[论文解读] The IRAM-30m line survey of the Horsehead PDR: IV. Comparative chemistry of H2CO and CH3OH

Viviana V. Guzmán, J. R. Goicoechea+7 more|arXiv (Cornell University)|Oct 23, 2013

Spectroscopy and Laser Applications被引用 34

一句话总结

本研究利用深空IRAM-30m和PdBI观测，调查了猎户座分子云中马头星云PDR区及致密核内H₂CO和CH₃OH的形成与释放机制。研究发现，这两种分子主要在尘埃颗粒表面形成，并通过PDR区的光致脱附过程释放到气相中；而在致密核中，CH₃OH主要在颗粒表面形成并发生光致脱附，而H₂CO则主要在气相中形成，且H₂CO/CH₃OH的丰度比从PDR区的~2.3降至致密核中的~0.9。

ABSTRACT

Aims. We investigate the dominant formation mechanism of H2CO and CH3OH in the Horsehead PDR and its associated dense core. Methods. We performed deep integrations of several H2CO and CH3OH lines at two positions in the Horsehead, namely the PDR and dense core, with the IRAM-30m telescope. In addition, we observed one H2CO higher frequency line with the CSO telescope at both positions. We determine the H2CO and CH3OH column densities and abundances from the single-dish observations complemented with IRAM-PdBI high-angular resolution maps (6") of both species. We compare the observed abundances with PDR models including either pure gas-phase chemistry or both gas-phase and grain surface chemistry. Results. We derive CH3OH abundances relative to total number of hydrogen atoms of ~1.2e-10 and ~2.3e-10 in the PDR and dense core positions, respectively. These abundances are similar to the inferred H2CO abundance in both positions (~2e-10). We find an abundance ratio H2CO/CH3OH of ~2 in the PDR and ~1 in the dense core. Pure gas-phase models cannot reproduce the observed abundances of either H2CO or CH3OH at the PDR position. Both species are therefore formed on the surface of dust grains and are subsequently photodesorbed into the gas-phase at this position. At the dense core, on the other hand, photodesorption of ices is needed to explain the observed abundance of CH3OH, while a pure gas-phase model can reproduce the observed H2CO abundance. The high-resolution observations show that CH3OH is depleted onto grains at the dense core. CH3OH is thus present in an envelope around this position, while H2CO is present in both the envelope and the dense core itself. Conclusions. Photodesorption is an efficient mechanism to release complex molecules in low FUV-illuminated PDRs, where thermal desorption of ice mantles is ineffective.

研究动机与目标

• 确定在低紫外辐射照射的猎户座PDR区及其关联致密核中，H₂CO和CH₃OH的主要形成路径。
• 评估表面化学反应与非热脱附过程（尤其是光致脱附）在将复杂冰层物质释放至气相中的作用。
• 将观测到的分子丰度与纯气相化学模型及气相与表面化学结合模型的预测结果进行比较。
• 利用高分辨率干涉成像图谱研究H₂CO和CH₃OH的空间分布与激发条件。
• 通过转动谱线发射分析确定其正交-对位比与动能温度。

提出的方法

• 利用IRAM-30m望远镜在PDR区和致密核两个位置对多个H₂CO和CH₃OH转动跃迁进行深度单镜积分观测。
• 利用IRAM PdBI进行高角分辨率（6

实验结果

研究问题

• RQ1 在低FUV辐射照射的猎户座PDR区，H₂CO和CH₃OH的主要形成机制是什么？
• RQ2 冰层颗粒包膜的光致脱附在导致观测到的H₂CO和CH₃OH气相丰度中起到了多大作用？
• RQ3 为何H₂CO/CH₃OH的丰度比在PDR区（~2.3）高于致密核区（~0.9）？
• RQ4 H₂CO和CH₃OH的空间分布有何不同？它们揭示了何种形成与脱附机制？
• RQ5 纯气相化学模型能否再现两个区域中观测到的H₂CO和CH₃OH丰度？

主要发现

• 在PDR区，观测到的CH₃OH丰度约为相对于H的1.2×10⁻¹⁰，致密核中约为2.3×10⁻¹⁰，而H₂CO丰度在两个区域均约为2×10⁻¹⁰。
• PDR区的H₂CO/CH₃OH丰度比约为2.3±0.4，致密核中约为0.9±0.1，表明两个区域的化学路径存在显著差异。
• 纯气相模型在PDR区无法再现观测到的CH₃OH丰度，其数量级相差极大，表明颗粒表面形成与光致脱附过程至关重要。
• 在致密核中，纯气相模型可合理再现H₂CO的观测丰度，但对CH₃OH的预测结果低了约5个数量级，表明CH₃OH的形成主要源于颗粒表面化学过程。
• 高分辨率PdBI成像显示，CH₃OH在致密核中发生耗尽，主要存在于包层区域，而H₂CO则在核心与包层中均有检测到。
• PDR区CH₃OH的正交-对位比约为2，致密核中约为3，与不同的形成与激发条件一致。