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Understanding ionizing fluxes of stellar populations is crucial for variousastrophysical problems including the epoch of reionization. Massive short-livedWolf-Rayet stars are generally considered as the main ionizing sources. Weexamine the role of less massive stars that lose their envelope throughinteraction with a companion. We use the evolutionary code MESA and theradiative transfer code CMFGEN to investigate stripped stars as a function ofmetallicity (Z).  We show that typical progenitors, initially 12$M_{\odot}$, produce hot andcompact stars (~4$M_{\odot}$, 60-80 kK, ~1$R_{\odot}$) that copiously produceionizing photons, emitting 60-85% and 30-60% of their energy as HI and HeIionizing radiation, for Z=0.0001-0.02. This is comparable to a typical massiveWolf-Rayet star, when accounting for their longer lifetimes and the favoringinitial mass function. Stripped stars further emit their ionizing photons witha delay (~20Myrs after star formation for this progenitor), allowing time forstellar feedback to disperse the birth clouds. This increases the fraction thatcan escape and contribute to ionization of the intergalactic medium.  We further find that Roche stripping fails to fully remove the H-richenvelope at low Z. This questions the common treatment of stripped stars inrapid population synthesis simulations as pure helium stars. We expectimplications for the rate of type Ib/c supernova at low Z and for the advancedevolutionary channels for type Ia supernova and gravitational wave sources. Wediscuss how the characteristic spectral features of stripped stars can be usedto increase the observed sample, which is urgently needed to test the models.

Ionizing spectra of stars that lose their envelope through interaction with a binary companion: role of metallicity