of CN, HCN, H13CN, and HC15N 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 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.
Highlights
Protoplanetary disks are characterized by radial and vertical temperature and UV radiation gradients due to the exposure of disk surfaces to stellar and interstellar radiation, and attenuation of this radiation by dust and gas (Calvet et al 1991; Herczeg et al 2002)
And spectrally resolved Atacama Large (sub-)Millimeter Array (ALMA) observations toward the protoplanetary disk surrounding Herbig Ae star MWC 480 clearly show that its CN emission is ∼ 2 times more extended than its HCN emission
We constructed a parametric model of the disk to obtain the underlying abundance profile of the two species and found that the observed difference in emission profiles correspond to different abundance profiles for CN and HCN
Summary
Protoplanetary disks are characterized by radial and vertical temperature and UV radiation gradients due to the exposure of disk surfaces to stellar and interstellar radiation, and attenuation of this radiation by dust and gas (Calvet et al 1991; Herczeg et al 2002). This radiation and temperature structure should result in a chemically stratified disk structure. The existence of these cold disk regions may be conductive to efficient fractionation of heavy isotopes into specific molecules as is introduced below
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
