论文标题
3D化学结构的弥漫性动荡ISM II- ch $^+$的起源，80年神秘的新解决方案
3D chemical structure of the diffuse turbulent ISM II -- Origin of CH$^+$, new solution to an 80 years mystery
论文作者
论文摘要
目的：在漫射的星际介质（ISM）中，Ch $^+$的大量大量是我们理解气体热力学和化学状态的长期存在的问题。我们在这里研究了在湍流和多相环境中CH+的形成，其中几乎仅由光电效应驱动气体的加热。方法：使用磁性水力动力学（MHD）代码公羊模拟弥散性ISM，这些磁水动力学（MHD）代码将自动计算气体的动力学和热演化以及h $^+$，h和h $ _2 $的丰度的时间相关的演变。其余的化学物质，包括大量的Ch $^+$，按后处理，平衡，在H $^+$，H和H $ _2 $的超平衡的约束下计算。考虑到经常被忽略的但最重要的信息，即沿观察到的视线的截距散布物质的长度，进行了与观测值进行的比较。结果：CH $^+$的准总数源自不稳定的气体，在动力学温度大于600 K的环境中，密度在0.6至10 cm $^{ - 3} $之间，电子分数范围在3 x 10 $^{ - 4} $和6 x 10 $^resc in and rescy范围之间。它的形成是由最初在冷中性培养基（CNM）中形成的温暖和平衡的h $ _2 $驱动的，并通过对流和热不稳定性组合在更加漫射的环境中，甚至在更漫射的环境中注入温暖的中性培养基（WNM）。与Hi-to-H $ _2 $过渡显示最紧密的模拟以及在太阳能社区中观察到的热压分布自然可以重现观察到的ch $^+$的丰富性，观察的分散，大多数视力范围的可能性的可能性，CH $^+$ $^+$的分配的概率，以及该行分配的分配。 CH $^+$的数量和模拟视觉线的统计属性是由富含H $ _2 $的不稳定气体的部分设置的，这是通过银河尺度来控制的，通过弥漫性ISM的平均密度（或等效地，其总质量），平均UV辐射场的幅度，以及构成构成构成的强度。结论：这项工作为80年历史的化学谜语提供了一种新的自然解决方案。在弥漫性ISM中，几乎普遍存在的Ch $^+$存在可能是由于CNM和WNM之间的交换而导致的，而无需调用歧义性的湍流消散区域，而无需调用歧义性歧义性的脉冲扩散或区域。通过两相动荡的混合，Ch $^+$因此可能是h $ _2 $ $ _2 $ CNM云的质量损失率的示踪剂。
Aims: The large abundances of CH$^+$ in the diffuse interstellar medium (ISM) are a long standing issue of our understanding of the thermodynamical and chemical states of the gas. We investigate, here, the formation of CH+ in turbulent and multiphase environments, where the heating of the gas is almost solely driven by the photoelectric effect. Methods: The diffuse ISM is simulated using the magnetohydrodynamic (MHD) code RAMSES which self-consistently computes the dynamical and thermal evolution of the gas along with the time-dependent evolutions of the abundances of H$^+$, H, and H$_2$. The rest of the chemistry, including the abundance of CH$^+$, is computed in post-processing, at equilibrium, under the constraint of out-ofequilibrium of H$^+$, H, and H$_2$. The comparison with the observations is performed taking into account an often neglected, yet paramount, piece of information, namely the length of the intercepted diffuse matter along the observed lines of sight. Results: The quasi totality of the mass of CH$^+$ originates from the unstable gas, in environments where the kinetic temperature is larger than 600 K, the density ranges between 0.6 and 10 cm$^{-3}$, the electronic fraction ranges between 3 x 10$^{-4}$ and 6 x 10$^{-3}$, and the molecular fraction is smaller than 0.4. Its formation is driven by warm and out-of-equilibrium H$_2$ initially formed in the cold neutral medium (CNM) and injected in more diffuse environments and even the warm neutral medium (WNM) through a combination of advection and thermal instability. The simulation which displays the tightest agreement with the HI-to-H$_2$ transition and the thermal pressure distribution observed in the Solar Neighborhood is found to naturally reproduce the observed abundances of CH$^+$, the dispersion of observations, the probability of occurrence of most of the lines of sight, the fraction of non-detections of CH$^+$, and the distribution of its line profiles. The amount of CH$^+$ and the statistical properties of the simulated lines of sight are set by the fraction of unstable gas rich in H$_2$ which is controlled, on Galactic scales, by the mean density of the diffuse ISM (or, equivalently, its total mass), the amplitude of the mean UV radiation field, and the strength of the turbulent forcing. Conclusions: This work offers a new and natural solution to an 80 years old chemical riddle. The almost ubiquitous presence of CH$^+$ in the diffuse ISM likely results from the exchanges of matter between the CNM and the WNM induced by the combination of turbulent advection and thermal instability, without the need to invoke ambipolar diffusion or regions of intermittent turbulent dissipation. Through two phase turbulent mixing, CH$^+$ might thus be a tracer of the H$_2$ mass loss rate of CNM clouds.
