Line‐driven Winds, Ionizing Fluxes, and Ultraviolet Spectra of Hot Stars at Extremely Low Metallicity. I. Very Massive O Stars

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Abstract
Wind models of very massive stars with metallicities in a range from 10-4 to 1.0 solar are calculated using a new treatment of radiation-driven winds with depth-dependent radiative force multipliers and a comprehensive list of more than two million spectral lines in non-LTE (NLTE). The models are tested by a comparison with observed stellar wind properties of O stars in the Galaxy and the SMC. Satisfying agreement is found. The calculations yield mass-loss rates, wind velocities, wind momenta, and wind energies as functions of metallicity and can be used to discuss the influence of stellar winds on the evolution of very massive stars in the early universe and on the interstellar medium in the early phases of galaxy formation. It is shown that the normal scaling laws, which predict stellar mass-loss rates and wind momenta to decrease as a power law with metal abundance, break down at a certain threshold. Analytical fit formulae for mass-loss rates are provided as a function of stellar parameters and metallicity. Ionizing fluxes of hot stars depend crucially on the strengths of their stellar winds, which modify the absorption edges of hydrogen and helium (neutral and ionized) and the line blocking in the far-UV. The new wind models are therefore also applied to calculate ionizing fluxes and observable spectra of very massive stars as a function of metallicity using the new hydrodynamic, NLTE line-blanketed flux constant model atmosphere code developed by Pauldrach and coworkers. Numbers of ionizing photons for the crucial ionization stages are given. For a fixed effective temperature, the He II ionizing emergent flux depends very strongly on metallicity but also on stellar luminosity. A strong dependence on metallicity is also found for the C III, Ne II, and O II ionizing photons, whereas the H I and He I ionizing flux is almost independent of metallicity. We also calculate UV spectra for all the models and discuss the behavior of significant line features as a function of metallicity.
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