Horizontal spreading of planetary debris accreted by white dwarfs
Horizontal spreading of planetary debris accreted by white dwarfs
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DOI：
10.1093/mnras/stab553
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发表时间：
2021-03-22
影响因子：
4.8
通讯作者：
Veras, Dimitri
中科院分区：
文献类型：
--
作者：
Cunningham, Tim;Tremblay, Pier-Emmanuel;Veras, Dimitri
大气层被金属污染的白色矮星在演化行星系统的岩石碎片吸积的背景下得到了广泛的研究。一个悬而未决的问题是吸积的几何形状以及物质如何到达并混合在白色矮星的表面层中。利用三维辐射流体动力学程序（COBOLD）-B-5，我们给出了简并星星大气中的第一输运系数，它描述了被动标量在表面平面上的对流扩散。我们耦合新推导的水平扩散系数与先前公布的垂直扩散系数提供理论约束金属表面扩散的白色矮星。我们的三维模拟网格探测了对流白色矮星的绝大多数参数空间，纯氢大气的有效温度范围为6000- 18000 K，纯氦大气的有效温度范围为12000 - 34000 K。我们的研究结果表明，温暖的富氢气氛（DA;大于或类似于13 000 K）和富氦气氛（DB和DBA;大于或类似于30 000 K）是无法有效地传播的吸积金属在其表面，无论吸积的时间依赖性。这一结果可能与目前没有探测到具有碎片盘的白色矮星表面丰度变化的情况不一致。对于较冷的富氢和富氦大气，我们预测在垂直扩散时间尺度内金属在表面上的分布基本上是均匀的。这通常小于光盘寿命估计的0.1%，本文使用过冲结果重新访问了这个数量。这些结果具有相关的研究的大部分组成的演变行星系统和模型的吸积盘物理。
White dwarfs with metal-polluted atmospheres have been studied widely in the context of the accretion of rocky debris from evolved planetary systems. One open question is the geometry of accretion and how material arrives and mixes in the white dwarf surface layers. Using the three-dimensional (3D) radiation hydrodynamics code (COBOLD)-B-5, we present the first transport coefficients in degenerate star atmospheres that describe the advection-diffusion of a passive scalar across the surface plane. We couple newly derived horizontal diffusion coefficients with previously published vertical diffusion coefficients to provide theoretical constraints on surface spreading of metals in white dwarfs. Our grid of 3D simulations probes the vast majority of the parameter space of convective white dwarfs, with pure-hydrogen atmospheres in the effective temperature range of 6000-18 000K and pure-helium atmospheres in the range of 12 000-34 000 K. Our results suggest that warm hydrogen-rich atmospheres (DA; greater than or similar to 13 000 K) and helium-rich atmospheres (DB and DBA; greater than or similar to 30 000 K) are unable to efficiently spread the accreted metals across their surface, regardless of the time dependence of accretion. This result may be at odds with the current non-detection of surface abundance variations in white dwarfs with debris discs. For cooler hydrogen- and helium-rich atmospheres, we predict a largely homogeneous distribution of metals across the surface within a vertical diffusion time-scale. This is typically less than 0.1 per cent of disc lifetime estimates, a quantity that is revisited in this paper using the overshoot results. These results have relevance for studies of the bulk composition of evolved planetary systems and models of accretion disc physics.