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arXiv 2102.06338astro-ph.EPastro-ph.GAastro-ph.SR
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探测HNC和HCN线发射作为原行星盘温度的探针

Exploring HNC and HCN line emission as probes of the protoplanetary disk temperature

Feng Long, Arthur D. Bosman, Paolo Cazzoletti, Ewine F. van Dishoeck, Karin I. Oberg, Stefano Facchini, Marco Tazzari, Viviana V. Guzman, Leonardo Testi

备注：
accepted for publication in A&A
发表信息：
A&A 647, A118 (2021)
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摘要：

原行星盘中分子的分布和丰度是盘的物理和化学结构的强有力的示踪剂。已知HCN及其异构体HNC的丰度比对气体温度敏感。因此，它们的谱线比可能为探测发射气体的性质提供了一个独特的机会。使用2D热化学代码DALI，我们运行了一组磁盘模型，解释了不同的恒星和磁盘属性，并更新了氮化学的化学网络。然后将这些建模结果与观测结果进行比较，包括TW Hya盘的HNC $J=3-2$和狼疮中29个盘的HNC $J=1-0$的新ALMA观测结果。与CN类似，HCN和HNC在具有较大圆盘张开角和较高UV通量的模型中具有较亮的线发射。HNC和HCN的预测是丰富的温暖的地面层和外中面区域，这导致在环形的排放模式。然而，精确的发射区域和发射形态取决于探测的过渡，以及其他参数，如C和O丰度。模拟的HNC-HCN线强度比增加，从$<0.1$在内部磁盘的外盘区域，这可以解释为在高温下的高效HNC破坏。从目前稀缺的观测和TW Hya磁盘的径向分布，综合HNC线通量与我们的模型预测大致一致。HNC与HCN的通量比随半径（温度降低）而增大，但其作为圆盘中的化学温度计的使用受其他因素的影响，包括UV通量和C和O丰度。HNC和HCN的高空间分辨率ALMA磁盘观测，可以定位的发射层将有很大的潜力，以限制磁盘的热和紫外辐射结构，并验证我们的氮化学的理解。

英文摘要：

The distributions and abundances of molecules in protoplanetary disks are powerful tracers of the physical and chemical disk structures. The abundance ratios of HCN and its isomer HNC are known to be sensitive to gas temperature. Their line ratios might therefore offer a unique opportunity to probe the properties of the emitting gas. Using the 2D thermochemical code DALI, we ran a set of disk models accounting for different stellar and disk properties, with an updated chemical network for the nitrogen chemistry. These modeling results were then compared with observations, including new ALMA observations of HNC $J=3-2$ for the TW Hya disk and HNC $J=1-0$ for 29 disks in Lupus. Similar to CN, HCN and HNC have brighter line emission in models with larger disk flaring angles and higher UV fluxes. HNC and HCN are predicted to be abundant in the warm surface layer and outer midplane region, which results in ring-shaped emission patterns. However, the precise emitting regions and emission morphology depend on the probed transition, as well as on other parameters such as C and O abundances. The modeled HNC-to-HCN line intensity ratio increases from $<0.1$ in the inner disk to up to 0.8 in the outer disk regions, which can be explained by efficient HNC destruction at high temperatures. Disk-integrated HNC line fluxes from current scarce observations and its radial distribution in the TW Hya disk are broadly consistent with our model predictions. The HNC-to-HCN flux ratio robustly increases with radius (decreasing temperature), but its use as a chemical thermometer in disks is affected by other factors, including UV flux and C and O abundances. High-spatial resolution ALMA disk observations of HNC and HCN that can locate the emitting layers would have the great potential to constrain both the disk thermal and UV radiation structures, and also to verify our understanding of the nitrogen chemistry.

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