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Observations conducted using the Atacama Large Millimeter/submillimeter Arrayon the protoplanetary disk around TW Hya show the nitrogen fractionation of HCNmolecules in HC$^{14}$N/HC$^{15}$N $\sim$120 at a radius of $\sim$20 AU. Inthis study, we investigated the physical and chemical conditions that controlthis nitrogen fractionation process. To this end, a new disk model wasdeveloped, in which the isotope-selective photodissociation of N$_2$ andisotope-exchange chemical reactions have been incorporated. Our model cansuccessfully reproduce the observed HCN column density when the elementalabundances of the gas-phase carbon and oxygen are depleted by two orders ofmagnitude relative to those in the interstellar medium and carbon is moreabundant than oxygen ([C/O]$_{\rm elem}>$ 1). The isotope-selectivephotodissociation of N$_2$ is the dominant nitrogen fractionation process inour models. The observed HC$^{14}$N/HC$^{15}$N ratio, which increases outwards,can also be reproduced by the model by assuming that the small dust grains inthe atmosphere of the outer disk are depleted more than those in the innerdisk. This is consistent with grain evolution models, according to which smalldust grains are continuously replenished in the inner disk due to fragmentationof the large dust grains that radially drift from the outer disk.

astrophysical journal/the astrophysical journal

Modeling Nitrogen Fractionation in the Protoplanetary Disk around TW  Hya: Model Constraints on Grain Population and Carbon-to-Oxygen Elemental  Abundance Ratio