[论文解读] High-pressure, low-abundance water in bipolar outflows. Results from a Herschel-WISH survey

M. Tafalla, René Liseau+7 more|arXiv (Cornell University)|Jan 22, 2013

Astrophysics and Star Formation Studies参考文献 100被引用 27

一句话总结

本研究利用赫歇尔-WISH对低质量原恒星喷流中正水在557和1670 GHz处的观测，探测了其中的暖、激波气体。研究发现水的典型丰度为3×10⁻⁷，气体压力较高（nT ≈ 4×10⁹ cm⁻³K），激发温度较低（约25 K），由于高压缩和低丰度，与标准C-激波模型存在矛盾。

ABSTRACT

(Abridged) We present a survey of the water emission in a sample of more than 20 outflows from low mass young stellar objects with the goal of characterizing the physical and chemical conditions of the emitting gas. We have used the HIFI and PACS instruments on board the Herschel Space Observatory to observe the two fundamental lines of ortho-water at 557 and 1670 GHz. These observations were part of the "Water In Star-forming regions with Herschel" (WISH) key program, and have been complemented with CO and H2 data. We find that the emission from water has a different spatial and velocity distribution from that of the J=1-0 and 2-1 transitions of CO, but it has a similar spatial distribution to H2, and its intensity follows the H2 intensity derived from IRAC images. This suggests that water traces the outflow gas at hundreds of kelvins responsible for the H2 emission, and not the component at tens of kelvins typical of low-J CO emission. A warm origin of the water emission is confirmed by a remarkable correlation between the intensities of the 557 and 1670 GHz lines, which also indicates the emitting gas has a narrow range of excitations. A non-LTE radiative transfer analysis shows that while there is some ambiguity on the exact combination of density and temperature values, the gas thermal pressure nT is constrained within less than a factor of 2. The typical nT over the sample is 4 10^{9} cm^{-3}K, which represents an increase of 10^4 with respect to the ambient value. The data also constrain within a factor of 2 the water column density. When this quantity is combined with H2 column densities, the typical water abundance is only 3 10^{-7}, with an uncertainty of a factor of 3. Our data challenge current C-shock models of water production due to a combination of wing-line profiles, high gas compressions, and low abundances.

研究动机与目标

• 表征低质量原恒星喷流中水辐射气体的物理与化学条件。
• 利用高频水线确定H₂O辐射气体的激发温度、密度和压力。
• 评估喷流组分中水的丰度相对于标准激波模型的差异。
• 研究水与CO谱线轮廓之间的差异，暗示存在不同的气体组分。
• 评估观测到的线强度比与C-激波模型和J-激波模型的一致性。

提出的方法

• 利用赫歇尔望远镜的HIFI和PACS仪器，对正水557和1670 GHz谱线进行高分辨率光谱观测。
• 将水谱线轮廓与CO（J=1-0, 2-1）及IRAC图像中的H₂辐射进行比较，以识别空间与运动学上的关联。
• 采用大速度梯度（LVG）方法进行非局部热动平衡辐射转移建模，以约束气体条件。
• 利用557和1670 GHz谱线的强度比推断激发温度，并约束nT乘积（热压力）。
• 结合IRAC测得的H₂柱密度与H₂O柱密度，推导水的丰度。
• 评估模型与C-激波和J-激波化学的一致性，考虑H₂O的破坏机制。

实验结果

研究问题

• RQ1 低质量原恒星喷流中H₂O辐射气体的激发温度与热压力是多少？
• RQ2 水辐射的空间与运动学分布如何与CO和H₂示踪剂相比较？
• RQ3 暖喷流气体中水的丰度是多少？与C-激波模型的预测相比如何？
• RQ4 为何观测到的谱线轮廓与强度比偏离标准C-激波模型的预期？
• RQ5 J型激波是否能更好地解释观测到的高压力、低丰度条件？

主要发现

• 557与1670 GHz水谱线的强度比集中在2至4之间，表明激发温度范围较窄，约为25 K。
• 在样本中，数密度与温度的乘积（nT）被约束在~4×10⁹ cm⁻³K，较周围云团条件高出约10⁴倍。
• 水的柱密度在2×10¹²至10¹⁴ cm⁻²之间变化，典型水丰度为3×10⁻⁷（相对于H₂），不确定性为3倍。
• 水辐射在空间上与IRAC观测到的H₂辐射相关，表明其追踪的是温度约500 K的激波气体，而非冷的CO辐射气体。
• 高压力与低丰度与标准C-激波模型不一致，提示J型激波可能是成因。
• 冷喷流组分中可能存在一个潜在的隐藏水储存库，可通过H₂O光谱中的宽自吸收特征探测到。