[论文解读] Water in star-forming regions with Herschel (WISH): II. Evolution of 557 GHz 110-101 emission in low-mass protostars

L. E. Kristensen, E. F. van Dishoeck+23 more|arXiv (Cornell University)|Mar 30, 2012

Astrophysics and Star Formation Studies参考文献 111被引用 129

一句话总结

本研究利用赫歇尔/HEFI对29个低质量原恒星中的557 GHz H₂O 1₁₀–1₀₁发射线进行了首次系统性调查，揭示了水蒸气发射在Class 0源中主要由喷流成分主导，而在Class I源中则转变为较弱的、由膨胀包层主导的发射。关键发现是清晰的演化趋势：更年轻的Class 0源表现出强烈的喷流驱动水蒸气发射并伴有吸积特征，而Class I源则表现出较弱的膨胀包层和减弱的喷流活动。

ABSTRACT

(Abridged) Water is a key tracer of dynamics and chemistry in low-mass protostars, but spectrally resolved observations have so far been limited in sensitivity and angular resolution. In this first systematic survey of spectrally resolved water emission in low-mass protostellar objects, H2O was observed in the ground-state transition at 557 GHz with HIFI on Herschel in 29 embedded Class 0 and I protostars. Complementary far-IR and sub-mm continuum data (including PACS data from our program) are used to constrain the spectral energy distribution of each source. H2O intensities are compared to inferred envelope and outflow properties and CO 3-2 emission. H2O emission is detected in all objects except one. The line profiles are complex and consist of several kinematic components. The profiles are typically dominated by a broad Gaussian emission feature, indicating that the bulk of the water emission arises in outflows, not the quiescent envelope. Several sources show multiple shock components in either emission or absorption, thus constraining the internal geometry of the system. Furthermore, the components include inverse P-Cygni profiles in 7 sources (6 Class 0, 1 Class I) indicative of infalling envelopes, and regular P-Cygni profiles in 4 sources (3 Class I, 1 Class 0) indicative of expanding envelopes. "Bullets" moving at >50 km/s are seen in 4 Class 0 sources; 3 of these are new detections. In the outflow, the H2O/CO abundance ratio as a function of velocity is nearly the same for all sources, increasing from 10^-3 at <5 km/s to >10^-1 at >10 km/s. The H2O abundance in the outer envelope is low, ~10^-10. The different H2O profile components show a clear evolutionary trend: in the Class 0 sources, emission is dominated by outflow components originating inside an infalling envelope. When the infall diminishes during the Class I phase, the outflow weakens and H2O emission disappears.

研究动机与目标

• 表征不同演化阶段低质量原恒星中水蒸气发射的运动学结构。
• 区分由H₂O谱线轮廓追踪的不同物理组分（吸积、喷流、膨胀）。
• 研究水蒸气丰度和激发条件如何随原恒星演化而变化。
• 确定激波和包层密度在塑造星形成区域H₂O发射中的作用。
• 将H₂O谱线轮廓与包层动力学及喷流活动联系起来，尤其关注Class 0与Class I源之间的差异。

提出的方法

• 利用赫歇尔空间望远镜上的太赫兹本地振荡器仪器（HIFI）对557 GHz H₂O 1₁₀–1₀₁跃迁进行单点光谱测光观测。
• 结合PACS及其他仪器的远红外和亚毫米波连续成分数据，为每个源构建光谱能量分布（SED）。
• 通过将谱线轮廓分解为运动学组分（宽喷流、窄吸积/膨胀、吸收）以识别物理区域。
• 对反P-Cygni和常规P-Cygni轮廓进行建模，以推断吸积和膨胀速度及质量吸积率。
• 在假设H₂O与CO具有相似激发条件的前提下，推导出H₂O/CO丰度比随速度的变化关系。
• 使用包层密度和喷流作用力作为代理变量，评估物理条件对H₂O发射的影响。

实验结果

研究问题

• RQ1 H₂O 557 GHz谱线发射的形态在从Class 0到Class I原恒星的演化序列中如何变化？
• RQ2 H₂O谱线轮廓中的不同特征分别追踪了哪些物理组分（吸积、喷流、膨胀）？
• RQ3 H₂O/CO丰度比在谱线翼部随速度如何变化？其在Class 0与Class I源之间是否存在差异？
• RQ4 激波在增强喷流中H₂O丰度方面起什么作用？这与包层密度有何关联？
• RQ5 从P-Cygni轮廓推断出的吸积与膨胀速度如何随原恒星年龄和光度演化？

主要发现

• 在29个低质量原恒星中除TMC1A外均检测到H₂O发射，复杂谱线轮廓主要由宽高斯组分主导，表明存在喷流发射。
• 在六个Class 0源和一个Class I源中观测到吸积特征（反P-Cygni轮廓），推断的吸积速度为~0.1–1.0 km s⁻¹，质量吸积率为~10⁻⁶–10⁻⁴ M☉ yr⁻¹。
• 膨胀特征（常规P-Cygni轮廓）主要出现在Class I源中，膨胀速度约为~0.5 km s⁻¹，且驰豫 timescale 约为~10⁴年，表明其为短暂阶段。
• 在四个Class 0源中检测到速度≥50 km s⁻¹的分子弹丸，其中三个此前未被发现具有此类特征。
• H₂O/CO丰度比从低速区（<5 km s⁻¹）的~10⁻³增加至高速区（>10 km s⁻¹）的≥10⁻¹，且Class 0与Class I源之间无显著差异。
• 外层冷包层中水蒸气丰度较低，约为~10⁻¹⁰，且Class 0源中的吸收特征主要为饱和状态，证实早期阶段外包层丰度低且吸积占主导地位。