Title: Effects of chemically homogeneous evolution of the first stars on the 21-cm signal and reionization Show Chinese title

Title: 第一代恒星化学均匀演化对21-cm信号和再电离的影响

Abstract: The first generation of stars, known as Population III (Pop III), played a crucial role in the early Universe through their unique formation environment and metal-free composition. These stars can undergo chemically homogeneous evolution (CHE) due to fast rotation, becoming more compact and hotter/bluer than their (commonly assumed) non-rotating counterparts. In this study, we investigate the impact of Pop III CHE on the 21-cm signal and cosmic reionization under various assumptions on Pop III star formation, such as their formation efficiency, initial mass function, and transition to metal-enriched star formation. We combine stellar spectra computed by detailed atmosphere models with semi-numerical simulations of Cosmic Dawn and the Epoch of Reionization ($z\sim 6-30$). The key effect of CHE arises from the boosted ionizing power of Pop III stars, which reduces the Pop III stellar mass density required to reproduce the observed Thomson scattering optical depth by a factor of $\sim 2$. Meanwhile, the maximum 21-cm global absorption signal is shallower by up to $\sim 15$ mK (11%), partly due to the reduced Lyman-band emission from CHE, and the large-scale ($k\sim 0.2\ \rm cMpc^{-1}$) power drops by a factor of a few at $z\gtrsim 25$. In general, the effects of CHE can be comparable to those of Pop III star formation parameters, showing an interesting interplay with distinct features in different epochs. These results highlight the importance of metal-free/poor stellar evolution in understanding the early Universe and suggest that future studies should consider joint constraints on the physics of star/galaxy formation and stellar evolution. Show Chinese abstract

Abstract: 第一代恒星，被称为第三星族（Pop III），在早期宇宙中发挥了关键作用，这是由于它们独特的形成环境和无金属的组成。 这些恒星由于快速旋转可以经历化学均匀演化（CHE），变得比通常假设的非旋转对应物更紧凑、更热/更蓝。 在本研究中，我们调查了在各种关于Pop III恒星形成的假设下，如它们的形成效率、初始质量函数以及向金属富集恒星形成的过渡，Pop III CHE对21-cm信号和宇宙再电离的影响。 我们将详细大气模型计算的恒星光谱与宇宙黎明和再电离时代（$z\sim 6-30$）的半数值模拟相结合。 CHE的关键影响来自于Pop III恒星增强的电离能力，这使得重现观测到的汤姆孙散射光学深度所需的Pop III恒星质量密度减少了一个因子$\sim 2$。 同时，最大21-cm全局吸收信号最多浅了$\sim 15$mK（11%），部分原因是由于CHE导致的Lyman带发射减少，而大尺度（$k\sim 0.2\ \rm cMpc^{-1}$）功率在$z\gtrsim 25$处下降了几倍。 一般来说，CHE的影响可以与Pop III恒星形成参数的影响相媲美，显示出不同时期的不同特征之间的有趣相互作用。 这些结果突显了无金属/贫金属恒星演化在理解早期宇宙中的重要性，并表明未来的研究应考虑对恒星/星系形成和恒星演化的物理机制进行联合约束。