Measuring H2 spin temperature in molecular cloud by near-infrared absorption spectroscopy

我们首次提出在静态云中测量分子氢的核自旋温度。H2的形成是星际介质中表面化学的原型，但我们的理解仍然存在薄弱环节和未知因素。氢的核自旋在分子云中冷却得非常慢。因此，氢的自旋温度保留了它在尘埃颗粒表面形成的原始遗迹。静止星云中新形成的氢的高自旋温度将是我们理解分子如何填充星际介质的支柱之一。我们将通过观测H2 v=1-0，S(0)和S(1)在2.223微米和2.122微米吸收时相对于管道星云中致密半透明云背后的恒星的振动跃迁，来定量地限制H2的自旋温度。到目前为止，这种尝试超出了天文观测的范围，因为缺乏针对静止云的像样的连续谱源，也因为缺乏大口径望远镜上具有高观测效率的高分辨率光谱仪。后一个障碍将由CRIRES消除，它将在VLT进行重大升级后于2017年进入VLT。为了克服前者，我们将利用密集的晚型恒星，并应用我们最新开发的技术来最精确地消除恒星吸收线。我们将在计划中改进的观测方案，也将是首次用其主要成分--分子氢绘制分子云质量图的重要资产。

We propose to measure the nuclear spin temperature of the molecular hydrogen for the first time in a quiescent cloud. The formation of H2 is the archetype of the surface chemistry in the interstellar medium, and yet our understanding is plagued with weak links and unknowns. The nuclear spin of H2 cools down only very slowly in a molecular cloud. The spin temperature of H2 therefore retains the pristine relic of its formation on a dust grain surface. The high spin temperature of newly formed H2 in a quiescent cloud will be one of the pillars of our understanding how the molecules come to fill the interstellar medium. We will quantitatively constrain the H2 spin temperature by the observation of vibrational transitions of H2 v=1-0 S(0) and S(1) at 2.223 and 2.122 um in absorption against the stars behind the dense and translucent clouds in the Pipe Nebula. Such attempts were beyond the scope of astronomical observations until now, for the lack of decent continuum sources toward quiescent clouds, and for the lack of high-resolution spectrographs at large aperture telescopes with a high observing efficiency. The latter hurdle will be lifted by CRIRES+, which will come to the VLT in 2017 after its major upgrade. To overcome the former, we will make use of the populous late type stars, and apply our newly developed technique to remove the stellar absorption lines to the finest precision. The observing scheme, which we will refine in the program, will also be an important asset to chart the mass of a molecular cloud for the first time with its principal constituent, the molecular hydrogen.