[论文解读] Water in star forming regions with Herschel (WISH) III. Far-infrared cooling lines in low-mass young stellar objects

A. Karska, Gregory J. Herczeg+23 more|arXiv (Cornell University)|Jan 21, 2013

Astrophysics and Star Formation Studies参考文献 80被引用 62

一句话总结

本研究利用赫歇尔-PACS光谱仪在WISH重点计划框架下，对18个低质量原恒星的远红外谱线发射进行了分析，结果表明水蒸气和CO主导了气体冷却预算，其激发成分与喷流壁面处的非解离激波相关。结果显示出高-J CO与H2O谱线流量与总辐射光度之间存在强烈相关性，表明其具有共同的物理起源；而[O i]和OH则追踪内包层中的解离激波，其冷却效率从Class 0源向Class I源逐渐降低。

ABSTRACT

(Abridged) Far-infrared Herschel-PACS spectra of 18 low-mass protostars of various luminosities and evolutionary stages are studied. We quantify their far-infrared line emission and the contribution of different atomic and molecular species to the gas cooling budget during protostellar evolution. We also determine the spatial extent of the emission and investigate the underlying excitation conditions. Most of the protostars in our sample show strong atomic and molecular far-infrared emission. Water is detected in 17 objects, including 5 Class I sources. The high-excitation H2O line at 63.3 micron is detected in 7 sources. CO transitions from J=14-13 up to 49-48 are found and show two distinct temperature components on Boltzmann diagrams with rotational temperatures of ~350 K and ~700 K. H2O has typical excitation temperatures of ~150 K. Emission from both Class 0 and I sources is usually spatially extended along the outflow direction but with a pattern depending on the species and the transition. The H2O line fluxes correlate strongly with those of the high-J CO lines, as well as with the bolometric luminosity and envelope mass. They correlate less strongly with OH and not with [OI] fluxes. The PACS data probe at least two physical components. The H2O and CO emission likely arises in non-dissociative (irradiated) shocks along the outflow walls with a range of pre-shock densities. Some OH is also associated with this component, likely resulting from H2O photodissociation. UV-heated gas contributes only a minor fraction to the CO emission observed by PACS, based on the strong correlation between the shock-dominated CO 24-23 line and the CO 14-13 line. [OI] and some of the OH emission probe dissociative shocks in the inner envelope. The total far-infrared cooling is dominated by H2O and CO, with [OI] increasing for Class I sources.

研究动机与目标

• 量化低质量原恒星的远红外谱线发射，并确定原子和分子物种对气体冷却预算的贡献。
• 研究Class 0和I源中谱线发射的空间分布与激发条件。
• 理解原恒星演化嵌入阶段中能量过程与冷却机制的演化。
• 厘清激波与紫外加热在激发年轻恒星物体远红外谱线中的作用。

提出的方法

• 利用赫歇尔-PACS光谱仪对18个低质量原恒星进行观测，覆盖不同光度和演化阶段。
• 为CO和H2O构建转动能级图，以推导转动能温和评估激发条件。
• 在多个空间元（spaxels）上进行空间分辨的流量测量，以区分紧凑与延展发射成分。
• 分析谱线流量、总辐射光度（Lbol）和包层质量（Menv）之间的相关性，以推断物理关联。
• 将谱线轮廓与空间分布与低-J CO喷流图进行比较，将谱线发射与喷流结构关联。
• 利用激波和紫外加热模型解释观测到的谱线比值与空间分布模式。

实验结果

研究问题

• RQ1 在低质量原恒星中，H2O、CO、[O i]和OH对远红外气体冷却预算的相对贡献是什么？
• RQ2 不同物种的发射空间分布如何与喷流结构及物理组分相关？
• RQ3 激发观测到的高-J CO和H2O谱线的物理条件（温度、密度、激波）是什么？
• RQ4 这些谱线的激发与冷却特性如何从Class 0演化到Class I原恒星？
• RQ5 紫外加热与激波激发在产生观测到的远红外谱线发射中各自起什么作用？

主要发现

• 在18个低质量原恒星中检测到水蒸气的有17个，包括5个Class I源；179.5 µm处的212−101跃迁线在所有源中均被检测到，63.3 µm处的818−707跃迁线在7个源中被检测到。
• CO谱线从J = 14−13延伸至J = 48−47，转动能级图揭示了两个分别约为350 K和700 K的独立组分。
• 在样本的一半中，H2O和CO发射在喷流方向上呈空间延展，覆盖至少10,000 AU的尺度；另一半则显示在1,000 AU以内的紧凑发射。
• H2O和高-J CO谱线流量与总辐射光度及包层质量强烈相关，表明其共同起源于激波激发的气体。
• [O i]和OH谱线在中心位置达到峰值，且彼此相关，但与H2O或CO流量无关，表明其追踪的是不同的内区，如解离激波区域。
• H2O对总远红外冷却的贡献为25–50%，CO为5–50%，[O i]贡献5–30%，且在Class I源中其相对重要性增加。