[论文解读] Water in star-forming regions with Herschel (WISH). IV. A survey of low-J H2O line profiles toward high-mass protostars

F. van der Tak, L. Chavarría+5 more|Chalmers Publication Library (Chalmers University of Technology)|Apr 10, 2013

Astrophysics and Star Formation Studies参考文献 1被引用 33

一句话总结

本研究基于赫歇尔/HIFI对19个大质量原恒星的低-J水线进行光谱观测，通过多高斯拟合分离出包层、喷流和前景云的发射与吸收成分。主要发现表明，基态水线追踪外层包层物质，其辐射强度与光度或质量无显著趋势；而987 GHz激发态线的辐射通量随总光度和远红外辐射通量增加，表明其可作为温度示踪剂及高红移探测的指标。

ABSTRACT

To understand the origin of water line emission and absorption during high-mass star formation, we decompose high-resolution Herschel-HIFI line spectra toward 19 high-mass star-forming regions into three distinct physical components. Protostellar envelopes are usually seen as narrow absorptions or emissions in the H2O 1113 and 1669 GHz ground-state lines, the H2O 987 GHz excited-state line, and the H2O-18 1102 GHz ground-state line. Broader features due to outflows are usually seen in absorption in the H2O 1113 and 1669 GHz lines, in 987 GHz emission, and not seen in H2O-18, indicating a low column density and a high excitation temperature. The H2O 1113 and 1669 GHz spectra show narrow absorptions by foreground clouds along the line of sight, which have a low column density and a low excitation temperature, although their H2O ortho/para ratios are close to 3. The intensities of the H2O 1113 and 1669 GHz lines do not show significant trends with luminosity, mass, or age. In contrast, the 987 GHz line flux increases with luminosity and the H2O-18 line flux decreases with mass. Furthermore, appearance of the envelope in absorption in the 987 GHz and H2O-18 lines seems to be a sign of an early evolutionary stage. We conclude that the ground state transitions of H2O trace the outer parts of the envelopes, so that the effects of star formation are mostly noticeable in the outflow wings. These lines are heavily affected by absorption, so that line ratios of H2O involving the ground states must be treated with caution. The average H2O abundance in high-mass protostellar envelopes does not change much with time. The 987 GHz line appears to be a good tracer of the mean weighted dust temperature of the source, which may explain why it is readily seen in distant galaxies.

研究动机与目标

• 利用低-J水线轮廓特征表征大质量原恒星环境的物理与运动学组分。
• 从水线轮廓中分离出原恒星包层、双极喷流和前景云的贡献。
• 评估水线特性对总光度、包层质量及远红外通量密度的依赖性。
• 研究水线轮廓在原恒星演化阶段中的演化趋势。
• 基于线流量与激发行为，评估水线在高红移星系中的可探测性。

提出的方法

• 利用赫歇尔/HIFI仪器对低-J水跃迁（1113、1669和987 GHz）进行光谱观测。
• 采用多高斯拟合方法，将复杂线轮廓分解为包层、喷流和前景云的贡献。
• 分析1113和1669 GHz基态线与总光度、包层质量及远红外通量密度之间的趋势。
• 检查987 GHz激发态线与光度及60和100 µm波段远红外通量的相关性，以评估其对源温度的敏感性。
• 通过水分子18O基态线轮廓随包层质量的变化，推断水丰度在时间上的稳定性。
• 通过中红外亮度及L_bol / M_env比值探测演化阶段，将线轮廓形态与源年龄关联。

实验结果

研究问题

• RQ1 大质量原恒星中水线轮廓如何反映包层、喷流和前景云的贡献？
• RQ2 基态水线的强度或线宽是否与总光度、包层质量或远红外通量密度相关？
• RQ3 987 GHz激发态水线如何随源光度和远红外通量变化？这对高红移探测有何启示？
• RQ4 随着包层质量增加，H2 18O基态线从发射转为吸收，是否表明水丰度在时间上保持恒定？
• RQ5 水线轮廓形态（如吸收与发射）如何随原恒星演化阶段演变？

主要发现

• 1113和1669 GHz基态水线在总光度、包层质量或远红外通量密度方面均无显著趋势，表明其追踪的是恒星形成效应较弱的外层包层。
• 基态线中的宽线翼明显与双极喷流相关，证实其作为喷流运动学的示踪剂。
• 987 GHz激发态水线的辐射通量系统性地随总光度和60与100 µm波段远红外通量增加，表明其追踪的是更温暖、更活跃的区域。
• 随着包层质量增加，H2 18O基态线从发射转为吸收，表明原恒星包层中的水丰度在时间上保持相对恒定。
• 年轻原恒星（L_bol / M_env较高）在所有水线中均呈现纯吸收轮廓，而更成熟的源则在一条或多条线中表现出发射，表明线轮廓形态存在明显演化。
• 吸积或膨胀特征（P Cygni轮廓）与演化阶段无相关性，表明此类运动在本样本中并非可靠的演化指标。