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arXiv 0912.0701astro-ph.EP
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热的和稠密气体中作为高HDO/H2O比率的可能起源的热气相化学:应用于内部原行星盘

Warm gas phase chemistry as possible origin of high HDO/H2O ratios in hot and dense gases: application to inner protoplanetary discs

Wing-Fai Thi, Peter Woitke, Inga Kamp

摘要:

地球海洋的起源是有争议的。地球可以从水合硅酸盐(湿地球情景)或彗星(干地球情景)中获得水。[HDO]/[H2O]比率用于区分各种情况。在地球海洋中发现了高[HDO]/[H2O]比率。这些高比率通常归因于富含氘的彗星水冰的释放,这些水冰是在低气体和尘埃温度下形成的。观测没有显示星际冰中的高[HDO]/[H2O]。我们研究了在致密气体(nH> 1 E6 cm^{-3})和热气体(T=100-1000 K)中,在没有颗粒表面化学的情况下,通过气相光化学可能形成高[HDO]/[H2O]比的情况。考虑到T>100 K时气体的主要中性-中性反应,我们导出了解析解。化学网络由光解离和中性-中性反应主导。尽管气体温度高,氘分馏发生,因为氘富集和反反应之间的活化能的差异。分析的解决方案,证实了在一个典型的TTauri星周围的1 E-3 MSun光盘使用光化学代码ProDiMo的时间依赖性的化学结果。ProDiMo代码包括与频率相关的二维尘埃连续辐射转移、详细的非LTE气体加热和冷却以及圆盘结构的流体静力学计算。分析和时间相关的模型预测高的[HDO]/[H2O]比的类地行星形成区(< 3 AU)的拱星盘。因此,[HDO]/[H2O]比值可能不是区分地球上不同来源的水的唯一标准。

英文摘要:

The origin of Earth oceans is controversial. Earth could have acquired its water either from hydrated silicates (wet Earth scenario) or from comets (dry Earth scenario). [HDO]/[H2O] ratios are used to discriminate between the scenarios. High [HDO]/[H2O] ratios are found in Earth oceans. These high ratios are often attributed to the release of deuterium enriched cometary water ice, which was formed at low gas and dust temperatures. Observations do not show high [HDO]/[H2O] in interstellar ices. We investigate the possible formation of high [HDO]/[H2O] ratios in dense (nH> 1E6 cm^{-3}) and warm gas (T=100-1000 K) by gas-phase photochemistry in the absence of grain surface chemistry. We derive analytical solutions, taking into account the major neutral-neutral reactions for gases at T>100 K. The chemical network is dominated by photodissociation and neutral-neutral reactions. Despite the high gas temperature, deuterium fractionation occurs because of the difference in activation energy between deuteration enrichment and the back reactions. The analytical solutions were confirmed by the time-dependent chemical results in a 1E-3 MSun disc around a typical TTauri star using the photochemical code ProDiMo. The ProDiMo code includes frequency-dependent 2D dust-continuum radiative transfer, detailed non-LTE gas heating and cooling, and hydrostatic calculation of the disc structure. Both analytical and time-dependent models predict high [HDO]/[H2O] ratios in the terrestrial planet forming region (< 3 AU) of circumstellar discs. Therefore the [HDO]/[H2O] ratio may not be an unique criterion to discriminate between the different origins of water on Earth.

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