MASSIVE STAR FORMATION AT THE PERIPHERY OF THE EVOLVED GIANT H II REGION W 39

We present the first detailed study of the large, {approx}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{sub LSR} = +65.4 {+-} 0.5 km s{sup -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) {approx} 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.