[论文解读] Origin of the hot gas in low-mass protostars: Herschel-PACS spectroscopy of HH 46

T. A. van Kempen, L. E. Kristensen+63 more|UvA-DARE (University of Amsterdam)|May 12, 2010

Astrophysics and Star Formation Studies参考文献 25被引用 62

一句话总结

本研究利用赫歇尔-PACS光谱仪对低质量原恒星HH 46中的热气体起源进行研究，发现激波加热（尤其是J型和C型激波）主导了远红外谱线冷却，占总冷却的至少60%，而紫外加温的腔壁和被动包层加热则贡献其余部分。数据排除了仅靠被动加热的可能性，并支持激波驱动的H₂O、OH和[O i]发射。

ABSTRACT

'Water in Star-forming regions with Herschel' (WISH) is a Herschel Key Programme aimed at understanding the physical and chemical structure of young stellar objects (YSOs) with a focus on water and related species. The low-mass protostar HH 46 was observed with the Photodetector Array Camera and Spectrometer (PACS) on the Herschel Space Observatory to measure emission in H2O, CO, OH, [OI], and [CII] lines located between 63 and 186 um. The excitation and spatial distribution of emission can disentangle the different heating mechanisms of YSOs, with better spatial resolution and sensitivity than previously possible. Far-IR line emission is detected at the position of the protostar and along the outflow axis. The OH emission is concentrated at the central position, CO emission is bright at the central position and along the outflow, and H2O emission is concentrated in the outflow. In addition, [OI] emission is seen in low-velocity gas, assumed to be related to the envelope, and is also seen shifted up to 170 km/s in both the red- and blue-shifted jets. Envelope models are constructed based on previous observational constraints. They indicate that passive heating of a spherical envelope by the protostellar luminosity cannot explain the high-excitation molecular gas detected with PACS, including CO lines with upper levels at >2500 K above the ground state. Instead, warm CO and H2O emission is probably produced in the walls of an outflow-carved cavity in the envelope, which are heated by UV photons and non-dissociative C-type shocks. The bright OH and [OI] emission is attributed to J-type shocks in dense gas close to the protostar. In the scenario described here, the combined cooling by far-IR lines within the central spatial pixel is estimated to be 2 imes 10-2 L_sun, with 60-80% attributed to J- and C-type shocks produced by interactions between the jet and the envelope.

研究动机与目标

• 确定低质量原恒星HH 46中观测到的远红外谱线发射的主要加热机制。
• 区分被动包层加热、紫外加温腔壁和激波特性的贡献，以解释谱线发射的来源。
• 量化CO、H₂O、OH和[O i]在中心原恒星区域的相对冷却贡献。
• 检验激波模型或被动包层模型是否能重现观测到的谱线强度比和强度。

提出的方法

• 利用赫歇尔-PACS仪器对HH 46进行远红外光谱观测，检测波长63至186 μm范围内的H₂O、CO、OH、[O i]和[C ii]发射线。
• 通过光谱元（spaxels）进行空间分辨的谱线发射分析，以区分来自中心原恒星、喷流和包层的发射。
• 使用单组分平板模型结合逃逸概率代码，对谱线强度比和强度进行建模，包含尘埃连续谱吸收和发射。
• 将观测到的谱线强度比（如H₂O 183 μm / 119 μm，[O i] 63 μm / 146 μm）与J型和C型激波模型及被动包层模型的预测进行比较。
• 通过积分谱线光度并对比原恒星光度，估算各物种的冷却速率。
• 利用激波模型（Neufeld & Dalgarno, 1989；Snell et al., 2005）约束发射区域的物理条件，如密度（n_H ~ 10⁷ cm⁻³）和温度（T_gas > 800 K）。

实验结果

研究问题

• RQ1 在HH 46中，导致观测到的高激发分子气体的主要加热机制是被动包层加热还是激波特性的？
• RQ2 OH、H₂O和[O i]的观测谱线强度比能否由激波模型再现？这些模型暗示了怎样的物理条件（密度、温度）？
• RQ3 紫外加温腔壁、C型激波和J型激波在驱动总远红外谱线冷却中的贡献如何比较？
• RQ4 观测到的OH发射是否与腔壁或喷流撞击区一致？这对激波形态有何启示？
• RQ5 中心光谱元中的观测谱线发射在总远红外冷却中占多大比例？与以往ISO-LWS测量相比如何？

主要发现

• 中心光谱元中[O i] 63 μm / 146 μm谱线强度比约为16，与低密度（~10⁴ cm⁻³）下的快速解离激波一致，排除了被动包层加热的可能性。
• OH发射来自高密度（n_H ~ 10⁷ cm⁻³）和高温（T_gas > 800 K）区域，物理尺度约为0.5″（~250 AU），与被动加热或腔壁处的C型激波不一致。
• 激波模型（J型和C型）在中心光谱元中贡献了至少60%的总远红外谱线冷却，仅激波的总冷却速率为1.5 × 10⁻² L☉。
• CO冷却分布在三个组分中：被动包层（0.1 × 10⁻³ L☉）、紫外加温腔壁（3.8 × 10⁻³ L☉）和C型激波（2.8 × 10⁻³ L☉），表明存在多种加热机制。
• H₂O发射最可能由紫外加热或C型激波解释，这些组分的总冷却贡献达5.0 × 10⁻³ L☉，无法仅由被动加热解释。
• 中心光谱元的总远红外谱线冷却超过23.8 × 10⁻³ L☉，与ISO-LWS测量结果一致，表明激波是谱线发射的主要能量来源。