[论文解读] Shock excited far-infrared molecular emission around T Tau

L. Spinoglio, T. Giannini+10 more|Open Research Online (The Open University)|Sep 29, 1999

Astrophysics and Star Formation Studies参考文献 1被引用 4

一句话总结

本研究首次获得了T Tauri的完整远红外光谱，揭示了在约300–400 AU的致密区域（温度为300–900 K）存在强烈的高J态CO、H2O和OH发射线。数据表明，恒星风在喷流基底区域引发激波激发，水和OH的丰度较环境水平提高逾10倍，与辐射解离过程一致，且冷却速率与喷流的机械光度相当。

ABSTRACT

The first complete far-infrared spectrum of T Tau has been obtained with the LWS spectrometer on-board the Infrared Space Observatory, which detected strong emission from high-J (J=14-25) CO, para- and ortho-H2O and OH transitions over the wavelength range from 40 to 190 micron. Most of the observed molecular emission can be explained by a single emission region at T~300-900 K and n(H_2)~10^(5-6) cm^(-3),with a diameter of about 2-3 arcsec. This corresponds to a very compact region of 300-400 AU at the distance of 140 pc. An higher temperature component seems to be needed to explain the highest excitation CO and water lines. We derive a water abundance of 1-7x10^(-5) and an OH abundance of ~3x10^(-5) with respect to molecular hydrogen, implying water and OH enhancements by more than a factor of 10 with respect to the expected ambient gas abundance. The observed cooling in the various species amounts to 0.04 L(sun),comparable to the mechanical luminosity of the outflow, indicating that the stellar winds could be responsible of the line excitation through shocks. In order to explain the observed molecular cooling in T Tau in terms of C-type shock models, we hypothesise that the strong far-ultraviolet radiation field photodissociates water in favour of OH. This would explain the large overabundance of OH observed. The estimated relatively high density and compactness of the observed emission suggest that it originates from the shocks taking place at the base of the molecular outflow emission, in the region where the action of the stellar winds from the two stars of the binary system is important.

研究动机与目标

• 利用高分辨率光谱学表征T Tauri双星系统周围的远红外分子辐射。
• 确定原行星环境发射区域的物理条件（温度、密度、尺度）。
• 研究水和OH丰度相对于平静星际介质值显著增强的成因。
• 评估恒星风驱动激波是否可解释观测到的谱线激发与冷却速率。
• 评估远紫外辐射在光致离解作用下对分子丰度变化的影响。

提出的方法

• 利用红外空间天文台（ISO）上的LWS光谱仪获取了T Tauri的完整远红外光谱。
• 分析了CO、正-和仲-态H2O以及OH在40–190 μm波段的高J（J=14–25）转动能级跃迁的发射线。
• 通过C型激波模型推断发射气体的温度、密度和柱密度。
• 通过将观测到的谱线流量与辐射转移计算结果对比，估算分子相对于H2的丰度。
• 通过比较总分子冷却光度与喷流的机械光度，评估能量收支平衡。
• 评估远紫外辐射对水和OH化学的影响，特别是H2O光致离解为OH的作用。

实验结果

研究问题

• RQ1 T Tauri周围远红外发射区域的物理条件（温度、密度、尺度）是什么？
• RQ2 CO、H2O和OH观测到的高J分子跃迁由何种机制激发？
• RQ3 为何水和OH的丰度显著高于典型星际丰度？
• RQ4 观测到的分子冷却速率在多大程度上与喷流的机械光度相匹配？
• RQ5 远紫外辐射在激波激发气体中如何影响H2O/OH丰度比？

主要发现

• 远红外光谱显示在40–190 μm波段存在强烈的高J CO（J=14–25）、正-和仲-态H2O以及OH跃迁发射。
• 发射来自一个大小为300–400 AU的紧凑区域，温度为300–900 K，H2密度为10^5–10^6 cm^-3。
• 水的丰度估计为相对于H2的1–7×10^-5，OH丰度约为3×10^-5，两者均较环境值提高逾10倍。
• 总分子冷却光度为0.04 L⊙，与喷流的机械光度相当，支持激波激发的解释。
• 高OH丰度可由强远紫外辐射场中H2O的光致离解解释。
• 数据最符合喷流基底处的C型激波模型，该区域由双星系统的恒星风驱动激波。