A Candidate for the Most Luminous OB Association in the Galaxy

TL;DR

This paper identifies a highly luminous OB association in the Milky Way by analyzing near-infrared data, suggesting it could be the most luminous of its kind in our galaxy, and providing insights into massive star formation.

Contribution

It presents the first identification of a candidate for the most luminous OB association in the Galaxy using near-infrared color selection techniques.

Findings

Detected an overdensity of sources consistent with massive stars

Sources are compatible with the extinction levels expected for such stars

The association's stellar content can ionize the surrounding H II region

Abstract

The Milky Way harbors giant H II regions which may be powered by star complexes more luminous than any Galactic OB association known. Being across the disk of the Galaxy, however, these brightest associations are severely extinguished and confused. We present a search for one such association toward the most luminous H II region in the recent catalog by Murray and Rahman, which, at ~9.7 kpc, has recombination rate of ~7x10^{51} sec$^{-1}$. Prior searches have identified only small scale clustering around the rim of this shell-like region, but the primary association has not previously been identified. We apply a near-infrared color selection and find an overdensity of point sources toward its southern central part. The colors and magnitudes of these excess sources are consistent with O- and early B-type stars at extinctions $0.96 < A_{K} < 1.2$, and they are sufficiently numerous (406 +/- 102 after subtraction of field sources) to ionize the surrounding H II region, making this a candidate for the most luminous OB association in the Galaxy. We reject an alternate theory, in which the apparent excess is caused by localized extinction, as inconsistent with source demographics.