Abstract
Abstract Nitrogen fractionation is commonly used to assess the thermal history of solar system volatiles. With ALMA it is for the first time possible to directly measure 14 N / 15 N ratios in common molecules during the assembly of planetary systems. We present ALMA observations of the H 13 CN and HC 15 N J = 3 − 2 lines at 0.″5 angular resolution, toward a sample of six protoplanetary disks, selected to span a range of stellar and disk structure properties. Adopting a typical 12 C / 13 C ratio of 70, we find comet-like 14 N / 15 N ratios of 80–160 in five of the disks (3 T Tauri and 2 Herbig Ae disks) and lack constraints for one of the T Tauri disks (IM Lup). There are no systematic differences between T Tauri and Herbig Ae disks, or between full and transition disks within the sample. In addition, no correlation is observed between disk-averaged D/H and 14 N / 15 N ratios in the sample. One of the disks, V4046 Sgr, presents unusually bright HCN isotopologue emission, enabling us to model the radial profiles of H 13 CN and HC 15 N . We find tentative evidence of an increasing 14 N / 15 N ratio with radius, indicating that selective photodissociation in the inner disk is important in setting the 14 N / 15 N ratio during planet formation.
Highlights
The origin of Solar System organics is a fundamental and highly debated topic
While the similar nitrogen fractionation ratios found in the cold Interstellar Medium (ISM), comets and disks is consistent with an inheritance scenario for the origin of organics in the Solar System, as we discuss the increasing 14N/15N ratio in the disk of V4046 Sgr suggests that in situ disk chemistry contributes to the observed fractionation patterns
We have presented assembly of the Solar System.Array (ALMA) observations at ∼ 0 .5 angular resolution of the HCN isotopologues in a diverse sample of six protoplanetary disks
Summary
The origin of Solar System organics is a fundamental and highly debated topic. It is unclear whether the organics in the different Solar System bodies were inherited from the cold and dense molecular parent cloud of our Sun, or if they are the result of chemical processing within the Solar nebula protoplanetary disk. Guzman et al (2015) presented the first detection of H13CN and HC15N in the disk around Herbig Ae star MWC 480, and provided the first measurement of the 14N/15N in a disk They found an isotopic ratio of 200±100, which is similar to what is observed in the cold ISM and in comets. Both mechanisms can reduce the HCN/HC15N ratio in protoplanetary disks (Lyons et al 2009; Heays et al 2014) Distinguishing between these different origins of nitrogen fractionation levels in disks and between inheritance and in situ disk fractionation chemistry (and further in comets and planets) requires more disk measurements, and constraints on the radial profiles of the 14N/15N ratio in disks with different structures and around stars with different radiation fields.
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