McCall B J, Huneycutt A J, Saykally R J, Geballe T R, Djuric N, Dunn G H, Semaniak J, Novotny O, Al-Khalili A, Ehlerding A, Hellberg F, Kalhori S, Neau A, Thomas R, Osterdahl F, Larsson M
Department of Chemistry, Department of Astronomy, University of California at Berkeley, Berkeley, California 94720, USA.
Nature. 2003 Apr 3;422(6931):500-2. doi: 10.1038/nature01498.
The H3+ molecular ion plays a fundamental role in interstellar chemistry, as it initiates a network of chemical reactions that produce many molecules. In dense interstellar clouds, the H3+ abundance is understood using a simple chemical model, from which observations of H3+ yield valuable estimates of cloud path length, density and temperature. But observations of diffuse clouds have suggested that H3+ is considerably more abundant than expected from the chemical models. Models of diffuse clouds have, however, been hampered by the uncertain values of three key parameters: the rate of H3+ destruction by electrons (e-), the electron fraction, and the cosmic-ray ionization rate. Here we report a direct experimental measurement of the H3+ destruction rate under nearly interstellar conditions. We also report the observation of H3+ in a diffuse cloud (towards Persei) where the electron fraction is already known. From these, we find that the cosmic-ray ionization rate along this line of sight is 40 times faster than previously assumed. If such a high cosmic-ray flux is ubiquitous in diffuse clouds, the discrepancy between chemical models and the previous observations of H3+ can be resolved.
H₃⁺分子离子在星际化学中起着基础性作用，因为它引发了一系列化学反应，从而产生了许多分子。在致密的星际云中，H₃⁺的丰度可通过一个简单的化学模型来理解，通过对H₃⁺的观测能够得到关于云的路径长度、密度和温度的有价值的估计。但是对弥漫星云的观测表明，H₃⁺的丰度比化学模型预期的要高得多。然而，弥漫星云模型一直受到三个关键参数不确定值的阻碍：电子（e⁻）对H₃⁺的破坏率、电子分数和宇宙射线电离率。在此，我们报告了在近乎星际条件下对H₃⁺破坏率的直接实验测量。我们还报告了在一个弥漫星云（朝向英仙座）中对H₃⁺的观测，该星云的电子分数是已知的。据此，我们发现沿着这条视线方向的宇宙射线电离率比之前假设的快40倍。如果如此高的宇宙射线通量在弥漫星云中普遍存在，那么化学模型与之前对H₃⁺的观测之间的差异就可以得到解决。
