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arXiv 1802.06753astro-ph.SR
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最冷的极低质量白矮星

The coolest extremely low-mass white dwarfs

Leila M. Calcaferro, Leandro G. Althaus, Alejandro H. Córsico

备注：
7 pages, 4 figures, 1 table. Accepted for publication in Astronomy & Astrophysics
发表信息：
A&A 614, A49 (2018)
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摘要：

极低质量白矮星（ELM WD; $M_{\star}$ $\lesssim$ $0.18-0.20\ $ $M_{\odot}$）被认为是通过稳定或不稳定的质量转移在双星系统中形成的。虽然稳定的质量转移预测形成的ELM WD厚的氢（H）信封，因此其特点是占主导地位的剩余核燃烧沿冷却支，ELM WD形成较薄的H信封不稳定的质量损失不能被丢弃。我们计算新的演化序列的氦（He）核心WD星薄H信封的主要目的是评估最低的$T_{\rm eff}$，可以达到这种类型的恒星。我们生成一个新的网格的演化序列的He核心WD星薄H信封的质量范围从$0.1554$到$0.2025 M_{\odot}$，并评估的变化，在冷却时间和表面重力引起的减少的H信封。我们还确定，考虑到祖先进化的预测，最低的$T_{\rm eff}$达到所产生的ELM WD。我们发现，略有减少的H信封产生显着增加ELM WD的冷却速率。正因为如此，具有薄H包封的ELM WD可以冷却到$\sim 2500\ $ K，与冷却到$\sim 7000\ $ K的典型对应物形成对比。此外，我们发现，减少的厚度的H信封显着增加的表面重力（$g$）的这些恒星。如果ELM WD形成有薄的H包封，则它们可以在非常低的$T_{\rm eff}$下被检测到。这种冷ELM WD的检测将表明它们是由薄的H包层形成的，从而打开了限制这种类型恒星形成的可能机制的可能性。最后但并非最不重要的是，由于H包层的减少，$g$的增加导致了这些恒星光谱测定的后果。

英文摘要：

Extremely low-mass white dwarf (ELM WD; $M_{\star}$ $\lesssim$ $0.18-0.20\ $ $M_{\odot}$) stars are thought to be formed in binary systems via stable or unstable mass transfer. Although stable mass transfer predicts the formation of ELM WDs with thick hydrogen (H) envelopes, and hence characterized by dominant residual nuclear burning along the cooling branch, the formation of ELM WDs with thinner H envelopes from unstable mass loss cannot be discarded. We compute new evolutionary sequences for helium (He) core WD stars with thin H envelope with the main aim of assessing the lowest $T_{\rm eff}$ that could be reached by this type of stars. We generate a new grid of evolutionary sequences of He core WD stars with thin H envelope in the mass range from $0.1554$ to $0.2025 M_{\odot}$, and assess the changes in both the cooling times and surface gravity induced by a reduction of the H envelope. We also determine, taking into account the predictions of progenitor evolution, the lowest $T_{\rm eff}$ reached by the resulting ELM WDs. We find that a slight reduction in the H envelope yields a significant increase in the cooling rate of ELM WDs. Because of this, ELM WDs with thin H envelope could cool down to $\sim 2500\ $K, in contrast with their canonical counterparts that cool down to $\sim 7000\ $K. In addition, we find that a reduction of the thickness of the H envelope increases markedly the surface gravity ($g$) of these stars. If ELM WDs are formed with thin H envelopes, they could be detected at very low $T_{\rm eff}$. The detection of such cool ELM WDs would be indicative that they were formed with thin H envelopes, thus opening the possibility of placing constraints to the possible mechanisms of formation of this type of stars. Last but not least, the increase in $g$ due to the reduction of the H envelope leads to consequences in the spectroscopic determinations of these stars.

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