Title: Chemistry and Radiative Transfer of Water in Cold, Dense Clouds Show Chinese title

Title: 冷密云中水的化学性质和辐射传输

Abstract: The Herschel Space Observatory's recent detections of water vapor in the cold, dense cloud L1544 allow a direct comparison between observations and chemical models for oxygen species in conditions just before star formation. We explain a chemical model for gas phase water, simplified for the limited number of reactions or processes that are active in extreme cold ($<$ 15 K). In this model, water is removed from the gas phase by freezing onto grains and by photodissociation. Water is formed as ice on the surface of dust grains from O and OH and released into the gas phase by photodesorption. The reactions are fast enough with respect to the slow dynamical evolution of L1544 that the gas phase water is in equilibrium for the local conditions thoughout the cloud. We explain the paradoxical radiative transfer of the H$_2$O ($1_{10}-1_{01}$) line. Despite discouragingly high optical depth caused by the large Einstein A coefficient, the subcritical excitation in the cold, rarefied H$_2$ causes the line brightness to scale linearly with column density. Thus the water line can provide information on the chemical and dynamical processes in the darkest region in the center of a cold, dense cloud. The inverse P-Cygni profile of the observed water line generally indicates a contracting cloud. This profile is reproduced with a dynamical model of slow contraction from unstable quasi-static hydrodynamic equilibrium (an unstable Bonnor-Ebert sphere). Show Chinese abstract

Abstract: 赫歇尔空间天文台最近在寒冷致密的云团L1544中探测到水蒸气，这使得我们可以直接比较观测结果与恒星形成前条件下氧物种的化学模型。 我们解释了一个针对气相水的化学模型，该模型简化了在极端低温（<$<$ 15 K）下活跃的有限数量的反应或过程。 在这个模型中，水通过冻结到尘埃颗粒上以及光解离从气相中去除。水作为冰在尘埃颗粒表面由O和OH形成，并通过光解吸释放到气相中。这些反应相对于L1544缓慢的动力学演化足够快，因此对于云中的局部条件而言，气相水处于平衡状态。 我们解释了H$_2$O ($1_{10}-1_{01}$)线的矛盾性辐射转移现象。尽管由于较大的爱因斯坦A系数导致令人沮丧的高光学深度，但在冷稀薄的H$_2$中的亚临界激发使线亮度随柱密度线性变化。因此，水线可以提供关于冷密云中心最黑暗区域中的化学和动力学过程的信息。 观测到的水线的反向P-Cygni谱通常指示一个收缩的云。这一谱线轮廓可以通过从不稳定准静态流体静力平衡（一个不稳定的Bonnor-Ebert球体）缓慢收缩的动力学模型来再现。