A Multiwavelength Look at Galactic Massive Star Forming Regions

TL;DR

This study analyzes 28 Galactic massive star-forming regions across multiple wavelengths, revealing how dust absorption affects ionization and luminosity, and calibrating star formation rates based on infrared luminosities.

Contribution

It provides new distance measurements for 17 regions and offers detailed insights into dust absorption, PAH destruction, and SFR calibration in massive star-forming regions.

Findings

Approximately 34% of Lyman continuum photons are absorbed by dust.

Luminous regions absorb about 51% of ionizing photons and reprocess 82% of starlight.

70 μm luminosity captures about 52% of the total infrared luminosity.

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 $\mu$m through 10 mm spectral energy distributions, we find that ${\sim}34$% of Lyman continuum photons emitted by massive stars are absorbed by dust before contributing to the ionization of H II regions, while ${\sim}68$% 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 $N_C \ge 10^{50}~{\rm s}^{-1}$ and dust-processed $L_{\rm TIR}\ge 10^{6.8}$ L$_{\odot}$) have on average higher percentages of absorbed Lyman continuum photons ($\sim$51%) and reprocessed starlight ($\sim$82%) compared to less luminous regions. Luminous H II regions show lower average 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 $\mu$m combined carry 94% of the dust-reprocessed $L_{\rm TIR}$. $L_{70}$ captures ${\sim}52$% of $L_{\rm TIR}$, and is therefore the preferred choice to infer the bolometric luminosity of dusty star-forming regions. We calibrate 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.