The First Spatially Resolved Detection of <sup>13</sup>CN in a Protoplanetary Disk and Evidence for Complex Carbon Isotope Fractionation
Tomohiro C. Yoshida, Hideko Nomura, Kenji Furuya, Richard Teague, Charles J. Law, Takashi Tsukagoshi, Seokho Lee, Christian Rab, Karin I. Öberg, Ryan A. Loomis
Abstract
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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>70</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>6</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>9</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> 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.