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arXiv 2109.13403astro-ph.SRastro-ph.HE
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AM CVn双星中白矮星的质量传递和恒星演化

Mass Transfer and Stellar Evolution of the White Dwarfs in AM CVn Binaries

Tin Long Sunny Wong, Lars Bildsten

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15 pages, 10 figures; Accepted to ApJ
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摘要：

计算了轨道周期为$P_{\mathrm{orb}} \approx 5-70$分钟的AM CVn双星中两颗白矮星的恒星演化。我们专注于的情况下，捐助者开始作为一个$M_{\mathrm{He}} < 0.2 \, M_{\odot}$氦WD和吸积是一个$M_{\mathrm{WD}} > 0.6 \, M_{\odot}$ WD。使用恒星天体物理学实验模块（MESA），我们同时发展两个WD假设保守的质量转移和角动量损失的引力辐射。这种自洽的演变产生的重要反馈的质量传递速率，$\dot{M}$，以及吸积WD的热演化的施主的属性。与先前的工作一致，我们发现，在早期的高$\dot{M}$的强制绝热演化的施主$P_{\mathrm{orb}} < 30$分钟，使其质量半径关系主要取决于其初始熵。当供体在$P_{\mathrm{orb}} \simeq 30 $分钟到达$ M_{\mathrm{He}} \approx 0.02-0.03 \, M_{\odot}$时，其变得完全对流，并且在进一步的质量损失下可能损失熵并且膨胀得比预期小得多。然而，我们表明，缺乏可靠的不透明度的捐助者的表面抑制这种可能的冷却安全的预测。我们的计算捕获核心加热，发生在第一个$\approx 10^7$年的吸积和继续到WD冷却阶段的演变如下。当与现有的吸积WD数据相比时，正如Cheng和合作者对孤立WD所看到的那样，我们还发现，考虑到它们必须冷却的时间，吸积WD并不像我们预期的那么酷。

英文摘要：

We calculate the stellar evolution of both white dwarfs (WDs) in AM CVn binaries with orbital periods of $P_{\mathrm{orb}} \approx 5-70$ minutes. We focus on the cases where the donor starts as a $M_{\mathrm{He}} < 0.2 \, M_{\odot}$ Helium WD and the accretor is a $M_{\mathrm{WD}} > 0.6 \, M_{\odot}$ WD. Using Modules for Experiments in Stellar Astrophysics (MESA), we simultaneously evolve both WDs assuming conservative mass transfer and angular momentum loss from gravitational radiation. This self-consistent evolution yields the important feedback of the properties of the donor on the mass transfer rate, $\dot{M}$, as well as the thermal evolution of the accreting WD. Consistent with earlier work, we find that the high $\dot{M}$'s at early times forces an adiabatic evolution of the donor for $P_{\mathrm{orb}} < 30$ minutes so that its mass-radius relation depends primarily on its initial entropy. As the donor reaches $ M_{\mathrm{He}} \approx 0.02-0.03 \, M_{\odot}$ at $P_{\mathrm{orb}} \simeq 30 $ minutes, it becomes fully convective and could lose entropy and expand much less than expected under further mass loss. However, we show that the lack of reliable opacities for the donor's surface inhibit a secure prediction for this possible cooling. Our calculations capture the core heating that occurs during the first $\approx 10^7$ years of accretion and continue the evolution into the phase of WD cooling that follows. When compared to existing data for accreting WDs, as seen by Cheng and collaborators for isolated WDs, we also find that the accreting WDs are not as cool as we would expect given the amount of time they have had to cool.

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