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arXiv 1406.0695astro-ph.EPastro-ph.SR
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在孤立和外部辐照的原行星盘中沿吸积流的复杂有机分子

Complex organic molecules along the accretion flow in isolated and externally irradiated protoplanetary disks

Catherine Walsh, Eric Herbst, Hideko Nomura, T. J. Millar, Susanna Widicus Weaver

备注：
Accepted for publication in Faraday Discussion, 168 "Astrochemistry of Dust, Gas, and Ice"
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摘要：

太阳诞生的环境将影响前太阳星云的条件，包括可达到的化学复杂性，这对生命前化学很重要。在两种情况下研究了金牛座T星周围盘中复杂有机分子（COMs）的形成和分布：（i）孤立盘，和（ii）被附近大质量恒星外部辐照的盘.化学计算沿吸积流从外盘向内使用一个全面的网络。进行了两个模拟，一个开始与复杂的冰和一个简单的冰。对于孤立的磁盘，COMs被运送到内部磁盘，在那里他们热解吸到气体达到一个丰度的初始假定的冰丰度的代表没有重大变化。对于简单的冰，COM有效地形成在粮食表面的条件下，在外盘。气相COM通过光脱附释放到分子层中。对于照射盘，复杂的冰也向内运输，然而，他们经历的热处理所造成的温暖的条件下照射盘，往往会减少其丰富的吸积流。对于简单的冰，颗粒表面化学不能在外盘合成COM，因为必要的颗粒表面自由基，往往是特别挥发性的，在颗粒表面上不够丰富。气相COM形成在内部区域的照射磁盘通过气相化学诱导的强烈结合的分子，如甲醇的解吸，因此，丰度是不代表的初始分子丰度注入到外盘。这些结果表明，在孤立的磁盘中形成的彗星的组成可能不同于在外部照射磁盘中形成的彗星，后者由更简单的冰组成。

英文摘要：

(Abridged) The birth environment of the Sun will have influenced the conditions in the pre-solar nebula, including the attainable chemical complexity, important for prebiotic chemistry. The formation and distribution of complex organic molecules (COMs) in a disk around a T Tauri star is investigated for two scenarios: (i) an isolated disk, and (ii) a disk irradiated externally by a nearby massive star. The chemistry is calculated along the accretion flow from the outer disk inwards using a comprehensive network. Two simulations are performed, one beginning with complex ices and one with simple ices only. For the isolated disk, COMs are transported without major alteration into the inner disk where they thermally desorb into the gas reaching an abundance representative of the initial assumed ice abundance. For simple ices, COMs efficiently form on grain surfaces under the conditions in the outer disk. Gas-phase COMs are released into the molecular layer via photodesorption. For the irradiated disk, complex ices are also transported inwards; however, they undergo thermal processing caused by the warmer conditions in the irradiated disk which tends to reduce their abundance along the accretion flow. For simple ices, grain-surface chemistry cannot synthesise COMs in the outer disk because the necessary grain-surface radicals, which tend to be particularly volatile, are not sufficiently abundant on the grain surfaces. Gas-phase COMs are formed in the inner region of the irradiated disk via gas-phase chemistry induced by the desorption of strongly bound molecules such as methanol; hence, the abundances are not representative of the initial molecular abundances injected into the outer disk. These results suggest that the composition of comets formed in isolated disks may differ from those formed in externally irradiated disks with the latter composed of more simple ices.

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