[论文解读] The warm gas atmosphere of the HD 100546 disk seen by Herschel (Evidence of a gas-rich, carbon-poor atmosphere?)

S. Bruderer, E. F. van Dishoeck+2 more|arXiv (Cornell University)|Jan 23, 2012

Astrophysics and Star Formation Studies参考文献 146被引用 114

一句话总结

本研究利用赫歇尔/PACS对HD 100546的观测，对原行星盘中温暖气体大气层进行建模，结合辐射转移、化学网络和气体能量平衡。结果表明，高气体/尘埃比和低挥发性碳丰度最能重现CO跃迁阶梯、[O I]和[C II]发射，表明该大气层为富气体、碳贫乏，且$ T_{\text{gas}} \gg T_{\text{dust}} $，可能由紫外辐射加热及碳被锁存在难熔化合物中所致。

ABSTRACT

(Abridged) Context. With the Herschel Space Observatory, lines of simple molecules (C+, O, and CO) have been observed in the atmosphere of protoplanetary disks. When combined with ground-based [CI], all principle forms of carbon can be studied. The absence of neutral carbon [CI], which is predicted by models to be strong, can then be interpreted together with [CII] and carbon monoxide. Aims. We study the gas temperature, excitation, and chemical abundance of the simple carbon-bearing species by the method of chemical-physical modeling. We explore the sensitivity of the lines to the entering parameters and constrain the region from which the line radiation emerges. Methods. Numerical models of the radiative transfer are used together with a chemical network simulation and a calculation of the gas energetics to obtain the gas temperature. We present our new model, which is based on our previous models but includes several improvements. Results. A model of the disk around the Herbig Be star HD 100546 is able to reproduce the CO ladder together with the atomic fine-structure lines of [OI] and either [CI] or [CII]. We find that the high-J lines of CO can only be reproduced by a warm atmosphere with Tgas>>Tdust. The high-J CO observable with PACS are dominated from regions within some tens of AU. Conclusions. Only a warm atmosphere with Tgas>>Tdust can reproduce the CO ladder. The CO ladder together with [O I] and the upper limit to [CI] can be reproduced by models with a high gas/dust ratio and a low abundance of volatile carbon. These models however produce too small amounts of [CII]. Models with a low gas/dust ratio and more volatile carbon also reproduce CO and [OI], are in closer agreement with observations of [CII], but overproduce [CI]. Due to the uncertain origin of the [CII] emission, we prefer the high gas/dust ratio models, indicating a low abundance of volatile carbon.

研究动机与目标

• 利用赫歇尔观测，确定HD 100546原行星盘中温暖气体大气层的物理与化学条件。
• 检验观测到的高-$J$ CO谱线及原子精细结构谱线（[O I]、[C II]、[C I]）是否能通过自洽的物化模型重现。
• 通过将模型预测与多谱线观测对比，约束气体/尘埃比、挥发性碳丰度及尘埃性质。
• 评估[C II]发射的起源，其可能源自残余包层或前景物质。
• 理解不同$J$-能级CO转动能级跃迁的谱线空间起源与激发机制。

提出的方法

• 采用二维轴对称辐射转移模型，计算CO、[O I]、[C II]和[C I]的谱线发射，同时耦合详细的化学网络。
• 通过求解能量平衡方程，自洽计算气体温度，包括紫外辐射加热和分子谱线冷却。
• 模型包含尘埃再辐射、紫外屏蔽，以及温度依赖的附着系数的非热H2形成过程。
• 改变尘埃消光定律（如$ R_V = 5.5 $），以测试紫外辐射的穿透深度及其对气体加热和化学反应的影响。
• 计算不同盘结构配置下的谱线轮廓与流量，包括变化的气体/尘埃比、碳丰度及尘粒大小分布。
• 通过基准测试验证模型的一致性，并将结果与赫歇尔-PACS及地面观测的CO和[C I]谱线进行比较。

实验结果

研究问题

• RQ1 为重现HD 100546中观测到的CO转动能级阶梯，需要怎样的气体温度结构？
• RQ2 不同气体/尘埃比和挥发性碳丰度的模型能否重现观测到的CO、[O I]、[C II]和[C I]谱线流量？
• RQ3 赫歇尔探测到的高-$J$ CO谱线的空间起源是什么？与地面观测的低-$J$谱线相比有何差异？
• RQ4 尽管模型预测[C I]发射较强，但为何未被探测到？这对碳化学的解释有何影响？
• RQ5 [C II]发射在多大程度上可归因于盘本身，而非残余包层或前景物质？

主要发现

• 为重现高-$J$ CO谱线，必须存在$ T_{\text{gas}} \gg T_{\text{dust}} $的温暖气体大气层；若$ T_{\text{gas}} = T_{\text{dust}} $，则模型会将这些谱线的流量低估数个数量级。
• 高气体/尘埃比（100）和低挥发性碳分数（$ \delta_{\text{C}} = 0.05 $）的模型能较好重现CO跃迁阶梯、[O I]和[C II]发射，但对[C I]的预测略高于观测值。
• 低气体/尘埃比（20）和较高挥发性碳（$ \delta_{\text{C}} = 0.5 $）的模型虽更符合[C I]的上限，但会低估[C II]流量。
• 高气体/尘埃比模型中的[C II]/[C I]线比值更低，更接近观测上限，且在外盘中若存在大颗粒或无PAHs，该比值会进一步降低。
• 最高-$J$ CO谱线（J=30-29）源自约20–50 AU的半径区域，中等-$J$谱线（如J=16-15）来自约40–90 AU，而低-$J$谱线则来自外盘（>100 AU）。
• 由于内盘中湍流和速度梯度更高，高-$J$ CO谱线的线宽显著宽于低-$J$谱线。