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Outbursts of luminous blue variable stars from variations in the helium opacity

机译：氦气不透明性的变化使蓝色的可变恒星爆发

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摘要

Luminous blue variables are massive, evolved stars that exhibit large variations in luminosity and size on timescales from months to years, with high associated rates of mass loss(1-5). In addition to this on-going variability, these stars exhibit outburst phases, during which their size increases and as a result their effective temperature decreases, typically to about 9,000 kelvin(3,6). Outbursts are believed to be caused by the radiation force on the cooler, more opaque, outer layers of the star balancing or even exceeding the force of gravity, although the exact mechanisms are unknown and cannot be determined using one-dimensional, spherically symmetric models of stars because such models cannot determine the physical processes that occur in this regime(7). Here we report three-dimensional simulations of massive, radiation-dominated stars, which show that helium opacity has an important role in triggering outbursts and setting the observed effective temperature during outbursts of about 9,000 kelvin. It probably also triggers the episodic mass loss at rates of 10(-7) to 10(-5) solar masses per year. The peak in helium opacity is evident in our three-dimensional simulations only because the density and temperature of the stellar envelope (the outer part of the star near the photosphere) need to be determined self-consistently with convection, which cannot be done in one-dimensional models that assume spherical symmetry. The simulations reproduce observations of long-timescale variability, and predict that convection causes irregular oscillations in the radii of the stars and variations in brightness of 10-30 per cent on a typical timescale of a few days. The amplitudes of these short-timescale variations are predicted to be even larger for cooler stars (in the outburst phase). This short-timescale variability should be observable with high-cadence observations.
机译：发光的蓝色变量是巨大的，不断演化的恒星，其光度和大小在数月至数年的时间尺度上表现出很大的变化，并且伴随着较高的质量损失率（1-5）。除了这种持续的变异性外，这些恒星还表现出爆发期，在此期间它们的大小增加，结果其有效温度降低，通常降低到大约9,000开尔文（3,6）。爆发是由于较冷的，更不透明的恒星外层上的辐射力引起的，该辐射层平衡甚至超过了重力，尽管确切的机理尚不清楚，无法使用一维的球对称模型确定之所以如此之多，是因为此类模型无法确定在该状态下发生的物理过程（7）。在这里，我们报告了由辐射为主的大质量恒星的三维模拟，结果表明，氦气不透明在触发爆发和设定约9000开尔文的爆发期间观察到的有效温度方面具有重要作用。它也可能以每年10（-7）到10（-5）太阳质量的速率引发间歇性质量损失。氦不透明度的峰值在我们的三维模拟中很明显，这仅是因为恒星包壳（靠近光球的恒星外部）的密度和温度需要通过对流自洽确定，而这不可能一次完成。球形对称的三维模型。这些模拟再现了长时间尺度变化的观察结果，并预测对流会导致恒星半径的不规则振荡，并且在几天的典型时间尺度上亮度会发生10％至30％的变化。对于较短的恒星，这些短时标变化的幅度预计会更大（在爆发阶段）。这种短时程变化应通过高频率的观测观察到。

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