The Galactic interstellar medium abounds in shocks with low velocities v _s ≲ 70 km s⁻¹. Some are descendants of higher velocity shocks, while others start off at low velocity (e.g., stellar bow shocks, intermediate velocity clouds, spiral density waves). Low-velocity shocks cool primarily via Lyα and two-photon continuum, augmented by optical recombination lines (e.g., Hα), forbidden lines of metals and free-bound emission, free–free emission. The dark far-ultraviolet (FUV) sky, aided by the fact that the two-photon continuum peaks at 1400 Å, makes the FUV band an ideal tracer of low-velocity shocks. GALEX FUV images reaffirm this expectation, discovering faint and large interstellar structure in old supernova remnants and thin arcs stretching across the sky. Interstellar bow shocks are expected from fast stars from the Galactic disk passing through the numerous gas clouds in the local interstellar medium within 15 pc of the Sun. Using the bests atomic data available to date, we present convenient fitting formulae for yields of Lyα, two-photon continuum, and Hα for pure hydrogen plasma in the temperature range of 10⁴–10⁵ K. The formulae presented here can be readily incorporated into time-dependent cooling models as well as collisional ionization equilibrium models.

中文翻译：

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低速冲击中的双光子产生

银河系星际介质充满低速冲击v _s ≲ 70 公里秒⁻¹。有些是高速激波的后代，而另一些则以低速开始（例如，恒星弓激波、中速云、螺旋密度波）。低速冲击主要通过 Ly 冷却α和双光子连续谱，通过光学复合线增强（例如，Hα）、金属禁线和自由结合排放、自由-自由排放。黑暗的远紫外线 (FUV) 天空，加上双光子连续谱峰值为 1400 Å，使 FUV 波段成为低速冲击的理想示踪剂。GALEX FUV 图像再次证实了这一预期，在古老的超新星遗迹中发现了微弱而巨大的星际结构，以及横跨天空的细弧。星际弓激波预计来自银河盘中的快速恒星，穿过距太阳 15 度以内的当地星际介质中的众多气体云。使用迄今为止最好的原子数据，我们提出了 Ly 产率的方便拟合公式α，双光子连续谱，和 Hα^{适用于温度范围为 10 4} –10 ⁵ K的纯氢等离子体。此处提出的公式可以轻松地合并到随时间变化的冷却模型以及碰撞电离平衡模型中。