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arXiv 0812.3664astro-ph
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对低质量嵌入的年轻恒星天体缺乏PAH发射

Lack of PAH emission toward low-mass embedded young stellar objects

V. C. Geers, E. F. van Dishoeck, K. M. Pontoppidan, F. Lahuis, A. Crapsi, C. P. Dullemond, G. A. Blake

备注：
13 pages, 9 figures, accepted for publication in A&A
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摘要：

多环芳烃已经在分子云和一些有圆盘的年轻恒星中被检测到，但还没有与嵌入的年轻恒星联系起来。我们提出了一个敏感的中红外光谱调查PAH功能对低质量嵌入YSO的样本。其目的是利用辐射转移模型对恒星形成的嵌入阶段中的PAH丰度施加约束。给出了39个源的VLT-ISAAC L波段谱和53个源的Spitzer IRS谱。线强度相比，最近的调查赫比格Ae/Be和金牛T星。用辐射转移程序RADMC和RADCAL模拟了包层中包埋有拱星盘的PMS星包埋YSO的PAH辐射。研究了多环芳烃的丰度、恒星辐射场、倾角和周围包层消光对多环芳烃特征的影响。大多数样品（97%）未检测到3.3 μ m PAH特征，典型上限为5E-16 W/m^2。紧凑的11.2微米PAH发射被视为直接朝向53个斯皮策短-高光谱中的1个，对于边界嵌入的源。对于所有12个源的VLT和斯皮策光谱，没有PAH功能检测。总的来说，PAH特征在63颗（候选）嵌入原恒星中最多有1颗被检测到（<~ 2%），甚至低于具有盘的II类金牛座T星（11-14%）。假设典型的I类恒星和信封参数，多环芳烃排放的情况下，最有可能解释的情况下，通过多环芳烃丰度至少一个数量级低于分子云，但类似于磁盘中发现的发射载体。因此，大多数多环芳烃可能进入原行星盘冻结在尘埃颗粒的冰层和/或凝结的形式。

英文摘要：

PAHs have been detected toward molecular clouds and some young stars with disks, but have not yet been associated with embedded young stars. We present a sensitive mid-IR spectroscopic survey of PAH features toward a sample of low-mass embedded YSOs. The aim is to put constraints on the PAH abundance in the embedded phase of star formation using radiative transfer modeling. VLT-ISAAC L-band spectra for 39 sources and Spitzer IRS spectra for 53 sources are presented. Line intensities are compared to recent surveys of Herbig Ae/Be and T Tauri stars. The radiative transfer codes RADMC and RADICAL are used to model the PAH emission from embedded YSOs consisting of a PMS star with a circumstellar disk embedded in an envelope. The dependence of the PAH feature on PAH abundance, stellar radiation field, inclination and the extinction by the surrounding envelope is studied. The 3.3 micron PAH feature is undetected for the majority of the sample (97%), with typical upper limits of 5E-16 W/m^2. Compact 11.2 micron PAH emission is seen directly towards 1 out of the 53 Spitzer Short-High spectra, for a source that is borderline embedded. For all 12 sources with both VLT and Spitzer spectra, no PAH features are detected in either. In total, PAH features are detected toward at most 1 out of 63 (candidate) embedded protostars (<~ 2%), even lower than observed for class II T Tauri stars with disks (11-14%). Assuming typical class I stellar and envelope parameters, the absence of PAHs emission is most likely explained by the absence of emitting carriers through a PAH abundance at least an order of magnitude lower than in molecular clouds but similar to that found in disks. Thus, most PAHs likely enter the protoplanetary disks frozen out in icy layers on dust grains and/or in coagulated form.

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