费米LAT方法研究分子云中的宇宙射线和星际介质
Fermi LAT Study of the Cosmic-rays and the Interstellar Medium in Nearby Molecular Clouds
摘要：
我们报告了用{ \it Fermi}大视场望远镜（LAT）对来自附近分子云Chamaeleon、R Coronae Australis（R CrA）以及Cepheus和Polaris耀斑区域的星际伽马射线发射的分析。它们是最近的分子云复合体之一，距离太阳系300 pc以内。这些分子云中的宇宙射线（CR）与星际气体相互作用产生的γ射线辐射对研究太阳系附近分子气体的CR密度和分布是有用的。从250 MeV到10 GeV，这三个区域的γ射线发射率约为（6- 10）$\times$ 10 $^{-27}$光子s $^{-1}$ sr $^{-1}$ H原子$^{-1}$，表明即使考虑系统不确定性，CR密度的变化也为$\sim$的20%.分子质量校正比$X_{\rm CO} = N{\rm (H_2)}/W_{\rm CO}$在三个区域之间约为（0.6- 1.0）$\times$ 10 $^{20}$ H $_2$ -molecule cm $^{-2}$（K km s $^{-1}$）$^{-1}$，表明$X_{\rm CO}$在太阳系附近存在变化。根据所得到的$X_{\rm CO}$值，我们计算了这些分子云中由\Wco \示踪的分子气体的质量。此外，在原子和分子气体之间的界面处也推断出类似数量的暗气体。
英文摘要：
We report an analysis of the interstellar gamma-ray emission from nearby molecular clouds Chamaeleon, R Coronae Australis (R CrA), and Cepheus and Polaris flare regions with the {\it Fermi} Large Area Telescope (LAT). They are among the nearest molecular cloud complexes, within $\sim$ 300 pc from the solar system. The gamma-ray emission produced by interactions of cosmic-rays (CRs) and interstellar gas in those molecular clouds is useful to study the CR densities and distributions of molecular gas close to the solar system. The obtained gamma-ray emissivities from 250 MeV to 10 GeV for the three regions are about (6--10) $\times$ 10$^{-27}$ photons s$^{-1}$ sr$^{-1}$ H-atom$^{-1}$, indicating a variation of the CR density by $\sim$ 20% even if we consider the systematic uncertainties. The molecular mass calibration ratio, $X_{\rm CO} = N{\rm (H_2)}/W_{\rm CO}$, is found to be about (0.6--1.0) $\times$ 10$^{20}$ H$_2$-molecule cm$^{-2}$ (K km s$^{-1}$)$^{-1}$ among the three regions, suggesting a variation of $X_{\rm CO}$ in the vicinity of the solar system. From the obtained values of $X_{\rm CO}$, we calculated masses of molecular gas traced by \Wco\ in these molecular clouds. In addition, similar amounts of dark gas at the interface between the atomic and molecular gas are inferred.
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