Three-dimensional simulations of the atmosphere of an AGB star
Three-dimensional simulations of the atmosphere of an AGB star
复制标题
AGB 恒星大气的三维模拟
DOI：
10.1051/0004-6361:20078096
复制
发表时间：
2008
影响因子：
6.5
通讯作者：
S. Höfner
中科院分区：
文献类型：
--
作者：
B. Freytag;S. Höfner
上下文渐近巨星分支星的风被假定为由尘埃上的辐射压力驱动。以前，这个过程已经模拟了详细的时间依赖性模拟大气和风假设球对称流。在这种模型中，动能是由一个可变的内边界（“活塞”）模拟恒星脉动的影响。然而，这些大气中的动力学过程-对流和脉动-实际上是三维的。目标。我们提出并分析了第一个三维辐射流体动力学模拟对流内部和大气的一个典型的AGB星星。特别是，我们检查是否在1D风模型中的活塞描述是兼容的3D结果。方法.我们使用了两个不同的RHD代码，一个（CO 5 BOLD）产生的三维模型的外部对流信封和内部大气的AGB星星，其他描述的大气和风加速区，包括尘埃的形成和非灰色的辐射传输，但假设球对称流。从三维模型中恒星表面层的运动出发，导出了一维模型中可变内边界的描述。结果3D模型显示了大的对流细胞和脉动，这些对流细胞和脉动在大气中产生了大致球形的膨胀冲击波，将物质悬浮到足够冷的区域，以允许尘埃形成。大气速度场的振幅和时间尺度接近一维风模型中开始尘埃形成所需的值。结论.三维模拟中的对流单元非常大，以至于相关的激波阵面几乎呈球形，证明了球对称假设和在风模型中使用活塞边界条件的合理性。然而，某些非径向结构存在于尘埃壳发展中的3D模拟，这应该是检测与当前的干涉技术。
Context. Winds of asymptotic giant branch stars are assumed to be driven by radiation pressure on dust. Previously, this process has been modeled with detailed time-dependent simulations of atmospheres and winds assuming spherically symmetric flows. In such models kinetic energy is injected by a variable inner boundary (“piston”) simulating the effects of stellar pulsation. However, the dynamical processes in these atmospheres – convection and pulsations – are actually three-dimensional. Aims. We present and analyze first 3D radiation hydrodynamics simulations of the convective interior and the atmosphere of a typical AGB star. In particular, we check whether the piston description in the 1D wind models is compatible with the 3D results. Methods. We used two different RHD codes, one (CO5BOLD) to produce 3D models of the outer convective envelope and the inner atmosphere of an AGB star, the other to describe the atmosphere and the wind acceleration region, including dust formation and non-grey radiative transfer, but assuming spherically symmetric flows. From the movements of stellar surface layers in the 3D models, we derived a description for the variable inner boundary in the 1D models. Results. The 3D models show large convection cells and pulsations that give rise to roughly spherically expanding shock waves in the atmosphere, levitating material into regions which are cool enough to allow for dust formation. The atmospheric velocity fields have amplitudes and time scales close to the values that are necessary to start dust formation in the 1D wind models. Conclusions. The convection cells in the 3D simulations are so large that the associated shock fronts appear almost spherical, justifying the assumption of spherical symmetry and the use of a piston boundary condition in the context of wind models. Nevertheless, certain non-radial structures exist in the dust shell developing in the 3D simulations which should be detectable with current interferometric techniques.