Lunch talk on Jun. 27, 2025

Bridging the scales: an analytical model for Population III star formation

Speaker: Boyuan Liu (Heidelberg University)

Venue: SWIFAR Building 2111

Time: 12:45 PM, Friday, Jun. 27, 2025

Abstract: 

The first generation of so-called Population III (Pop III) stars, formed in extremely metal-poor primordial gas with inefficient cooling, are expected to have distinct features compared with present-day metal-enriched stars (more massive, compact, and less mass loss) and play important roles in the first billion years of cosmic history through their radiation, metal enrichment, and by seeding massive black holes (BHs). Although their detailed properties are largely unknown, direct and indirect probes (e.g., galaxy surveys by JWST, stellar archaeology, binary BH mergers, and 21-cm signal) are providing us with increasingly more clues. Self-consistent theoretical predictions of the formation rates, sites, and masses of Pop III stars are crucial for interpreting observations, but are challenging due to complex physical processes operating over the large range of length scales involved. One solution is to combine analytical models for the small-scale star formation process with cosmological simulations that capture the large-scale physics, such as structure formation, radiation backgrounds, and baryon-dark matter streaming motion that regulate the conditions of Pop III star formation. To bridge the scales, we build an analytical model to predict the final masses of Pop III stars/clusters from the properties of star-forming clouds, based on the key results of small-scale star formation simulations and stellar evolution models. Our model for the first time considers the interplay between feedback and fragmentation and covers different modes of Pop III star formation ranging from ordinary small (∼ 10 − 2000 M⊙) clusters in molecular-cooling clouds to massive (≳ 10^4 M⊙) clusters containing supermassive (∼ 10^4 − 3 × 10^5 M⊙) stars under violent collapse of atomic-cooling clouds with large gas accretion rates of ≳ 0.1 M⊙/yr. As an example, the model is applied to the Pop III star-forming clouds in the progenitors of typical haloes hosting high- luminous quasars _h ∼ 10^12 M⊙ at  ∼ 6), which shows that formation of Pop III massive clusters is common (∼ 20 − 70%) in such biased (∼ 4) regions, and the resulting heavy black hole seeds from supermassive stars can account for a significant fraction of observed luminous (≳ 10^46 erg/s) quasars at  ∼ 6.