We present the first detailed study of the large, ~30 pc diameter, inner-Galaxy H II region W?39. Radio recombination line observations combined with H I absorption spectra and Galactic rotation models show that the region lies at V LSR = +65.4 ± 0.5?km?s–1, corresponding to a near kinematic distance of 4.5 ± 0.2 kpc. Analysis of radio continuum emission shows that the H II region is being powered by a cluster of OB stars with a combined hydrogen-ionizing luminosity of log (Q) ≥ 50, and that there are three compact H II regions located on the periphery of W?39, each with log (Q) ~ 48.5 (single O7-O9?V star equivalent). In the infrared, W?39 has a hierarchical bubble morphology, and is a likely site of sequential star formation involving massive stars. Kinematic models of the expansion of W?39 yield timescales of the order of Myr, consistent with a scenario where the formation of the smaller H II regions has been triggered by the expansion of W?39. Using Spitzer GLIMPSE and MIPSGAL data, we show that star formation activity is not distributed uniformly around the periphery of W?39 but is concentrated in two areas that include the compact H II regions as well as a number of intermediate-mass Class I and Class II young stellar objects.
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