[论文解读] Warm water deuterium fractionation in IRAS 16293-2422 - The high-resolution ALMA and SMA view

M. V. Persson, J. K. Jørgensen+1 more|Leiden Repository (Leiden University)|Nov 28, 2012

Astrophysics and Star Formation Studies参考文献 34被引用 33

一句话总结

本研究利用高分辨率ALMA和SMA观测，测量了原恒星双星IRAS 16293-2422内区的暖水氘同位素分馏比（HDO/H₂O）。测得比值为(9.2 ± 2.6) × 10⁻⁴，显著低于以往估计值，且与地球海洋及彗星中的值一致，表明原恒星内区的化学处理程度极低。

ABSTRACT

Measuring the water deuterium fractionation in the inner warm regions of low-mass protostars has so far been hampered by poor angular resolution obtainable with single-dish ground- and space-based telescopes. Observations of water isotopologues using (sub)millimeter wavelength interferometers have the potential to shed light on this matter. Observations toward IRAS 16293-2422 of the 5(3,2)-4(4,1) transition of H2-18O at 692.07914 GHz from Atacama Large Millimeter/submillimeter Array (ALMA) as well as the 3(1,3)-2(2,0) of H2-18O at 203.40752 GHz and the 3(1,2)-2(2,1) transition of HDO at 225.89672 GHz from the Submillimeter Array (SMA) are presented. The 692 GHz H2-18O line is seen toward both components of the binary protostar. Toward one of the components, "source B", the line is seen in absorption toward the continuum, slightly red-shifted from the systemic velocity, whereas emission is seen off-source at the systemic velocity. Toward the other component, "source A", the two HDO and H2-18O lines are detected as well with the SMA. From the H2-18O transitions the excitation temperature is estimated at 124 +/- 12 K. The calculated HDO/H2O ratio is (9.2 +/- 2.6)*10^(-4) - significantly lower than previous estimates in the warm gas close to the source. It is also lower by a factor of ~5 than the ratio deduced in the outer envelope. Our observations reveal the physical and chemical structure of water vapor close to the protostars on solar-system scales. The red-shifted absorption detected toward source B is indicative of infall. The excitation temperature is consistent with the picture of water ice evaporation close to the protostar. The low HDO/H2O ratio deduced here suggests that the differences between the inner regions of the protostars and the Earth's oceans and comets are smaller than previously thought.

研究动机与目标

• 利用高角分辨率测量原恒星双星IRAS 16293-2422内区暖气体中的水氘同位素分馏比（HDO/H₂O）。
• 克服以往单天线观测的局限性，这些观测在小尺度上受光束弥散和物理建模不确定性的影响。
• 确定原恒星内区暖区域的HDO/H₂O比值是否与地球海洋或彗星冰中的值一致。
• 利用干涉测量数据确定内包层中水同位素体的物理条件（激发温度、柱密度）。
• 通过在25–280 AU尺度上解析发射分布，提供对原恒星最内区氘化水平的模型无关约束。

提出的方法

• 在ALMA上对IRAS 16293-2422的两个组分进行了692.07914 GHz处H₂¹⁸O 5₃,₂–4₄,₁跃迁的观测。
• 利用SMA对源A进行了203.40752 GHz处H₂¹⁸O 3₁,₃–2₂,₀跃迁和225.89672 GHz处HDO 3₁,₂–2₂,₁跃迁的观测。
• 假设局部热动平衡，通过H₂¹⁸O跃迁的相对线强度推导激发温度。
• 利用转动图法估算HDO和H₂¹⁸O的柱密度，校正光束弥散，并假设¹⁶O/¹⁸O比值为560。
• 根据推导出的柱密度计算HDO/H₂O比值，不确定度估计基于流量校准和激发温度变化。
• 通过考虑HDO发射可能比H₂¹⁸O更弥散，测试了对空间扩展的敏感性。

实验结果

研究问题

• RQ1 通过高分辨率干涉测量，IRAS 16293-2422原恒星内区暖气体中的HDO/H₂O比值是多少？
• RQ2 测得的HDO/H₂O比值与地球海洋及彗星冰中的值相比如何？
• RQ3 哪些物理条件（如激发温度、空间扩展）控制着观测到的水同位素体发射？
• RQ4 观测到的HDO/H₂O比值是否表明在原恒星阶段与行星系统形成之间，水经历了显著的化学处理？
• RQ5 高分辨率干涉测量数据能否解决早期单天线观测与理论预期之间的差异？

主要发现

• 在IRAS 16293-2422的两个组分上均检测到692 GHz的H₂¹⁸O谱线，源B处红移吸收表明存在吸积运动。
• 源A中暖水蒸气的激发温度估计为124 ± 12 K，与原恒星附近冰的蒸发一致。
• 源A中暖气体的HDO/H₂O比值为(9.2 ± 2.6) × 10⁻⁴，显著低于以往单天线观测的估计值。
• 该比值仅略高于地球海洋中3 × 10⁻⁴的值，更接近于彗星Hartley 2中测得的3.2 × 10⁻⁴。
• 当激发温度为80 K时，HDO/H₂O比值保持在(7.8 ± 2.2) × 10⁻⁴，当温度为300 K时为(1.1 ± 0.3) × 10⁻³，表明在合理物理条件下具有鲁棒性。
• 观测到的HDO/H₂O比值对光束弥散和空间扩展假设具有鲁棒性，但若HDO发射比H₂¹⁸O更弥散，则该比值可能为上限。