A case study of the Galactic H II region M17 and environs: Implications for the galactic star formation rate

Determinations of star formation rates (SFRs) in the Milky Way and other galaxies are fundamentally based on diffuse emission tracers of ionized gas that are sensitive only to massive OB stars. OB stars dominate the ionization of H II regions, yet they make up <1% of young stellar populations. The primary goal of this Thesis is to obtain a detailed census of the young stellar population associated with a bright Galactic HII region and to compare the resulting star formation history with global SFR tracers. The main SFR tracer considered is infrared continuum, since it can be used to derive SFRs in both the Galactic and extragalactic cases. I focus this study on M17, one of the nearest giant H II regions to the Sun (d = 2.1 kpc). The giant molecular cloud associated with M17 is a significant star-forming structure on the Galactic scale, with a complicated star formation history. I report the discovery that M17 is located on the rim of a 20-pc diameter bubble outlining a ∼5 Myr old H II region, suggesting that the formation of the M17 ionizing cluster may have been triggered by the expansion of the bubble. Several dozen intermediate-mass young stellar objects (YSOs) are distributed around the rim of the bubble, indicating even more recent triggered star formation. The star formation rate in the central ionizing cluster is SFRX ∼ 0.01 M⊙ yr-1, derived from the observed X-ray luminosity function of the young stellar population. I conclude with comparisons between SFRX and the SFRs predicted for M17 by widely-used calibrations based on infrared and radio continuum emission. The results imply that current SFR determinations based on ionized gas tracers are uncertain by factors of ∼2--3, due chiefly to large uncertainties in the masses and ionizing photon production rates of O stars and the need to correct for the high binary fractions observed among massive stars.