The First Spatially Resolved Detection of 13CN in a Protoplanetary Disk and Evidence for Complex Carbon Isotope Fractionation

Abstract

Recent measurements of carbon isotope ratios in both protoplanetary disks and exoplanet atmospheres have suggested a possible transfer of significant carbon isotope fractionation from disks to planets. For a clearer understanding of the isotopic link between disks and planets, it is important to measure the carbon isotope ratios in various species. In this paper, we present a detection of the ^13 CN N = 2 − 1 hyperfine lines in the TW Hya disk with the Atacama Large Millimeter/submillimeter Array. This is the first spatially resolved detection of ^13 CN in disks, which enables us to measure the spatially resolved ^12 CN/ ^13 CN ratio for the first time. We conducted nonlocal thermal equilibrium modeling of the ^13 CN lines in conjunction with previously observed ^12 CN lines to derive the kinetic temperature, H _2 volume density, and column densities of ^12 CN and ^13 CN. The H _2 volume density is found to range between (4 − 10) × 10 ^7 cm ^−3 , suggesting that CN molecules mainly reside in the disk's upper layer. The ^12 CN/ ^13 CN ratio is measured to be ${70}_{-6}^{+9}$ at 30 < r < 80 au from the central star, which is similar to the ^12 C/ ^13 C ratio in the interstellar medium. However, this value differs from the previously reported values found for other carbon-bearing molecules (CO and HCN) in the TW Hya disk. This could be self-consistently explained by different emission layer heights for different molecules combined with preferential sequestration of ^12 C into the solid phase toward the disk midplane. This study reveals the complexity of the carbon isotope fractionation operating in disks.

Keywords

• Protoplanetary disks
• Astrochemistry
• Isotopic abundances