标题： 星际亚稳态氦吸收作为宇宙射线电离率的探针 显示英文标题

标题： Interstellar Metastable Helium Absorption as a Probe of the Cosmic-Ray Ionization Rate

摘要： 宇宙射线对星际物质的电离率一直是一个主要的争议点，不同的估算结果相差三个数量级。 这个电离率的观测限制都依赖于分析由宇宙射线电离后产生的各种分子的化学性质，在许多情况下，这些分析包含显著的不确定性。 即使在最简单的情况下（H3+），推导出的电离率也取决于吸收路径长度的（不确定）估计值。 本文研究了利用亚稳态氦的10830 Å吸收线来推断宇宙射线电离率的可行性。 我们展示了朝向HD 183143的弥散云的观测数据，但仅得到了亚稳态氦柱密度的一个上限值。 对He+化学性质的彻底调查表明，只有少量的He+会重新结合到三重态并占据亚稳态能级。 此外，必须考虑由次级电子激发到氦的三重态能级，因为在某些环境中这是产生He*的主要机制。 结合这些各种形成和破坏途径，我们推导出了亚稳态氦稳态丰度的新方程。 使用这些方程与我们的观测数据，我们得出zeta_He < 1.2*10^-15 s^-1，这比之前使用H3+对该视线推断的电离率高约5倍。 尽管目前对星际He*的观测非常困难... 显示英文摘要

摘要： The ionization rate of interstellar material by cosmic rays has been a major source of controversy, with different estimates varying by three orders of magnitude. Observational constraints of this rate have all depended on analyzing the chemistry of various molecules that are produced following cosmic-ray ionization, and in many cases these analyses contain significant uncertainties. Even in the simplest case (H3+) the derived ionization rate depends on an (uncertain) estimate of the absorption path length. In this paper we examine the feasibility of inferring the cosmic-ray ionization rate using the 10830 A absorption line of metastable helium. Observations through the diffuse clouds toward HD 183143 are presented, but yield only an upper limit on the metastable helium column density. A thorough investigation of He+ chemistry reveals that only a small fraction of He+ will recombine into the triplet state and populate the metastable level. In addition, excitation to the triplet manifold of helium by secondary electrons must be accounted for as it is the dominant mechanism which produces He* in some environments. Incorporating these various formation and destruction pathways, we derive new equations for the steady state abundance of metastable helium. Using these equations in concert with our observations, we find zeta_He < 1.2*10^-15 s^-1, an upper limit about 5 times larger than the ionization rate previously inferred for this sight line using H3+. While observations of interstellar He* are extremely difficult at present...