The detection and characterization of highly magnified stars with JWST: prospects of finding Population III

Abstract

ABSTRACT 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.