Numerical simulations of surface convection in a late M-dwarf
Numerical simulations of surface convection in a late M-dwarf
复制标题
晚期 M 矮星表面对流的数值模拟
DOI：
10.1051/0004-6361:20021153
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发表时间：
2002
影响因子：
6.5
通讯作者：
P. Hauschildt
中科院分区：
文献类型：
--
作者：
Hans;Hans;F. Allard;P. Hauschildt;P. Hauschildt
基于与时间相关的可压缩对流的详细 2D 和 3D 数值辐射流体动力学 (RHD) 模拟，我们研究了典型晚型 M 矮星（Teff ≈ 2800K，log g = 5.0，太阳化学成分）对流表面层的动力学和热结构。 RHD 模型预测的恒星粒化在质量上与熟悉的太阳模式类似。定量地看，颗粒细胞对流翻转时间尺度约为100s，水平尺度为80km；颗粒图案的相对强度对比度达1.1%，均方根垂直速度最大可达240ms -1 。人们发现，在较高的、形式上对流稳定的大气层中，与辐射平衡的偏差是微不足道的，从而可以使用一维标准模型大气对大气进行可靠的建模。 αMLT = 2.1 的混合长度参数提供了 RHD 模型大气的平均热结构的最佳表示，而当拟合恒星包层更深层中遇到的渐近熵（αMLT = 1.5）或匹配垂直速度（αMLT = 3.5）时，可以找到替代值。 RHD 和标准模型大气之间的密切对应意味着，目前在 M 矮星系统中观测到的恒星颜色与预测的恒星颜色之间存在的差异不能追溯到一维标准模型中对对流处理不充分。 RHD 模型预测对流混合区域将适度扩展，超出正式的史瓦西稳定边界，这为较冷（棕矮星）大气中尘埃颗粒的分布提供了线索。
Based on detailed 2D and 3D numerical radiation-hydrodynamics (RHD) simulations of time-dependent compress- ible convection, we have studied the dynamics and thermal structure of the convective surface layers of a prototypical late-type M-dwarf (Teff ≈ 2800K, log g = 5.0, solar chemical composition). The RHD models predict stellar granulation qualitatively similar to the familiar solar pattern. Quantitatively, the granular cells show a convective turn-over time scale of ≈100s, and a horizontal scale of 80km; the relative intensity contrast of the granular pattern amounts to 1.1%, and root-mean-square verti- cal velocities reach 240ms −1 at maximum. Deviations from radiative equilibrium in the higher, formally convectively stable atmospheric layers are found to be insignificant allowing a reliable modeling of the atmosphere with 1D standard model atmo- spheres. A mixing-length parameter of αMLT = 2.1 provides the best representation of the average thermal structure of the RHD model atmosphere while alternative values are found when fitting the asymptotic entropy encountered in deeper layers of the stellar envelope (αMLT = 1.5), or when matching the vertical velocity (αMLT = 3.5). The close correspondence between RHD and standard model atmospheres implies that presently existing discrepancies between observed and predicted stellar colors in the M-dwarf regime cannot be traced back to an inadequate treatment of convection in the 1D standard models. The RHD models predict a modest extension of the convectively mixed region beyond the formal Schwarzschild stability boundary which provides hints for the distribution of dust grains in cooler (brown dwarf) atmospheres.