[论文解读] Chemistry in Disks. V: CN and HCN in proto-planetary disks

Edwige Chapillon, S. Guilloteau+3 more|arXiv (Cornell University)|Sep 26, 2011

Astrophysics and Star Formation Studies参考文献 49被引用 34

一句话总结

本研究利用IRAM Plateau de Bure干涉仪的高分辨率ALMA类观测，对三颗原行星盘（DM Tau、LkCa 15、MWC 480）中的CN和HCN进行成图，发现其激发温度出人意料地低（8–10 K），CN/HCN比值显著偏高（5–10），这一结果挑战了现有的化学模型。结果表明，气相中的CN和HCN在冷盘中质平面中仍大量存在，可能由于冻结效率低下或Lyα辐射增强，暗示对低温表面化学过程的理解尚不完整。

ABSTRACT

The chemistry of proto-planetary disks is thought to be dominated by two major processes: photodissociation near the disk surface, and depletion on dust grains in the disk mid-plane, resulting in a layered structure with molecules located in a warm layer above the disk mid-plane. We attempt here to confront this warm molecular layer model prediction with the distribution of two key molecules for dissociation processes: CN and HCN. Using the IRAM Plateau de Bure interferometer, we obtained high spatial and spectral resolution images of the CN J=2-1 and HCN J=1-0 lines in the disks surrounding the two T-Tauri DM Tau and LkCa 15 and the Herbig Ae MWC 480. Disk properties are derived assuming power law distributions. The hyperfine structure of the observed transitions allows us to constrain the line opacities and excitation temperatures. We compare the observational results with predictions from existing chemical models, and use a simple PDR model (without freeze-out of molecules on grains and surface chemistry) to illustrate dependencies on UV field strength, grain size and gas-to-dust ratio. We also evaluate the impact of Lyman alpha radiation. The temperature ordering follows the trend found from CO lines, with DM Tau being the coldest object and MWC 480 the warmest. Although CN indicates somewhat higher excitation temperatures than HCN, the derived values in the T-Tauri disks are very low (8-10 K). They agree with results obtained from CCH, and are in contradiction with thermal and chemical model predictions. These very low temperatures, as well as geometrical constraints, suggest that substantial amounts of CN and HCN remain in the gas phase close to the disk mid-plane, and that this mid-plane is quite cold. The observed CN/HCN ratio (5-10) is in better agreement with the existence of large grains, and possibly also a substantial contribution of Lyman alpha radiation.

研究动机与目标

• 通过观测T Tauri和Herbig Ae盘中的CN和HCN，检验分层盘化学模型的预测。
• 利用超精细结构分析，研究CN和HCN的激发条件与空间分布。
• 评估紫外屏蔽、Lyα辐射、颗粒大小和气尘比对分子丰度的影响。
• 评估现有化学模型是否能重现观测到的柱密度和激发温度。

提出的方法

• 利用IRAM Plateau de Bure干涉仪对CN J=2-1和HCN J=1-0谱线进行高角分辨率和高 spectral 分辨率观测。
• 通过最小化方案分析超精细结构，推导光学深度、激发温度和柱密度。
• 使用PDR代码（不包含冻结或表面化学）对紫外场、颗粒大小和气尘比的依赖性进行建模。
• 将观测到的谱线轮廓和比值与现有化学模型及辐射转移的改进LVG近似预测结果进行比较。
• 假设CN与HCN在He中的碰撞速率相同（Green & Thaddeus, 1974）。
• 评估其他解释，如气尘比降低或尘埃消光增加，以调和低温观测与模型预期之间的矛盾。

实验结果

研究问题

• RQ1 原行星盘中CN和HCN的观测激发温度是否与分层盘化学模型的预测一致？
• RQ2 CN和HCN的空间分布如何？是否与盘中质平面上方暖分子层的预期位置相符？
• RQ3 为何观测到的CN/HCN比值显著高于预期（5–10）？何种物理条件可解释这一现象？
• RQ4 气尘比降低或尘埃消光增加是否能调和低推导激发温度与模型预期之间的矛盾？
• RQ5 Lyα辐射或颗粒表面化学在维持冷中质平面中CN和HCN方面发挥何种作用？

主要发现

• T Tauri盘中CN和HCN的推导激发温度极低，范围为8至10 K，与热力学和化学模型的预测相矛盾。
• 尽管CN的预测激发温度较高，但观测值与冷中质平面一致，表明CN并未局限于盘的上层。
• 观测到的约5–10的CN/HCN比值更可能由大颗粒的存在和Lyα辐射的显著贡献所解释。
• 气尘比降低（DM Tau中低至~0.003 M⊙）可调和低温观测，但无法在化学模型中重现观测到的柱密度。
• CN和HCN在冷中质平面的持续存在表明，对低温气相和表面化学过程的理解尚不完整，尤其是在约10 K的温度下。
• 需要更高激发跃迁的CN和HCN观测，以更全面地约束激发条件，并区分不同物理情景。