[论文解读] The impact of shocks on the chemistry of molecular clouds: high resolution images of chemical differentiation along the NGC1333-IRAS2A outflow

J. K. Jørgensen, M. R. Hogerheijde+4 more|ArXiv.org|Nov 5, 2003

Astrophysics and Star Formation Studies被引用 52

一句话总结

本研究利用高分辨率干涉仪和单镜面亚毫米波观测，调查了NGC1333-IRAS2A出流中由激波驱动的化学过程。结果表明，在原恒星出流与致密凝聚体之间的15,000 AU界面处，激波加热引发显著的化学分异，CH₃OH、SiO及含硫物种在激波气体中的丰度提高了2–4个数量级，而HCO⁺仅在激波之后被探测到，表明其经历了尘埃冰层的化学处理，并在高速条件下选择性地存活下来。

ABSTRACT

This paper presents a detailed study of the chemistry in the outflow associated with the low-mass protostar NGC1333-IRAS2A down to 3" (650 AU) scales. Millimeter-wavelength aperture-synthesis observations from the OVRO and BIMA interferometers and (sub)millimeter single-dish observations from the Onsala 20m telescope and CSO are presented. The interaction of the highly collimated protostellar outflow with a molecular condensation ~15000 AU from the central protostar is clearly traced by molecular species such as HCN, SiO, SO, CS, and CH3OH. Especially SiO traces a narrow high velocity component at the interface between the outflow and the molecular condensation. Multi-transition single-dish observations are used to distinguish the chemistry of the shock from that of the molecular condensation and to address the physical conditions therein. Statistical equilibrium calculations reveal temperatures of 20 and 70 K for the quiescent and shocked components, respectively, and densities near 10^6 cm^{-3}. Significant abundance enhancements of two to four orders of magnitude are found in the shocked region for molecules such as CH3OH, SiO and the sulfur-bearing molecules. HCO+ is seen only in the aftermath of the shock consistent with models where it is destroyed through release of H2O from grain mantles in the shock. N2H+ shows narrow lines, not affected by the outflow but rather probing the ambient cloud. Differences in abundances of HCN, H2CO and CS are seen between different outflow regions and are suggested to be related to differences in the atomic carbon abundance. Compared to the warm inner parts of protostellar envelopes, higher abundances of in particular CH3OH and SiO are found in the outflows, which may be related to density differences between the regions.

研究动机与目标

• 通过高分辨率空间与光谱分析，研究原恒星出流对分子云化学的影响。
• 在低质量原恒星环境的激波与静止包层化学之间进行分离。
• 确定激波气体与周围气体组分的物理条件（温度、密度）及化学丰度。
• 将NGC1333-IRAS2A中的分子丰度模式与其它出流区域及原恒星包层进行比较，以评估环境与动力学因素的影响。
• 探讨原子碳丰度与密度在塑造不同出流中观测到的化学差异中的作用。

提出的方法

• 利用欧文斯山谷与BIMA干涉仪的综合孔径干涉测量，实现3″（650 AU）尺度的空间结构解析。
• 利用翁索拉空间天文台与加州理工亚毫米波天文台的多跃迁单镜面谱线观测，探测激发态与物理条件。
• 对分子谱线轮廓进行统计平衡建模，推导静止与激波气体组分的温度与密度。
• 比较不同跃迁与仪器的谱线轮廓，评估各组分内物理条件的均匀性。
• 分析激波示踪分子（如SiO、CH₃OH）相对于静止物种（如HCO⁺、N₂H⁺）的丰度增强，以推断激波化学机制。
• 利用N₂H⁺作为周围非激波气体的示踪剂，因其谱线窄且在出流物质中无增强。

实验结果

研究问题

• RQ1 与周围分子云相比，激波过程如何改变出流气体中的分子丰度？
• RQ2 NGC1333-IRAS2A出流中激波组分与静止组分的物理条件（温度、密度）如何？
• RQ3 为何CH₃OH与SiO在激波区域显著增强，而HCO⁺仅在激波之后被探测到？
• RQ4 NGC1333-IRAS2A中的化学丰度与L1157等其它出流区域相比如何？差异可能由何原因解释？
• RQ5 原子碳丰度或局部密度的差异在多大程度上影响了出流中观测到的分子丰度分布？

主要发现

• 激波气体的温度约为70 K，密度约为2×10⁶ cm⁻³，而静止组分温度更低（约20 K），密度也较低（约1×10⁶ cm⁻³）。
• 在激波区域，CH₃OH的丰度可高达CO丰度的约5%，相比静止气体提高了2–4个数量级。
• SiO在激波界面处追踪到一个狭窄的高速组分，表明其高效地从尘埃颗粒中释放出易挥发性物质。
• HCO⁺仅在激波后区被探测到，与模型一致，即其在激波加热过程中因冰层释放的H₂O而被破坏。
• N₂H⁺表现出窄线且不受出流影响，证实其有效追踪了周围非激波云气。
• NGC1333-IRAS2A中HCN、H₂CO与CS的丰度低于L1157出流，可能与预激波气体中原子碳丰度的差异有关。