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 ¹³CN 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 ¹³CN in disks, which enables us to measure the spatially resolved ¹²CN/¹³CN ratio for the first time. We conducted nonlocal thermal equilibrium modeling of the ¹³CN lines in conjunction with previously observed ¹²CN lines to derive the kinetic temperature, H₂ volume density, and column densities of ¹²CN and ¹³CN. The H₂ volume density is found to range between (4 − 10) × 10⁷ cm⁻³, suggesting that CN molecules mainly reside in the disk's upper layer. The ¹²CN/¹³CN ratio is measured to be 70−6+9 at 30 < r < 80 au from the central star, which is similar to the ¹²C/¹³C 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 ¹²C into the solid phase toward the disk midplane. This study reveals the complexity of the carbon isotope fractionation operating in disks.

中文翻译：

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原行星盘中 13CN 的首次空间分辨探测以及复杂碳同位素分馏的证据

最近对原行星盘和系外行星大气中碳同位素比率的测量表明，重要的碳同位素分馏可能从盘转移到行星。为了更清楚地了解圆盘和行星之间的同位素联系，测量不同物种的碳同位素比率非常重要。在本文中，我们提出了¹³ CN的检测氮= TW Hya 圆盘中阿塔卡马大型毫米/亚毫米阵列中的 2 − 1 条超细线。这是对磁盘中¹³ CN的首次空间分辨检测，这使我们能够首次测量空间分辨的¹² CN/ ¹³ CN 比率。我们结合先前观察到的^{12 CN 线对13} CN 线进行非局部热平衡建模，以推导出¹² CN 和¹³ CN 的动力学温度、H ₂体积密度和柱密度。发现H _{2体积密度范围在(4 − 10) × 10} ⁷ cm ^-3之间，表明CN分子主要存在于盘的上层。¹² CN/ ¹³ CN比率经测量为 70-6+9 30 时 <r距离中心恒星 < 80 au，与星际介质中的¹² C/ ¹³ C 比率相似。然而，该值与之前报道的 TW Hya 盘中其他含碳分子（CO 和 HCN）的值不同。这可以通过不同分子的不同发射层高度以及¹² C 朝向盘中面优先隔离到固相中来自洽地解释。这项研究揭示了圆盘中碳同位素分馏的复杂性。