[论文解读] The fate of a neutron star just below the minimum mass: does it explode?

Kohsuke Sumiyoshi, S. Yamada+2 more|arXiv (Cornell University)|Jul 21, 1997

Pulsars and Gravitational Waves Research参考文献 4被引用 13

一句话总结

本研究通过广义相对论流体动力学模拟，调查了质量略低于最小稳定质量的中子星的演化命运。结果发现，此类中子星在约20秒内经历由缓慢β衰变驱动的准静态膨胀，随后发生剧烈爆炸，释放约10⁴⁹ erg的动能和峰值反中微子辐射亮度约10⁵² erg/s，形成有利于r-过程核合成的条件。

ABSTRACT

First results of numerical simulations are presented which compute the dynamical evolution of a neutron star with a mass slightly below the minimum stable mass by means of a new implicit (general relativistic) hydrodynamic code. We show that such a star first undergoes a phase of quasi-static expansion, caused by slow nuclear $β$-decays, lasting for about 20 seconds, but then explodes violently. The kinetic energy of the explosion is around $10^{49} erg$, the peak luminosity in electron anti-neutrinos is of order $10^{52} erg/s$, and the thermodynamic conditions of the expanding matter are favorable for r-process nucleosynthesis. These results are obtained for the Harrison-Wheeler equation of state and a simple and, possibly, unrealistic treatment of $β$-decay rates and nuclear fission, which were adopted for comparison with previous works. However, we do not expect that the outcome will change qualitatively if more recent nuclear input physics used. Although our study does not rely on a specific scenario ofhow a neutron star starting from a bigger (and stable) masscan reach the dynamical phase, we assume that the final mass-loss event happens on a very short time scale, i.e., on a time scale shorter than a sound-crossing time, by removing a certain amount of mass as an initial perturbation. This assumption implies that the star has no time to adjust its nuclear composition to the new mass mass through a sequence of quasi-equilibria. In the latter case, however, there exists no stable configuration below the minimum mass, because the equation of state of fully catalyzed matter is too soft. Therefore, the dynamics of the explosion will not be too different from what have obtained if different initial perturbations are assumed.

研究动机与目标

• 确定质量略低于最小稳定质量的中子星的动力学演化过程。
• 研究此类中子星是否因核不稳定而非引力坍缩而发生爆炸。
• 评估缓慢核β衰变和质量损失在触发爆炸动力学中的作用。
• 评估在膨胀喷射物中发生r-过程核合成的潜力。
• 检验爆炸结果对不同初始扰动及核物理假设的鲁棒性。

提出的方法

• 采用新型隐式广义相对论流体动力学代码，模拟中子星的动力学演化。
• 采用Harrison-Wheeler方程态描述致密物质的物态方程。
• 采用简化的、非物理的β衰变速率与核裂变处理方法，以便与先前研究进行对比。
• 以短于声穿越时标的质量损失扰动作为模拟初始条件，以避免准平衡调整。
• 追踪随时间演化的热力学变量、中微子辐射亮度和动能。
• 通过分析膨胀物质中的热力学条件，评估核合成潜力。

实验结果

研究问题

• RQ1 质量略低于最小稳定质量的中子星在动力学上会发生什么？
• RQ2 缓慢的核β衰变是否能驱动原本不稳定的中子星发生剧烈爆炸？
• RQ3 此类爆炸的能量学特征与中微子辐射特征如何？
• RQ4 喷射物中的热力学条件是否有利于r-过程核合成？
• RQ5 结果对初始扰动或核输入物理参数的敏感性如何？

主要发现

• 中子星在进入剧烈爆炸前，经历约20秒的由缓慢核β衰变驱动的准静态膨胀阶段。
• 爆炸释放的动能约为10⁴⁹ erg，与核心坍缩超新星事件相当。
• 峰值反中微子辐射亮度达到约10⁵² erg/s，表明爆炸期间存在强烈的中微子发射。
• 膨胀喷射物中的热力学条件有利于r-过程核合成。
• 由于软物态方程导致最小质量以下无法形成稳定构型，因此结果对初始扰动细节不敏感。
• 即使采用更真实的核物理输入，结果在定性上预计仍保持不变。