ABSTRACT Explicit radiation hydrodynamic simulations of the atmospheres of massive stars and of convection in accretion discs around white dwarfs suffer from prohibitively short time steps due to radiation. This constraint is related to the cooling time rather than the radiative pressure, which also becomes important in hot stars and discs. We show that the radiative time step constraint is governed by the minimum of the sum of the optically thick and thin contributions rather than the smaller one of the two. In simulations with the Pencil Code, their weighting fractions are found empirically. In three-dimensional convective accretion disc simulations, the Deardorff term is found to be the main contributor to the enthalpy flux rather than the superadiabatic gradient. We conclude with a discussion of how the radiative time step problem could be mitigated in certain types of investigations.

中文翻译：

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辐射流体力学中的时间步长约束

摘要 大质量恒星大气层和白矮星周围吸积盘中对流的显式辐射流体动力学模拟由于辐射而受到非常短的时间步长的影响。这种约束与冷却时间有关，而不是与辐射压力有关，这在热恒星和圆盘中也很重要。我们表明，辐射时间步长约束受光学厚薄贡献之和的最小值控制，而不是两者中较小的一个。在 Pencil Code 的模拟中，它们的权重分数是凭经验找到的。在三维对流吸积盘模拟中，发现迪尔多夫项是焓通量的主要贡献者，而不是超绝热梯度。