恒星氦在其他宇宙中燃烧：三阿尔法微调问题的解决方案
Stellar Helium Burning in Other Universes: A solution to the triple alpha fine-tuning problem
摘要：
受其他宇宙可能存在的启发，不同的基本常数的值，本文考虑恒星模型在宇宙中，$^8$是稳定的。许多以前的作者已经注意到，在我们宇宙中的恒星在没有众所周知的7.6 MeV的$^{12}$ C共振的情况下很难产生碳和其他重元素。这种共振是必要的，因为$^8$ Be在我们的宇宙中是不稳定的，所以碳必须通过三重α反应产生，以达到必要的丰度。虽然共振能量的适度变化（200 - 300 keV）确实会影响碳的产生，但铍的结合能（$\sim100$ keV）的更小变化将使$^8$ Be稳定。具有$A=8$的稳定同位素通常会消除三α过程的需要，特别是用于碳生产的$^{12}$ C共振。本文探讨了$^8$ Be在其他宇宙中稳定存在的可能性。简单的核考虑表明，如果基本常数变化$\sim {\rm few}-10$ %，则可以实现结合能为$\sim0.1-1$ MeV的束缚态。在这种情况下，$^8$ Be可以通过氦燃烧合成，而$^{12}$ C可以随后通过铍的核燃烧产生。这篇论文的重点是将氦燃烧成铍的恒星模型;一旦所讨论的宇宙有稳定的铍供应，碳的产生就可以在同一颗恒星或以后的恒星世代的后续演化中发生。使用半解析恒星结构模型以及一个国家的最先进的恒星演化代码，我们发现，可行的恒星配置，产生铍存在于一个广泛的参数空间。最后，我们证明了碳可以在以后的进化阶段产生。
英文摘要：
Motivated by the possible existence of other universes, with different values for the fundamental constants, this paper considers stellar models in universes where $^8$Be is stable. Many previous authors have noted that stars in our universe would have difficulty producing carbon and other heavy elements in the absence of the well-known $^{12}$C resonance at 7.6 MeV. This resonance is necessary because $^8$Be is unstable in our universe, so that carbon must be produced via the triple alpha reaction to achieve the requisite abundance. Although a moderate change in the energy of the resonance (200 -- 300 keV) will indeed affect carbon production, an even smaller change in the binding energy of beryllium ($\sim100$ keV) would allow $^8$Be to be stable. A stable isotope with $A=8$ would obviate the need for the triple alpha process in general, and the $^{12}$C resonance in particular, for carbon production. This paper explores the possibility that $^8$Be can be stable in other universes. Simple nuclear considerations indicate that bound states can be realized, with binding energy $\sim0.1-1$ MeV, if the fundamental constants vary by a $\sim {\rm few}-10$ percent. In such cases, $^8$Be can be synthesized through helium burning, and $^{12}$C can be produced later through nuclear burning of beryllium. This paper focuses on stellar models that burn helium into beryllium; once the universe in question has a supply of stable beryllium, carbon production can take place during subsequent evolution in the same star or in later stellar generations. Using both a semi-analytic stellar structure model as well as a state-of-the-art stellar evolution code, we find that viable stellar configurations that produce beryllium exist over a wide range of parameter space. Finally, we demonstrate that carbon can be produced during later evolutionary stages.
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