[论文解读] Water in Low-Mass Star-Forming Regions with Herschel: The Link Between Water Gas and Ice in Protostellar Envelopes

M. Schmalzl, R. Visser+5 more|arXiv (Cornell University)|Sep 23, 2014

Astrophysics and Star Formation Studies参考文献 95被引用 21

一句话总结

本研究利用赫歇尔望远镜观测，调查低质量原恒星中水的化学性质，建模原恒星包层中水冰与气相之间的相互作用。研究发现，由于预恒星核寿命较短（<0.1 Myr）且在温度高于15 K时原子氧无法冻结，仅有10–30%的氧预算以水冰形式存在，而FUV光致解吸与动力学过程则主导了水蒸气谱线轮廓的形成。

ABSTRACT

Aims: Our aim is to determine the critical parameters in water chemistry and the contribution of water to the oxygen budget by observing and modelling water gas and ice for a sample of eleven low-mass protostars, for which both forms of water have been observed. Methods: A simplified chemistry network, which is benchmarked against more sophisticated chemical networks, is developed that includes the necessary ingredients to determine the water vapour and ice abundance profiles in the cold, outer envelope in which the temperature increases towards the protostar. Comparing the results from this chemical network to observations of water emission lines and previously published water ice column densities, allows us to probe the influence of various agents (e.g., FUV field, initial abundances, timescales, and kinematics). Results: The observed water ice abundances with respect to hydrogen nuclei in our sample are 30-80ppm, and therefore contain only 10-30% of the volatile oxygen budget of 320ppm. The keys to reproduce this result are a low initial water ice abundance after the pre-collapse phase together with the fact that atomic oxygen cannot freeze-out and form water ice in regions with T(dust)&gt;15 K. This requires short prestellar core lifetimes of less than about 0.1Myr. The water vapour profile is shaped through the interplay of FUV photodesorption, photodissociation, and freeze-out. The water vapour line profiles are an invaluable tracer for the FUV photon flux and envelope kinematics. Conclusions: The finding that only a fraction of the oxygen budget is locked in water ice can be explained either by a short pre-collapse time of less than 0.1 Myr at densities of n(H)~1e4 cm-3, or by some other process that resets the initial water ice abundance for the post-collapse phase. A key for the understanding of the water ice abundance is the binding energy of atomic oxygen on ice.

研究动机与目标

• 确定控制低质量恒星形成区中水化学的关键参数，特别是原恒星包层中水气与冰之间的关联。
• 通过将观测到的水冰与气相丰度与理论模型进行比较，量化水对氧预算的贡献。
• 研究物理条件（如FUV辐射、初始丰度、时标和包层动力学）在塑造水丰度分布中的作用。
• 解决模型预测的高水冰丰度与观测中低水冰丰度之间的矛盾。

提出的方法

• 开发并校准了一个简化的化学网络，用于模拟冷、外层原恒星包层中水蒸气与冰的丰度分布，与复杂网络进行对比。
• 模型包含关键过程：水蒸气在尘埃颗粒上的冻结、FUV光致解吸、光致离解以及非热解吸。
• 通过改变模型参数（如FUV场、初始冰丰度、坍缩后时标、速度场）来拟合观测到的水发射线与冰柱密度。
• 采用贝叶斯模型比较方法评估参数空间，通过计算贝叶斯因子来评估不同参数值的相对可能性。
• 通过对其余自由参数进行边缘化，计算每个模型的证据，从而实现对模型拟合的统计比较。
• 该分析应用于11个同时具备赫歇尔水线观测与先前发表的冰柱密度数据的低质量原恒星。

实验结果

研究问题

• RQ1 为何在冷区中水冰形成效率高，但低质量原恒星中观测到的水冰丰度仅占总氧预算的10–30%？
• RQ2 哪些物理条件（如核寿命、FUV场或初始冰丰度）最能再现观测到的水冰与气相丰度？
• RQ3 FUV光致解吸与包层动力学如何塑造观测到的水蒸气谱线轮廓？
• RQ4 为何在温度高于约15 K时原子氧无法形成水冰，这对冰形成效率有何影响？
• RQ5 观测到的低冰丰度是否可由短暂的预恒星核寿命或坍缩后初始冰丰度的重置来解释？

主要发现

• 样本中水冰丰度相对于氢核的范围为30至80 ppm，仅占总挥发性氧预算320 ppm的10–30%。
• 低冰丰度最合理的解释是预恒星核寿命极短（≤0.1 Myr），在密度约10⁴ cm⁻³的条件下，无法实现氧的完全冻结。
• 在尘埃温度高于15 K的区域，原子氧无法有效形成水冰，这限制了包层较暖区域的冰形成。
• 水蒸气谱线轮廓强烈受FUV光致解吸与包层动力学影响，使其成为FUV通量与喷流动力学的敏感示踪。
• 坍缩后初始水冰丰度较低是重现观测冰丰度的关键，表明冰化学在坍缩后可能被重置。
• 原子氧在冰表面的结合能是控制高温下冰形成效率的关键未知参数。