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Gas-grain Modeling of Interstellar O2
DonghuiQuan
Xinjiang Astronomical Observatory, Chinese Academy of Sciences, 150 Science 1-Street, Urumqi, 830011, China
Abstract:
Molecular oxygen (O2) is essential to human beings on the earth. Although elemental oxygen is rather abundant, O2 is rare in the interstellar medium (ISM). It was only detected in two galactic and one extra-galactic region. The inconsistency between observations and theoretical studies is a big challenge for astrochemical models. Here we report a two-phase modeling research of molecular oxygen, using the Nautilus gas-grain code. We apply the isothermal cold dense models in the ISM with two typical sets of initial elemental abundances, as well as the warm-up models with various physical conditions. Under cold dense conditions, we find that the timescales for gas-phase CO, O2 and H2O to reach peak values are dependent on the hydrogen density and are shortened when hydrogen density increases. In warm-up models, O2 abundances are in good agreement with observations at temperatures rising after 105 yr. In both isothermal and warm-up models, the steady-state O2 fractional abundance is independent of the hydrogen density, as long as the temperature is high enough (> 30 K), at which O2 is prevented from significant depleting onto grain surface. In addition, low density is preferable for the formation of O2, whether molecular oxygen is under cold conditions or in warm regions.
Key words:  Astrochemistry, Models, ISM, Molecules, Abundances
FundProject:
星际氧气分子的气相-尘埃模型研究
DonghuiQuan
Xinjiang Astronomical Observatory, Chinese Academy of Sciences, 150 Science 1-Street, Urumqi, 830011, China
摘要:
氧气(O2)对地球上的人类是必不可少的。然而,尽管地球上氧元素十分丰富,但在星际空间中却很少被探测到。目前科学家们仅在两个银河系内及一个银河系外区域中观测到氧气的存在。理论模型计算结果同观测结果间存在较大偏差,这使得天体化学理论模型的发展面临巨大挑战。本文采用气相-尘埃模型Nautilus研究了星际氧气的演化过程,使用了两种典型初始丰度值下的恒温模型和变化物理条件下的加热模型进行模拟计算。结果表明,在冷致密云的条件下,CO、O2和H2O达到峰值的时间依赖于氢核密度的多少,其随氢核密度的增大而减小。在加热模型中,在温度升高后的105年后,氧气的丰度值与观测结果符合较好。在温度大于30 K后,氧气的稳态丰度值将不再随氢核密度而变化,且大于此温度可以阻止氧气大量的沉降到尘埃表面。此外,无论在恒温模型,还是在加热模型中,低氢核密度更有利于O2的生成。
关键词:  天体化学,模型,星际介质,分子,丰度
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