Previous near-IR surveys have been relatively shallow. We used the UKIDSS Galactic Plane Survey (GPS) Data Release 10 to generate a more complete sample of young stars in the W40 region. The uniform survey depths (about 1–1.5 mags deeper in JHK than previous near-IR surveys of the region) and accessibility of nearby control fields allow us to estimate background contamination.

Wide-field near-IR surveys have been used to find young stars in star-forming regions. However, not correcting for background contamination can lead to significant overestimations of K-band excess sources. We find far fewer Class II and III sources in the W40 field than previous studies, suggesting that the star formation rate and efficiency across the W40 field may be lower than previously thought. Stellar density maps of the W40 region that have an estimated background density map subtracted show the W40 cluster clearly but also additional groupings of excess sources. They coincide with regions of relatively lower extinction, which suggests that they are on the far side of the W40 cloud complex but still related to W40.

The W40 field star density map created from a j=30 Nearest Neighbor algorithm shows excess stars above background, after having a normalized and rescaled off-source stellar density map subtracted. The contours denote densities of 0, 1, 2, ... stars arcmin^-2. The red box shows a strip of low extinction along the eastern edge of the field, which was used to rescaled the off-source star density map.

Stars with K-band excess plotted as points in the W40 field with colors representing H-K colors, against a backdrop of the Herschel PACS 160 μm emission.

Overlaid are contours from the j=30 Nearest Neighbor analysis of the K-band excess stars representing stellar clumping minus a normalized and rescaled background density map, with contour levels in 0.5, 1.0, 1.5, 2.0, ... stars arcmin^-2. The slight over-density of K-band excess stars to the southeast, northeast, and south of the W40 cluster is evident from the stellar density contours. Locations of some of these excess stars appear to be slightly anticorrelated with the bright dust emission.