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arXiv 1407.0248astro-ph.SR
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混合C-O-Ne白矮星作为Ia型超新星的祖先:依赖于Urca过程和混合假设

Hybrid C-O-Ne white dwarfs as progenitors of type Ia supernovae: dependence on Urca process and mixing assumptions

P. Denissenkov, J. W. Truran, F. Herwig, S. Jones, B. Paxton, K. Nomoto, T. Suzuki, H. Toki

摘要:

当碳在超AGB星的CO核心偏离中心被点燃时,它在对流壳层中的燃烧倾向于传播到中心。C火焰实际上是否能够到达中心取决于C对流壳层下额外混合的效率。而温盐混合是太低效的干扰C-火焰传播,对流边界混合可以防止C燃烧到达中心。结果,在恒星失去其外壳后,形成了一颗C-O-Ne白矮星(WD)。这种“混合”WD具有被厚ONe区包围的小CO核。在我们的一维恒星演化计算中,混合WD被允许吸积富C物质,就像它在一个近距离双星系统中一样,并以足够高的速率从它的伴星吸积富H物质,以这种速率,吸积的H将在静止条件下被处理成He,假设He可以转化成C。当质量的吸积WD接近的Ekrasekhar极限,我们发现一系列的对流Urca壳闪光与高丰度的23 Na和25 Mg。随后是偏离中心的C点火,导致几乎占据整个恒星的对流。为了模拟Urca过程,我们使用最新的反应和中微子能量损失率的高分辨率数据。由于强调对流Urca过程中的不确定性,在我们的混合WD模型的超新星Ia祖先,我们考虑了一些潜在的可能的替代实例不同的混合假设,所有这些都达到了一个阶段的爆炸性C点火,无论是关闭或在中心。我们的混合超新星Ia祖模型有更低的C到O的丰度比在爆炸性的C点火的时刻比他们的纯CO同行,这可以解释所观察到的多样性的超新星Ia。

英文摘要:

When carbon is ignited off-centre in a CO core of a super-AGB star, its burning in a convective shell tends to propagate to the centre. Whether the C flame will actually be able to reach the centre depends on the efficiency of extra mixing beneath the C convective shell. Whereas thermohaline mixing is too inefficient to interfere with the C-flame propagation, convective boundary mixing can prevent the C burning from reaching the centre. As a result, a C-O-Ne white dwarf (WD) is formed, after the star has lost its envelope. Such a "hybrid" WD has a small CO core surrounded by a thick ONe zone. In our 1D stellar evolution computations, the hybrid WD is allowed to accrete C-rich material, as if it were in a close binary system and accreted H-rich material from its companion with a sufficiently high rate at which the accreted H would be processed into He under stationary conditions, assuming that He could then be transformed into C. When the mass of the accreting WD approaches the Chandrasekhar limit, we find a series of convective Urca shell flashes associated with high abundances of 23Na and 25Mg. They are followed by off-centre C ignition leading to convection that occupies almost the entire star. To model the Urca processes, we use the most recent well-resolved data for their reaction and neutrino-energy loss rates. Because of the emphasized uncertainty of the convective Urca process in our hybrid WD models of SN Ia progenitors, we consider a number of their potentially possible alternative instances for different mixing assumptions, all of which reach a phase of explosive C ignition, either off or in the centre. Our hybrid SN Ia progenitor models have much lower C to O abundance ratios at the moment of the explosive C ignition than their pure CO counterparts, which may explain the observed diversity of the SNe Ia.

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