Ultra-compact Embedded Clusters in the Galactic Plane

We have identified a previously unrecognized population of very compact, embedded low-mass Galactic stellar clusters. These tight (r$\approx$0.14 pc) groupings appear as bright singular objects at the few arcsec resolution of the Spitzer Space Telescope at 8 and 24 $\mu$m but become resolved in the sub-arcsecond UKIDSS images. They average six stars per cluster surrounded by diffuse infrared emission and coincide with 100 – 300 M${}_{\text{sun}}$ clumps of molecular material within a larger molecular cloud. The magnitudes of the brightest stars are consistent with mid- to early-B stars anchoring $\sim$80 M${}_{\text{sun}}$ star clusters. Their evolutionary descendants are likely to be Herbig Ae/Be pre-main sequence clusters. These ultra-compact embedded clusters (UCECs) may fill part of the low-mass void in the embedded cluster mass function. We provide an initial catalog of 18 UCECs drawn from infrared Galactic Plane surveys.