The Spatially Resolved HCN/HC15N Isotopic Ratio in the V4046 Sgr Protoplanetary Disk

Abstract

Molecular isotopic ratios can be used to investigate the origin of the organic material in the Solar System, as it is unclear if the organic molecules present in different Solar System bodies are inherited from the protosolar nebula or if these molecules are formed from chemical processing during the protoplanetary disk phase. One way to approach this problem is by characterizing isotopic compositions of N-bearing molecules. In particular, the 14N/15N ratio is commonly used to trace the origin of Solar System bodies, by assessing the thermal history of Solar System volatiles using nitrogen fractionation. We present ALMA spatially resolved observations of the HCN and its main isotopologues H13CN and HC15N J = 4 - 3 lines at 0.2′′ angular resolution toward the protoplanetary disk V4046 Sgr. The bright HCN isotopologues emission in V4046 Sgr allows us to obtain the radial profiles and trace the 14N/15N ratio profile across the disk. Adopting a typical 12C/13C ratio of 70, we find an increasing 14N/15N ratio of 70-220 across the disk, consistent with values observed in rocky planets, comets and meteorites. We also find a rather constant 12C/13C isotopic ratio as a function of radius, although a tentative decreasing profile is reported. The increasing 14N/15N profile across the disk indicates that nitrogen chemistry is altered by in-situ chemical fractionation in the disk and selective photodissociation is the dominant pathway to fractionate HCN in the inner part of the disk.