[论文解读] Dust processing in photodissociation regions - Mid-IR emission modelling

M. Compiègne, A. Abergel+2 more|ArXiv.org|Sep 29, 2008

Astrophysics and Star Formation Studies参考文献 72被引用 32

一句话总结

本研究利用斯皮策望远镜和ISO数据，对光致分解区（PDRs）中的中红外尘埃发射进行建模，将尘埃发射模型与辐射转移计算相结合，以区分尘埃激发效应与尘埃演化的影响。研究发现，在猎户座分子云的马头星云中，PAH/VSG丰度比从稀疏区到致密区降低了2.4倍；在NGC2023北区则降低了约5倍，表明在小空间尺度上尘埃经历了快速演化。

ABSTRACT

Mid-infrared spectroscopy of dense illuminated ridges (or photodissociation regions, PDRs) suggests dust evolution. Such evolution must be reflected in the gas physical properties through processes like photo-electric heating or H_2 formation. With Spitzer Infrared Spectrograph (IRS) and ISOCAM data, we study the mid-IR emission of closeby, well known PDRs. Focusing on the band and continuum dust emissions, we follow their relative contributions and analyze their variations in terms of abundance of dust populations. In order to disentangle dust evolution and excitation effects, we use a dust emission model that we couple to radiative transfer. Our dust model reproduces extinction and emission of the standard interstellar medium that we represent with diffuse high galactic latitude clouds called Cirrus. We take the properties of dust in Cirrus as a reference to which we compare the dust emission from more excited regions, namely the Horsehead and the reflection nebula NGC 2023 North. We show that in both regions, radiative transfer effects cannot account for the observed spectral variations. We interpret these variations in term of changes of the relative abundance between polycyclic aromatic hydrocarbons (PAHs, mid-IR band carriers) and very small grains (VSGs, mid-IR continuum carriers). We conclude that the PAH/VSG abundance ratio is 2.4 times smaller at the peak emission of the Horsehead nebula than in the Cirrus case. For NGC2023 North where spectral evolution is observed across the northern PDR, we conclude that this ratio is ~5 times lower in the dense, cold zones of the PDR than in its diffuse illuminated part where dust properties seem to be the same as in Cirrus. We conclude that dust in PDRs seems to evolve from "dense" to "diffuse" properties at the small spatial scale of the dense illuminated ridge.

研究动机与目标

• 利用斯皮策望远镜和ISO数据，理解猎户座分子云等致密照亮PDR中红外光谱的变化。
• 区分中红外发射光谱中由辐射转移激发引起的效应与实际尘埃演化的影响。
• 确定PAH与极小颗粒（VSGs）相对丰度的变化是否可解释观测到的光谱变化。
• 以高银纬的Cirrus尘埃为参考，量化PDR中尘埃性质的演化。
• 评估尘埃演化对利用PAH作为星系恒星形成率指标解释的影响。

提出的方法

• 采用基于Désert等人（1990）模型的尘埃发射模型，以重现弥散星际介质（Cirrus尘埃）的消光与发射特性。
• 将该模型与辐射转移代码耦合，模拟PDR中尘埃颗粒在非均匀紫外辐射场下的激发过程。
• 将模型预测与观测光谱的中红外谱型（PAH带为7–9 μm，连续谱为22–24 μm）进行比较。
• 通过调节PAH与VSG的相对丰度来拟合观测光谱形状，从而将尘埃演化效应与激发效应分离。
• 利用斯皮策-IRS的空间分辨光谱，比较NGC2023北区弥散照亮边缘与致密冷区的尘埃性质。
• 模型假设PAH与VSG主导5–35 μm波段的发射，背景连续谱仅在恒星附近的空腔区域变得显著。

实验结果

研究问题

• RQ1 辐射转移效应在多大程度上能解释PDR中7–9 μm（PAH）与22–24 μm（中红外连续谱）发射比的观测下降？
• RQ2 在NGC2023北区的致密照亮脊上，PAH与VSG的相对丰度如何变化？
• RQ3 马头星云中的尘埃是否与Cirrus尘埃性质一致，还是显示出显著的演化特征？
• RQ4 哪些物理过程可能驱动PDR中从稀疏区到致密区PAH/VSG丰度的观测下降？
• RQ5 PDR中尘埃演化如何影响将PAH发射作为星系恒星形成率指标的解释？

主要发现

• 仅靠辐射转移效应无法解释马头星云和NGC2023北区中红外发射的观测光谱变化。
• 在马头星云峰值发射区域，PAH/VSG相对丰度比Cirrus参考情况降低了2.4倍。
• 在NGC2023北区，PAH/VSG丰度比从弥散照亮边缘到PDR致密冷内区降低了约5倍。
• NGC2023北区弥散照亮部分的尘埃性质与Cirrus尘埃一致，表明该区域未发生显著处理。
• 观测到的光谱演化发生在极小的空间尺度（致密照亮脊）上，表明尘埃从‘致密’特性快速演化为‘稀疏’特性。
• PAH丰度的下降与多种情景一致，如PAH团簇的光致蒸发、尘埃聚集体的解聚，或紫外驱动的脂肪族烃类向芳香族烃类的转化。