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arXiv 0901.1313astro-ph.SR
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星周盘中分子的化学史。I. Ices

The chemical history of molecules in circumstellar disks. I. Ices

R. Visser, E. F. van Dishoeck, S. D. Doty, C. P. Dullemond

备注：
18 pages, 16 figures, accepted by A&A
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摘要：

（略）目的与方法。一个二维的，半解析模型如下，第一次，从一个崩溃的分子云（前恒星的核心）的原恒星和拱盘的化学演化。该模型计算从云中任何一点的下落轨迹，并跟踪粘性演化盘中物质的径向和垂直运动。它包括一个完整的时间依赖性的辐射转移处理的尘埃温度，这控制了大部分的化学。一个小的参数网格进行了探索，以了解声速和质量和旋转的云的影响。一氧化碳（CO）和水（H2O）的冻结和蒸发，以及在冰中形成复杂有机分子的可能性，被认为是说明完整化学的重要第一步。结果两个物种在崩塌开始前都向中心冻结。纯CO冰在下落阶段蒸发，并重新吸附在盘的那些冷却到低于CO解吸温度约18 K的部分。H2O在恒星的下沉和盘形成阶段几乎到处都是固体，并在距离恒星约10天文单位的范围内蒸发。混合的CO-H2O冰在保持某些固态CO高于18 K和解释彗星中CO的存在方面很重要。预计最终进入行星和彗星形成区的物质在坍缩期间将在温暖区度过足够的时间，以在颗粒上形成第一代复杂的有机物种。在热的内包层（热的核心或热corino）的动力学时间尺度太短，大量的第二代分子的高温气相化学形成。

英文摘要：

(Abridged) Aims & Methods. A two-dimensional, semi-analytical model is presented that follows, for the first time, the chemical evolution from a collapsing molecular cloud (a pre-stellar core) to a protostar and circumstellar disk. The model computes infall trajectories from any point in the cloud and tracks the radial and vertical motion of material in the viscously evolving disk. It includes a full time-dependent radiative transfer treatment of the dust temperature, which controls much of the chemistry. A small parameter grid is explored to understand the effects of the sound speed and the mass and rotation of the cloud. The freeze-out and evaporation of carbon monoxide (CO) and water (H2O), as well as the potential for forming complex organic molecules in ices, are considered as important first steps to illustrate the full chemistry. Results. Both species freeze out towards the centre before the collapse begins. Pure CO ice evaporates during the infall phase and re-adsorbs in those parts of the disk that cool below the CO desorption temperature of ~18 K. H2O remains solid almost everywhere during the infall and disk formation phases and evaporates within ~10 AU of the star. Mixed CO-H2O ices are important in keeping some solid CO above 18 K and in explaining the presence of CO in comets. Material that ends up in the planet- and comet-forming zones of the disk is predicted to spend enough time in a warm zone during the collapse to form first-generation complex organic species on the grains. The dynamical timescales in the hot inner envelope (hot core or hot corino) are too short for abundant formation of second-generation molecules by high-temperature gas-phase chemistry.

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