ALMA-IMF. XIV. Free–Free Templates Derived from H41α and Ionized Gas Content in 15 Massive Protoclusters

We use the H41α recombination line to create templates of the millimeter free–free emission in the ALMA-IMF continuum maps, which allows us to separate it from dust emission. This method complements spectral-index information and extrapolation from centimeter-wavelength maps. We use the derived maps to estimate the properties of up to 34 H ii regions across the ALMA-IMF protoclusters. The hydrogen ionizing photon rate Q ₀ and spectral types follow the evolutionary trend proposed by Motte et al. The youngest protoclusters lack detectable ionized gas, followed by protoclusters with increasing numbers of OB stars. The total Q ₀ increases from ∼10⁴⁵ s⁻¹ to >10⁴⁹ s⁻¹. We used the adjacent He41α line to measure the relative number abundances of helium, finding values consistent with the Galactic interstellar medium, although a few outliers are discussed. A search for sites of maser amplification of the H41α line returned negative results. We looked for possible correlations between the electron densities, emission measures, and Q ₀ with H ii region size D. The latter is the best correlated, with Q ₀ ∝ D ^{2.49 ± 0.18}. This favors interpretations in which smaller ultracompact H ii regions are not necessarily the less dynamically evolved versions of larger ones but rather are ionized by less massive stars. Moderate correlations were found between the dynamical width ΔV _dyn with D and Q ₀. ΔV _dyn increases from about 1 to 2 times the ionized-gas sound speed. Finally, an outlier H ii region south of W43-MM2 is discussed. We suggest that this source could harbor an embedded stellar or disk wind.