[论文解读] Outflows from young objects observed with the ISO-LWS I. Fine structure lines [O I] 63, [O I]145 and [C II]157 micron

R. Liseau, K. Justtanont+1 more|ArXiv.org|Sep 28, 2005

Spectroscopy and Laser Applications被引用 33

一句话总结

本研究利用ISO-LWS数据，分析了年轻恒星物体中远红外微结构线[O I] 63, 145和[C II] 157 μm，评估其在星形成区域的诊断能力。研究发现，[O I] 63/145 μm线强度比异常偏低，最合理的解释是冷而稀薄的气体对63 μm线产生前景吸收，而非放大或光学厚度效应，这显著限制了这些线在喷流区域的诊断实用性。

ABSTRACT

Far infrared fine structure line data from the ISO archive have been extracted for several hundred YSOs and their outflows, including molecular (CO) outflows, optical jets and Herbig-Haro (HH) objects. Given the importance of these lines to astrophysics, their excitation and transfer ought to be investigated in detail and, at this stage, the reliability of the diagnostic power of the fine structure transitions of O I and C II has been examined. Several issues, such as the extremely small intensity ratios of the oxygen 63 micron to 145 micron lines, are still awaiting an explanation. It is demonstrated that, in interstellar cloud conditions, the 145 micron line is prone to masing, but that this effect is likely an insufficient cause of the line ratio anomaly observed from cold dark clouds. Very optically thick emission could in principle also account for this, but would need similar, prohibitively high column densities and must therefore be abondoned as a viable explanation. One is left with [O I] 63 micron self absorption by cold and tenuous foreground gas, as has been advocated for distant luminous sources. Recent observations with the submillimeter observatory Odin support this scenario also in the case of nearby dark molecular clouds. On the basis of this large statistical material we are led to conclude that in star forming regions, the [O I] and [C II] lines generally have only limited diagnostic value.

研究动机与目标

• 评估[O I] 63, 145 μm和[C II] 157 μm微结构线在年轻恒星物体（YSO）喷流中的可靠性与诊断潜力。
• 解决在冷暗云中观测到的[O I] 63/145 μm强度比异常偏低的问题，该现象与标准的光学薄发射模型相矛盾。
• 检验竞争性解释，如145 μm线的放大效应、光学厚度效应以及前景吸收。
• 确定这些线是否能可靠地追踪喷流中密度和温度等物理条件。
• 评估这些线在星际介质恒星形成研究中的局限性。

提出的方法

• 分析552个YSO的ISO-LWS中等分辨率光谱观测，包括分子喷流、光学喷流和赫比希-哈罗天体。
• 通过在谱线中心处对谱通量密度进行高斯拟合提取线流量，并基于连续谱噪声采用保守的误差估计。
• 利用3σ上限，基于均方根噪声和光谱分辨率单元（63 μm为spl = 0.29 μm，145/157 μm为0.6 μm）。
• 将观测到的[O I] 63/145 μm和[O I] 63/[C II] 157 μm强度比与理论模型进行统计比较。
• 应用三能级O I基态（3P_J）系统的统计平衡方程，包括碰撞速率和辐射跃迁概率。
• 使用Schöier等人（2005）提供的原子数据进行数值建模与解析求解，特别关注反转与放大效应。

实验结果

研究问题

• RQ1 为何在光学薄条件下，观测到的[O I] 63 μm / 145 μm强度比显著低于预期？
• RQ2 仅靠145 μm线的放大效应是否足以解释冷暗云中观测到的线强度比异常？
• RQ3 考虑到所需柱密度，光学厚发射是否是63/145 μm比值偏低的可行解释？
• RQ4 冷而稀薄气体对63 μm线的前景吸收是否能为观测到的比值提供一致的解释？
• RQ5 O I和C II微结构线在YSO喷流中对物理条件的诊断价值在多大程度上得以保留？

主要发现

• 绝大多数观测到的[O I] 63/145 μm强度比与光学薄发射不一致，反而表明在低温下存在光学厚发射。
• 仅靠145 μm线的放大效应不足以解释冷暗云中观测到的线强度比异常。
• 光学厚发射需要不切实际的高柱密度，因此是不可行的解释。
• 冷而稀薄气体（N(O I) ≈ 1×10^17 cm⁻²）对63 μm线的前景吸收提供了一个可行的解释，且得到Odin对H2O线观测的支持。
• [O I] 63 μm / [C II] 157 μm线强度比也受到类似影响，降低了其诊断可靠性。
• 总体而言，由于吸收效应的影响，[O I]和[C II]微结构线在银河系星形成区域中的诊断价值有限。