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

Abstract

We use the H41 α 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 ∼10 ^45 s ^−1 to >10 ^49 s ^−1 . We used the adjacent He41 α 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 H41 α 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 Δ V _dyn with D and Q _0 . Δ V _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.

Keywords

• H II regions
• Star forming regions
• Millimeter astronomy