2024MNRAS.533.2727Z

Gravitational lensing may render individual high-mass stars detectable out to cosmological distances, and several extremely magnified stars have in recent years been detected out to redshifts $z\approx 6$. Here, we present Muspelheim, a model for the evolving spectral energy distributions of both metal-enriched and metal-free stars at high redshifts. Using this model, we argue that lensed stars will form a highly biased sample of the intrinsic distribution of stars across the Hertzsprung-Russell diagram, and that this bias will typically tend to favour the detection of lensed stars in evolved stages characterized by low effective temperatures, even though stars only spend a minor fraction of their lifetimes in such states. We also explore the prospects of detecting individual, lensed metal-free (Population III) stars at high redshifts using the James Webb Space Telescope (JWST). We find that very massive ($\gtrsim 100\ \mathrm{M}_\odot$) Population III stars at $z\gtrsim 6$ may potentially be detected by JWST in surveys covering large numbers of strong-lensing clusters, provided that the Population III stellar initial mass function is sufficiently top-heavy, that these stars evolve to effective temperatures $\le 15000$ K, and that the cosmic star formation rate density of Pop III stars reaches $\gtrsim 10^{-4}\ \mathrm{M}_\odot$ cMpc^{$^{-3}$} yr^{$^{-1}$} at $z\approx 6$-10. Various ways to distinguish metal-free lensed stars from metal-enriched ones are also discussed.