Massive Star Formation at the Periphery of the Evolved Giant HII Region W 39

TL;DR

This study provides a detailed analysis of the large HII region W 39, revealing its structure, star formation activity, and the influence of its expanding bubble on triggering new star formation in the surrounding areas.

Contribution

First detailed multi-wavelength study of W 39, combining radio, infrared, and kinematic data to understand its structure, star formation, and expansion-driven triggering mechanisms.

Findings

W 39 is at 4.5 kpc distance with a large 30 pc diameter.

The region hosts a cluster of OB stars and three peripheral compact HII regions.

Star formation activity is concentrated in specific areas, likely triggered by W 39's expansion.

Abstract

We present the first detailed study of the large, ~30 pc diameter, inner-Galaxy HII region W 39. Radio recombination line observations combined with HI absorption spectra and Galactic rotation models show that the region lies at V(LSR) = +65.4+/-0.5 km/s corresponding to a near kinematic distance of 4.5+/-0.2 kpc. Analysis of radio continuum emission shows that the HII 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 HII 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 order Myr consistent with a scenario where the formation of the smaller HII 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 HII regions as well as a number of intermediate-mass Class I and Class II YSOs.