The Efficiency of Stellar Reionization: Effects of Rotation, Metallicity, and Initial Mass Function

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We compute the production rate of photons in the ionizing Lyman continua (LyC) of H I (lambda < 912 A), He I (lambda < 504 A), and He II (lambda < 228 A) using recent stellar evolutionary tracks coupled to a grid of non-LTE, line-blanketed (WM-basic) model atmospheres. The median LyC production efficiency is QₗyC = (6+/-2)x10⁶0 LyC photons per Msun of star formation (range [3.1-9.4]x10⁶0) corresponding to a revised calibration of 10⁵³.³⁺/⁻⁰.² photons/s per Msun/yr. Efficiencies in the helium continua are QₕeI 10⁶0 photons/Msun and QₕeII 10⁵6 photons/Msun at solar metallicity and larger at low metallicity. The critical star formation rate needed to maintain reionization against recombinations at z = 7 is rhoₛFR = (0.012 Msun/yr/Mpc³) [(1+z)/8]³ [(Cₕ /3) (0.2/ fₑsc) for fiducial values of IGM clumping factor Cₕ = 3 and LyC escape fraction fₑsc = 0.2. The boost in LyC production efficiency is an important ingredient, together with metallicity, Cₕ, and fₑsc, in assessing whether IGM reionization was complete by z 7. Monte-Carlo sampled spectra of coeval starbursts during the first 5 Myr have intrinsic flux ratios of F(1500)/F(900) = 0.4-0.5 and F(912-)/F(912+) = 0.4-0.7 in the far-UV (1500A), the LyC (900A), and at the Lyman edge (912A). These ratios can be used to calibrate the LyC escape fractions in starbursts.