We study the expansion of neutron star (NS) matter containing hyperons $\mathrm{\Lambda }$ from high densities up to the end of nucleosynthesis. This process should occur in NS-NS or NS–black hole collisions. Hyperons $\mathrm{\Lambda }$ decay by mesonic and nonmesonic reactions on a very short timescale to form proton and neutron matter. When material becomes diluted enough, the formation of helium and heavier nuclei releases energy and increases its temperature to reach nuclear statistical equilibrium (NSE) conditions with high entropy per baryon. From then on, nucleosynthesis proceeds and the final composition is largely determined by the proton per baryon fraction $Y_p$. We consider a recent equation of state that considers the presence of $\mathrm{\Lambda }$ hyperons [D. Logoteta, I. Vidaña, and I. Bombaci, Eur. Phys. J. A 55, 207 (2019)] and simultaneously allows for the existence of NSs with masses $M\gtrsim 2M_{\odot }$. We assumed the composition of NS matter at densities above the threshold for the occurrence of $\mathrm{\Lambda }$ hyperons and computed its decay, finding that it appreciably increases $Y_p$. Then, we computed the subsequent nucleosynthesis starting from NSE assuming that the density falls exponentially. We find that, depending on the initial composition of the ejected material, the increase in $Y_p$ due to hyperon decay is sufficient to sizeably affect the final isotopic composition of ejected matter (for example, lanthanide production may be strongly inhibited). These results indicate that $\mathrm{\Lambda }$ hyperons may affect the final composition of matter ejected in kilonova events. We also discuss the case of pure $\mathrm{\Lambda }$ matter as a possible scenario for the collision by strange stars, where we have obtained values $Y_p\approx 0.4\text{–}0.5$, which leads to final isotopes far lighter than in the former case.

中文翻译：

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Λ超子对千新星核合成的贡献

我们研究包含超子的中子星 (NS) 物质的膨胀 $\mathrm{\Lambda }$从高密度到核合成结束。这个过程应该发生在 NS-NS 或 NS-黑洞碰撞中。超子$\mathrm{\Lambda }$在很短的时间尺度内通过介子和非介子反应衰变形成质子和中子物质。当材料变得足够稀释时，氦和更重的原子核的形成会释放能量并增加其温度以达到每个重子具有高熵的核统计平衡 (NSE) 条件。从那时起，核合成继续进行，最终组成在很大程度上取决于每个重子分数的质子${是}_{磷}$. 我们考虑一个最近的状态方程，它考虑了$\mathrm{\Lambda }$超子[D. Logoteta, I. Vidaña 和 I. Bombaci, Eur。物理。J. A 55 , 207 (2019)] 并同时允许存在具有质量的 NS$米\gtrsim 2{米}_{\odot }$. 我们假设 NS 物质的组成密度高于发生的阈值$\mathrm{\Lambda }$ 超子并计算它的衰变，发现它明显增加 ${是}_{磷}$. 然后，假设密度呈指数下降，我们计算了从 NSE 开始的后续核合成。我们发现，取决于喷射材料的初始成分，增加${是}_{磷}$由于超子衰变足以显着影响喷出物质的最终同位素组成（例如，镧系元素的产生可能受到强烈抑制）。这些结果表明$\mathrm{\Lambda }$超子可能会影响千新星事件中喷射出的物质的最终组成。我们也讨论纯的情况$\mathrm{\Lambda }$ 物质作为奇怪恒星碰撞的可能场景，我们已经获得了值 ${是}_{磷}\approx 0.4\text{——}0.5$，这导致最终同位素比前一种情况轻得多。