大质量恒星类氦离子中X射线谱线比率的变化：辐射驱动的情况
Variability in X-ray line ratios in helium-like ions of massive stars: the radiation-driven case
摘要：
类氦离子的“冷杉”三重态的谱线比率已被证明是大质量恒星周围X射线发射气体条件的有力诊断。最近的观察表明，这些比率可以随着时间而变化。线比变化的可能原因是有限的：辐射场的变化或密度的变化，以及质量损失或几何形状的变化。在本文中，我们调查的能力，在辐射场的变化，以诱导比值R=f/i的变化。为了隔离辐射效应，我们使用了一个启发式模型的温度和半径变化的变星在B和O范围内的低密度，稳态风。我们模拟的X射线发射气体的发射率的变化接近恒星，由于在气体的位置从可用的紫外光子的水平泵浦的差异。我们发现，在这些条件下，在R的变化是由恒星的温度。虽然在大多数温度下，大多数谱线的相对变化幅度大致相当，但可检测到的变化仅限于每种光谱类型的几条谱线。我们预测，由于恒星的变化，R中的变量值必须遵循我们的模拟中发现的可预测的趋势。我们的模型使用径向脉动作为恒星变化的模式，使R的变化幅度最大化。这个模型是足够强大的，以显示哪些离子将提供最好的机会，观察在不同的恒星温度下的f/i比的变化，以及该变化与其他可观察参数的相关性。在真实系统中，这种效应会比我们的模型更复杂，在存在非径向脉动的情况下，相位和抑制振幅会有所不同。这表明，在多条谱线上同时发生的R变化不太可能是由可变辐射场产生的。
英文摘要：
Line ratios in "fir" triplets of helium-like ions have proven to be a powerful diagnostic of conditions in X-ray emitting gas surrounding massive stars. Recent observations indicate that these ratios can be variable with time. The possible causes of variation in line ratios are limited: changes in the radiation field or changes in density, and changes in mass-loss or geometry. In this paper, we investigate the ability of changes in the radiation field to induce variability in the ratio R=f/i. To isolate the radiative effect, we use a heuristic model of temperature and radius changes in variable stars in the B and O range with low-density, steady-state winds. We model the changes in emissivity of X-ray emitting gas close to the star due to differences in level-pumping from available UV photons at the location of the gas. We find that under these conditions, variability in R is dominated by the stellar temperature. Although the relative amplitude of variability is roughly comparable for most lines at most temperatures, detectable variations are limited to a few lines for each spectral type. We predict that variable values in R due to stellar variability must follow predictable trends found in our simulations. Our model uses radial pulsations as a mode of stellar variability that maximizes the amplitude of variation in R. This model is robust enough to show which ions will provide the best opportunity for observing variability in the f/i ratio at different stellar temperatures, and the correlation of that variability with other observable parameters. In real systems, the effects would be more complex than in our model, with differences in phase and suppressed amplitude in the presence of non-radial pulsations. This suggests that changes in R across many lines concurrently are not likely to be produced by a variable radiation field.
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