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arXiv 1709.00296astro-ph.GAastro-ph.SR
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恒星形成区rho Oph A的气体和尘埃：II。PDR和致密核中的气体

Gas and dust in the star-forming region rho Oph A: II. The gas in the PDR and in the dense cores

Bengt Larsson, Rene Liseau

备注：
accepted for publication in Astronomy & Astrophysics (25/08/2017) 20 pages, 17 figures
发表信息：
A&A 608, A133 (2017)
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摘要：

我们调查到何种程度的本地物理和化学条件相关的恒星形成介质中的各种对象的演化状态。ρ Oph A显示了小空间中低质量恒星形成的整个序列。利用H2，H2O，NH3，N2 H+，O2，OI，CO和CS的分光光度线图，我们研究了原子和分子气体在这个致密的分子核中的分布。这些物种的物理参数的推导，因为是他们的相对丰度在rho Oph A。使用辐射转移模型，我们检查的冷致密的核心，从他们的解决线轮廓的基态线的H2O和NH3，后者没有污染的VLA 1623流出观察和线重叠的超精细组件被明确考虑。这个光子主导区（PDR）的分层结构，从侧面看，清楚地显示出来。多环芳烃（PAH）和OI在整个令人兴奋的恒星S1周围的区域都可以看到。在离分子核心0.05 pc的界面处，原子氢迅速转化为H2，而OI进一步突出到分子核心中。这为在芯SM 1中气相形成O2提供了氧原子，其中X（O2）~ 5.e-8。在那里，O2与H2O丰度之比[X（H2O）~ 5.e-9]显著高于1。远离核心，O2由于H2O形成的增加而急剧减少。在分子核心之外，从S1看去，在远侧，来自0.5 pc远的早期B型恒星HD 147889的强烈辐射破坏了分子。在暗核SM 1附近，观测到的丰度比X（O2）/X（H2O）>1，这表明该天体非常年轻，这可以解释为什么O2在太阳系外是如此难以捉摸的分子。

英文摘要：

We investigate to what degree local physical and chemical conditions are related to the evolutionary status of various objects in star-forming media. rho Oph A displays the entire sequence of low-mass star formation in a small volume of space. Using spectrophotometric line maps of H2, H2O, NH3, N2H+, O2, OI, CO, and CS, we examine the distribution of the atomic and molecular gas in this dense molecular core. The physical parameters of these species are derived, as are their relative abundances in rho Oph A. Using radiative transfer models, we examine the infall status of the cold dense cores from their resolved line profiles of the ground state lines of H2O and NH3, where for the latter no contamination from the VLA 1623 outflow is observed and line overlap of the hyperfine components is explicitly taken into account. The stratified structure of this photon dominated region (PDR), seen edge-on, is clearly displayed. Polycyclic aromatic hydrocarbons (PAHs) and OI are seen throughout the region around the exciting star S1. At the interface to the molecular core 0.05 pc away, atomic hydrogen is rapidly converted into H2, whereas OI protrudes further into the molecular core. This provides oxygen atoms for the gas-phase formation of O2 in the core SM1, where X(O2)~ 5.e-8. There, the ratio of the O2 to H2O abundance [X(H2O)~ 5.e-9] is significantly higher than unity. Away from the core, O2 experiences a dramatic decrease due to increasing H2O formation. Outside the molecular core, on the far side as seen from S1, the intense radiation from the 0.5 pc distant early B-type star HD147889 destroys the molecules. Towards the dark core SM1, the observed abundance ratio X(O2)/X(H2O)>1, which suggests that this object is extremely young, which would explain why O2 is such an elusive molecule outside the solar system.

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