Overshooting by differential heating
Overshooting by differential heating
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DOI：
10.1051/0004-6361/201425125
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发表时间：
2015-06-01
影响因子：
6.5
通讯作者：
Spruit, H. C.
中科院分区：
文献类型：
--
作者：
Andrassy, R.;Spruit, H. C.
在恒星主序演化的长核时间尺度上，即使是微弱的混合过程也可能与恒星中化学物质的重新分布有关。我们研究了一个“差热”过程，当温度波动通过辐射扩散从对流区边界传播到邻近的辐射区时，就会发生这种过程。由此引起的流体静力平衡的扰动使流动从对流区延伸出一段距离。研究了固体边界上施加静态温度波动的简化微分加热问题。高雷诺数和低佩莱特数（高热扩散率）的天体物理学相关极限被证明是有趣的非直觉性的。导出了稳态微分热流的一组标度关系。提出了一种适用于检测弱流所需的高动态范围流幅值的数值方法。二维模拟结果表明，流动达到稳态，并证实了解析标度关系。这意味着在热源上方的有限高度，流速突然下降到一个可以忽略不计的值。我们用高度相关的扩散系数近似计算了由于恒星内不同加热流引起的混合率，并表明这种混合在10 m圆点零年龄主序星的对流核心上方延伸了约4%的压力尺度高度。
On the long nuclear time scale of stellar main-sequence evolution, even weak mixing processes can become relevant for redistributing chemical species in a star. We investigate a process of "differential heating", which occurs when a temperature fluctuation propagates by radiative diffusion from the boundary of a convection zone into the adjacent radiative zone. The resulting perturbation of the hydrostatic equilibrium causes a flow that extends some distance from the convection zone. We study a simplified differential-heating problem with a static temperature fluctuation imposed on a solid boundary. The astrophysically relevant limit of a high Reynolds number and a low Peclet number (high thermal diffusivity) turns out to be interestingly non-intuitive. We derive a set of scaling relations for the stationary differential heating flow. A numerical method adapted to a high dynamic range in flow amplitude needed to detect weak flows is presented. Our two-dimensional simulations show that the flow reaches a stationary state and confirm the analytic scaling relations. These imply that the flow speed drops abruptly to a negligible value at a finite height above the source of heating. We approximate the mixing rate due to the differential heating flow in a star by a height-dependent diffusion coefficient and show that this mixing extends about 4% of the pressure scale height above the convective core of a 10 M-circle dot zero-age main sequence star.