Effective model for the 21-cm signal with population III stars

Observations of the 21-cm signal are opening a window to the cosmic-dawn epoch when the first stars formed. These observations are usually interpreted with seminumerical or hydrodynamical simulations, which are often computationally intensive and inflexible to changes in cosmological or astrophysical effects. Here, we present an effective, fully analytic model for the impact of the first stars on the 21-cm signal, using the modular code zeus21. zeus21 employs an analytic prescription of the star formation rate density (SFRD) to recover the fully nonlinear and nonlocal correlations of radiative fields that determine the 21-cm signal. We introduce the earliest population stars residing in low-mass molecular-cooling galaxies in zeus21, with distinct spectra from later population II stars. We also self-consistently model feedback in the form of ${H}_{2}$-dissociating Lyman-Werner (LW) radiation as well as dark matter-baryon relative velocities, both of which suppress star formation in the lowest-mass halos. LW feedback produces a scale dependence on the SFRD fluctuations, due to the long mean free path of LW photons. Relative velocities give rise to ``wiggles'' in the spatial distribution of the 21-cm signal; we present an improved calculation of the shape of these velocity-induced acoustic oscillations, showing they remain a standard ruler at cosmic dawn. Our improved version of zeus21 predicts the 21-cm global signal and power spectra in agreement with simulations at the $\ensuremath{\sim}10%$ level, yet is at least 3 orders of magnitude faster. This public code represents a step toward efficient and flexible parameter inference at cosmic dawn, allowing us to predict the first billion years of the Universe in mere seconds.