A Multiwavelength Look at Galactic Massive Star-forming Regions

Abstract

We present a multiwavelength study of 28 Galactic massive star-forming H II regions. For 17 of these regions, we present new distance measurements based on Gaia DR2 parallaxes. By fitting a multicomponent dust, blackbody, and power-law continuum model to the 3.6 μm through 10 mm spectral energy distributions, we find that ∼34% of Lyman continuum photons emitted by massive stars are absorbed by dust before contributing to the ionization of H II regions, while ∼68% of the stellar bolometric luminosity is absorbed and reprocessed by dust in the H II regions and surrounding photodissociation regions. The most luminous, infrared-bright regions that fully sample the upper stellar initial mass function (ionizing photon rates N _C ≥ 10⁵⁰ s^-1 and dust-processed L _TIR ≥ 10^6.8 L _⊙) have on average higher percentages of absorbed Lyman continuum photons (∼51%) and reprocessed starlight (∼82%) compared to less luminous regions. Luminous H II regions show lower average polycyclic aromatic hydrocarbon (PAH) fractions than less luminous regions, implying that the strong radiation fields from early-type massive stars are efficient at destroying PAH molecules. On average, the monochromatic luminosities at 8, 24, and 70 μm combined carry 94% of the dust-reprocessed L _TIR. L ₇₀ captures ∼52% of L _TIR, and is therefore the preferred choice to infer the bolometric luminosity of dusty star-forming regions. We calibrate star formation rates (SFRs) based on L ₂₄ and L ₇₀ against the Lyman continuum photon rates of the massive stars in each region. Standard extragalactic calibrations of monochromatic SFRs based on population synthesis models are generally consistent with our values.