[论文解读] Bright CO ro-vibrational emission lines in the class I source GSS 30 IRS1: Probing the inner disk of a young embedded star

K. M. Pontoppidan, F. L. Schoeier+2 more|ArXiv.org|Jul 18, 2002

Astrophysics and Star Formation Studies被引用 14

一句话总结

本研究首次探测到嵌入型I类原恒星GSS 30 IRS1在4.5–4.8 μm波段的强转振动能CO发射线。该发射源自温度为515 ± 5 K的单一温度气体，最合理的解释是：在内盘（10–50 AU）的致密非离解吸积激波中，通过双极腔体的散射机制产生，总气体质量为1–100 M⊕，物理尺度为20–100 AU。

ABSTRACT

We present a 4.5-4.85 micron R=5000 spectrum of the low mass class I young stellar object GSS 30 IRS1 (L=25L_sun) in the rho Ophiuchus core, observed with the infrared spectrometer (ISAAC) on the Very Large Telescope (VLT-UT1). Strong line emission from the ro-vibrational transitions of 12CO and 13CO is detected. In total more than 40 distinct lines are seen in the covered region. The line emission is spatially extended and detected up to 2" = 320 AU from the central source but is spectrally unresolved (Delta v < 30 km/s). This is the first time strong emission in the fundamental ro-vibrational band from CO has been observed from an embedded young stellar object. The line fluxes were modeled using a 1-dimensional full radiative transfer code, which shows that the emission is fully consistent with a gas in LTE at a single well constrained temperature (T=515+/-5 K). Furthermore, the ratios between lines from the two detected isotopic species of CO show that the 12CO lines must be optically thick. However, this is inconsistent with the observed spatial extent of the emission, since this implies such low CO column densities that the lines are optically thin. A likely solution to the discrepancy is that the lines are emitted by a smaller more dense region and then scattered in the bipolar cavity present around the central star. This gives a rough estimate of the total molecular gas mass of 1-100 M_earth and a physical extent of ~20-100 AU. We propose that the most likely origin of the line emission is post-shocked gas in a dense dissociative accretion shock from the inner 10-50 AU of a circumstellar disk. The presence of a shock capable of dissociating molecules in the disk will have implications for the chemical evolution of disks around young low mass stars.

研究动机与目标

• 研究嵌入型I类原恒星GSS 30 IRS1在4.5–4.8 μm波段强转振动能CO发射的起源。
• 确定内盘区域发射气体的物理条件（温度、柱密度、光学厚度）。
• 解决观测到的谱线光学厚度与空间尺度之间的矛盾，该矛盾无法由单一发射区域解释。
• 评估吸积激波在低质量原恒星周围原行星盘的化学与物理演化中的作用。

提出的方法

• 利用甚大望远镜VLT-UT1上的ISAAC光谱仪，获取GSS 30 IRS1的高分辨率（R = 5,000）近红外光谱。
• 在M波段（4.5–4.8 μm）中识别出超过40条12CO和13CO的转振动能跃迁。
• 应用一维全辐射转移代码对谱线发射进行建模，假设局部热动平衡（LTE）。
• 利用观测到的12CO/13CO谱线流量比推断光学厚度并约束柱密度。
• 探索散射模型，以调和观测到的空间尺度（最大2 角秒 = 320 AU）与高光学厚度之间的矛盾。
• 评估其他可能情景，包括双温度气体分布，发现其与观测结果不一致。

实验结果

研究问题

• RQ1 为重现GSS 30 IRS1中观测到的CO转振动能谱线，所需物理条件（温度、密度、光学厚度）为何？
• RQ2 为何发射在空间上呈扩展状（最大320 AU）却在光谱上未分辨（Δv < 30 km s⁻¹），且如何调和高光学厚度与该特征之间的矛盾？
• RQ3 何种机制可在低柱密度区域产生温度约为515 K、光学厚度高的单一温度气体？
• RQ4 观测到的发射是否与内盘激波一致？若是，对盘内化学演化与吸积动力学有何启示？

主要发现

• CO转振动能发射最符合温度为515 ± 5 K的单一温度气体模型，CO柱密度为2 ± 0.5 × 10¹⁸ cm⁻²。
• 12CO谱线光学厚，但观测到的空间尺度（320 AU）暗示柱密度较低，造成矛盾。
• 该矛盾通过散射模型得以解决：发射源自一小片致密区域，并在双极腔体内发生散射。
• 观测到的谱线流量与比值与双温度气体分布模型不一致，排除此类模型。
• 该发射最可能由内盘（10–50 AU）的致密吸积流中产生的J型激波解释。
• 激波情景暗示显著的分子解离，对低质量原恒星内盘的化学演化具有重大影响。