白矮星是中、小质量恒星演化到最终阶段的产物。根据它们光谱中所表现出的氢、氦、碳以及其他元素的谱线，可以将它们分为很多种光谱型。绝大多数白矮星分属以下三种光谱型：表面几乎全部由氢组成的DA型白矮星以及大气具有丰富氦元素的DO或DB型白矮星。奇怪的是，在45,000≥T_eff≥30,000K的范围内，只发现了数量极少的富氦大气的白矮星，这个温度范围就被称为DB gap。造成这一现象的原因尚不清楚，我们怀疑是引力分层和对流混合在演化过程中的相互竞争导致了这一现象的出现。白矮星上强大的引力场带来的引力沉淀效应会使白矮星大气分层。在DB gap 中，这一过程使得较轻的氢漂浮到白矮星表面，这样就只能看到DA型白矮星了。到了DB gap 的低温端(T_eff ≈ 30,000K)，氦对流区将表层的氢混合到氦中，于是DB型白矮星便出现了。在这篇论文里，我们通过计算一系列白矮星的演化模型来研究对流混合过程是如何改变白矮星表面元素丰度。
其他摘要
White dwarfs are the products of the final evolutionary states of all medium and low mass stars. They are classified into many spectral types based on the appearance of hydrogen, helium, carbon and other spectral lines. The vast majority of white dwarfs fall into three spectral types: the DA white dwarfs whose surfaces consist almost entirely of hydrogen, and DO or DB white dwarfs with helium-rich atmospheres. Remarkably, in the range 45,000 ≥ T_eff ≥ 30,000K, there are only few white dwarfs with helium-rich atmospheres to be found. This dearth of helium-rich atmospheres white dwarfs in this temperature range is known as the “DB gap”. The reason for this phenomenon is not clear. It is suspected to be due to a competition between gravitational separation and convective mixing in the evolutionary processes. The strong gravitational fields in white dwarfs are thought to stratify the atmospheres of white dwarfs by gravitational settling. This process allows the lighter hydrogen float to the surfaces of white dwarfs in the DB gap, so the stars are seen to be DA white dwarfs. At the cool ends of the gap (T_eff ≈ 30,000K), the helium convective zone mixes the upper hydrogen layer into helium, so the stars then appear as DB white dwarfs. In this thesis, we calculate white dwarf evolution models to investigate that convective mixing process how to change the surface abundances of white dwarfs.
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