[论文解读] Water in star-forming regions with Herschel (WISH) V. The physical conditions in low-mass protostellar outflows revealed by multi-transition water observations

J. C. Mottram, L. E. Kristensen+18 more|arXiv (Cornell University)|Sep 19, 2014

Astrophysics and Star Formation Studies参考文献 81被引用 30

一句话总结

本研究利用赫歇尔HIFI对29个低质量Class 0/I原恒星的多个水跃迁谱线进行高分辨率观测，以约束喷流激波中的物理条件。研究发现，原恒星处的水辐射源自两种不同的激波组分——喷流中的J-激波和腔壁中的C-激波，二者具有相似的激波后H2密度（10⁵–10⁸ cm⁻³）和H2O柱密度（10¹⁶–10¹⁸ cm⁻²），但几何结构和填充因子不同，分别表现为10–200 AU（点状激波）的紧凑发射区域和1–30 AU（腔壁激波）的薄层结构。

ABSTRACT

Context: Outflows are an important part of the star formation process as both the result of ongoing active accretion and one of the main sources of mechanical feedback on small scales. Water is the ideal tracer of these effects because it is present in high abundance in various parts of the protostar. Method: We present extit{Herschel} HIFI spectra of multiple water-transitions towards 29 nearby Class 0/I protostars as part of the WISH Survey. These are decomposed into different Gaussian components, with each related to one of three parts of the protostellar system; quiescent envelope, cavity shock and spot shocks in the jet and at the base of the outflow. We then constrain the excitation conditions present in the two outflow-related components. Results: Water emission is optically thick but effectively thin, with line ratios that do not vary with velocity, in contrast to CO. The physical conditions of the cavity and spot shocks are similar, with post-shock H$_{2}$ densities of order 10$^{5}-$10$^{8}$\,cm$^{-3}$ and H$_{2}$O column densities of order 10$^{16}-$10$^{18}$\,cm$^{-2}$. H$_{2}$O emission originates in compact emitting regions: for the spot shocks these correspond to point sources with radii of order 10-200\,AU, while for the cavity shocks these come from a thin layer along the outflow cavity wall with thickness of order 1-30\,AU. Conclusions: Water emission at the source position traces two distinct kinematic components in the outflow; J shocks at the base of the outflow or in the jet, and C shocks in a thin layer in the cavity wall. Class I sources have similar excitation conditions to Class 0 sources, but generally smaller line-widths and emitting region sizes. We suggest that it is the velocity of the wind driving the outflow, rather than the decrease in envelope density or mass, that is the cause of the decrease in H$_{2}$O intensity between Class 0 and I.

研究动机与目标

• 利用多跃迁水辐射约束低质量原恒星喷流中的物理条件。
• 通过速度分辨谱线区分喷流-喷流系统中的激波组分，特别是腔壁激波与点状激波。
• 通过非局部热动平衡（non-LTE）辐射转移建模量化喷流相关组分的激发条件。
• 比较Class 0与Class I原恒星之间的物理条件，并解释观测到的H2O辐射强度下降现象。
• 通过考虑几何结构与激波特性的差异，解决源上与源外激波诊断之间的矛盾。

提出的方法

• 在WISH关键计划中获取了29个Class 0/I原恒星的多条水跃迁的高分辨率赫歇尔HIFI速度分辨谱线。
• 将谱线分解为三个系统组分的高斯组分：静止包层、腔壁激波和喷流中的点状激波。
• 应用非局部热动平衡辐射转移建模（RADEX）以约束两个喷流相关组分的激发条件。
• 利用线强度比和光学深度估计（如988 GHz处的τ）评估水辐射的光学深度和激发状态。
• 将源上激波条件与源外测量结果进行比较，以识别几何结构与激波特性的差异。
• 评估风速、包层密度和质量对Class 0到Class I源中H2O辐射强度演化的影响。

实验结果

研究问题

• RQ1 低质量原恒星喷流中腔壁激波与点状激波组分的物理条件（温度、密度、柱密度）是什么？
• RQ2 源上水辐射的激发条件与源外激波区域的条件相比如何？
• RQ3 尽管激发条件相似，为何H2O谱线强度从Class 0到Class I源会下降？
• RQ4 激波几何结构与填充因子在塑造观测到的水谱线轮廓和光学深度方面起到何种作用？
• RQ5 J-激波（喷流）与C-激波（腔壁）在密度、温度和发射区域大小方面有何物理特性差异？

主要发现

• 原恒星处的水辐射光学厚度较高，但有效光学厚度较低，其线强度比与速度无关，与CO行为不同。
• 腔壁激波与点状激波具有相似的激波后H2密度（10⁵–10⁸ cm⁻³）和H2O柱密度（10¹⁶–10¹⁸ cm⁻²），表明激发条件相近。
• 点状激波起源于半径为10–200 AU的紧凑区域，而腔壁激波则来自沿喷流腔壁分布的薄层（厚度1–30 AU）。
• Class I源的激发条件与Class 0源相似，但谱线宽度更窄，发射区域尺寸更小。
• H2O辐射强度从Class 0到Class I的下降归因于喷流中的风速，而非包层密度或质量的减少。
• 源上与源外激波诊断结果的差异源于激波特性和几何结构的差异，其中源上区域具有更高的填充因子，而源外测量中柱密度更低。