Outer boundary conditions for evolving cool white dwarfs
Outer boundary conditions for evolving cool white dwarfs
复制标题
冷白矮星演化的外边界条件
DOI：
10.1051/0004-6361/201219292
复制
发表时间：
2012
影响因子：
6.5
通讯作者：
Bertolami
中科院分区：
文献类型：
--
作者：
R. Rohrmann;L. Althaus;E. García–Berro;A. Córsico;Marcelo M. Miller;Bertolami
白矮星演化本质上是一个重力热冷却过程，对于冷白矮星来说，这一过程敏感地依赖于外边界条件的处理。我们给出了适用于计算冷白矮星演化的详细的外边界条件，对于纯H组分，采用了详细的非灰色模型大气。我们还探讨了冷白矮星大气中能量传输的不同假设对白矮星冷却时间的影响。详细的非灰色模型大气的计算考虑了气体状态方程和化学平衡中的非理想效应，分子的碰撞诱导吸收，以及Lymanα准分子不透明度。结果表明，对于MSun分别为0.60和0.90的序列，采用详细的外边界条件后，有效温度分别低于5800和6100K。详细的模式大气在对数L/L_SUN=-4时预测的年龄比在tau_Ross=2/3时用类爱丁顿近似得到的年龄短约10%。我们还分析了各种假设和相关的物理过程在外边界条件计算中的影响。特别是，我们发现Ly_α红翼吸收对白矮星的演化没有实质性的影响。白矮星的冷却时间标度对6000K的表面边界条件很敏感。有趣的是，在可观察到的亮度下，非灰色效应对冷却时间几乎没有影响。事实上，碰撞诱导的吸收过程显著影响了富氢大气中古老的白矮星的光谱和颜色，但在整个Rosseland平均不透明度范围内，对它们的冷却速度没有明显的影响。
White dwarf evolution is essentially a gravothermal cooling process, which,for cool white dwarfs, sensitively depends on the treatment of the outer boundary conditions. We provide detailed outer boundary conditions appropriate for computing the evolution of cool white dwarfs employing detailed non-gray model atmospheres for pure H composition. We also explore the impact on the white dwarf cooling times of different assumptions for energy transfer in the atmosphere of cool white dwarfs. Detailed non-gray model atmospheres are computed taken into account non-ideal effects in the gas equation of state and chemical equilibrium, collision-induced absorption from molecules, and the Lyman alpha quasi-molecular opacity. Our results show that the use of detailed outer boundary conditions becomes relevant for effective temperatures lower than 5800 and 6100K for sequences with 0.60 and 0.90 M_sun, respectively. Detailed model atmospheres predict ages that are up to approx 10% shorter at log L/L_sun=-4 when compared with the ages derived using Eddington-like approximations at tau_Ross=2/3. We also analyze the effects of various assumptions and physical processes of relevance in the calculation of outer boundary conditions. In particular, we find that the Ly_alpha red wing absorption does not affect substantially the evolution of white dwarfs. White dwarf cooling timescales are sensitive to the surface boundary conditions for T_eff < 6000K. Interestingly enough, non-gray effects have little consequences on these cooling times at observable luminosities. In fact, collision-induced absorption processes, which significantly affect the spectra and colors of old white dwarfs with hydrogen-rich atmospheres, have not noticeable effects in their cooling rates, except throughout the Rosseland mean opacity.