[论文解读] Warm ISM in the Sgr A complex - II. The [C/N] abundance ratio traced by [CII] 158 μm and [NII] 205 μm observations toward the Arched Filaments at the Galactic center

Pablo García, N. Abel+3 more|arXiv (Cornell University)|Jun 1, 2021

Stellar, planetary, and galactic studies参考文献 81被引用 1

一句话总结

本研究通过利用 [NII] 205 μm 观测，对银河系中心弓形纤维（Arched Filaments, AF）中 [CII] 158 μm 辐射的贡献进行解耦，建模了电离区（H II regions）和光致电离区（PDRs）的贡献。结果表明，AF 中 [C/N] ≈ 1.13 ± 0.09 且 [N/H] ≈ 6.21 × 10⁻⁴，表明银河系银心区域因二级氮合成而出现氮元素增强，其中 PDRs 贡献了总 [CII] 辐射的 20%–75%。

ABSTRACT

Context. The Arches Cluster – Arched Filaments (AF) system is our Galaxy’s prime example of the complexity involved in the interaction between the strong radiation field of numerous OB stars and their surrounding ISM in extremely harsh environments such as the Galactic center (GC) of the Milky Way. It offers a unique opportunity to study the close relationship between photon-dominated regions (PDRs) and H II regions and their relative contributions to the observed [CII] emission.Aims. We aim to investigate the I ([CII]) versus I ([NII]) integrated intensity behavior in the AF region in order to assess the [CII] emission contribution from the H II region, which is traced by [NII] line observations, and PDR components in the high-metallicity environment of the GC.Methods. We used [CII] 158 μm and [NII] 205 μm fine structure line observations of the AF in the literature to compare their observational integrated intensity distribution to semi-theoretical predictions for the contribution of H II regions and adjacent PDRs to the observed [CII] emission. We explored variations in the [C/N] elemental abundance ratio to explain the overall behavior of the observed relationship. Based on our models, the H II region and PDR contributions to the observed [CII] emission is calculated for a few positions within and near to the AF. Estimates for the [C/N] abundance ratio and [N/H] nitrogen elemental abundance in the AF can then be derived.Results. The behavior of the I ([CII]) versus I ([NII]) relationship in the AF can be explained by model results satisfying 0.84 ⊙ to 2 Z ⊙ , hydrogen volume density log n (H) = 3.5, and ionization parameters log U from −1 to −2. A least-squares fit to the model data points yields log I ([CII]) = 1.068 × log I ([NII]) + 0.645 to predict the [CII] emission arising from the H II regions in the AF. The fraction of the total observed [CII] emission arising from within PDRs varies between ~0.20 and ~0.75. Our results yield average values for the carbon-to-nitrogen ratio and nitrogen elemental abundances of [C/N]AF = 1.13 ± 0.09 and [N/H]AF = 6.21 × 10−4 for the AF, respectively. They are a factor of ~0.4 smaller and ~7.5 larger than their corresponding Galactic disk values.Conclusions. The large spatial variation of the fraction of [CII] emission arising either from H II regions or PDRs suggests that both contributions must be disentangled before any modeling attempt is made to explain the observed [CII] emission in the AF. We suggest thatsecondary production of nitrogen from low- to intermediate-mass stars in the Galactic bulge is a plausible mechanism to explain the large abundance differences between the GC and the Galactic disk. The mass loss of such stars would enrich the GC ISM with nitrogen as the gas falls into the inner GC orbits where the AF are located. Overall, our results show that tight constraints are needed on the [C/N] abundance ratio for the GC, significantly tighter than previous abundance measurements have discerned.

研究动机与目标

• 解耦银河系中心弓形纤维中观测到的 [CII] 158 μm 辐射来自电离区与光致电离区的贡献。
• 利用精细结构线观测，研究银河系中心高金属丰度环境中 [C/N] 的丰度比。
• 评估电离气体与中性气体组分在极端环境中对 I([CII]) 与 I([NII])) 强度关系的相对作用。
• 推导弓形纤维中氮元素丰度与 [C/N] 的约束，并与银河系盘面值进行比较。
• 评估低至中等质量恒星在银河系银心区域通过二级氮合成解释观测到的丰度异常的合理性。

提出的方法

• 利用文献中已有的弓形纤维 [CII] 158 μm 与 [NII] 205 μm 精细结构线观测数据。
• 将观测到的 I([CII]) 与 I([NII]) 强度分布与电离区及其相邻 PDR 的半理论模型进行比较。
• 通过幂律拟合模型电离区的 [CII] 辐射贡献：log I([CII]) = 1.068 × log I([NII]) + 0.645。
• 评估不同位置处总 [CII] 辐射中来自 PDRs 的比例，范围约为 20% 至 75%。
• 通过将模型预测与观测强度比值拟合，估算 AF 中的 [C/N] 丰度比与 [N/H] 氮元素丰度。
• 探索电离参数（log U 从 −1 到 −2）与氢体积密度（log n(H) = 3.5）以约束物理条件。

实验结果

研究问题

• RQ1 弓形纤维中观测到的 [CII] 辐射有多少比例来自 PDRs，又有多少来自电离区？
• RQ2 弓形纤维中的 [C/N] 丰度比与银河系盘面相比如何？这对核合成意味着什么？
• RQ3 在不同电离与密度条件下，H II 区与 PDR 模型能否解释观测到的 I([CII]) 与 I([NII])) 强度关系？
• RQ4 弓形纤维中推断的氮元素丰度 [N/H] 是多少？与银河系盘面值相比如何？
• RQ5 低至中等质量恒星在银河系银心区域通过二级氮合成，是否是解释观测到的丰度增强的合理机制？

主要发现

• 弓形纤维中 I([CII]) 与 I([NII]) 的关系最符合 [C/N] 为太阳值 0.84 至 2.0 倍、氢密度 log n(H) = 3.5、电离参数 log U 为 −1 至 −2 的模型。
• 对数据进行幂律拟合得到 log I([CII]) = 1.068 × log I([NII]) + 0.645，为 AF 中电离区的 [CII] 辐射提供预测关系。
• 不同位置处来自 PDRs 的总 [CII] 辐射比例在约 20% 至 75% 之间变化，表明发射贡献存在显著的局域差异。
• 弓形纤维中推导出的 [C/N] 丰度比为 [C/N]AF = 1.13 ± 0.09，约为银河系盘面值的 0.4 倍。
• AF 中的氮元素丰度为 [N/H]AF = 6.21 × 10⁻⁴，约为银河系盘面值的 7.5 倍。
• 结果表明，银河系银心区域中演化后的低至中等质量恒星通过二级氮合成，是解释内银河系中心观测到的氮元素富集的合理机制。