2012ApJ...755L..30A

We have identified a previously unrecognized population of very compact, embedded low-mass Galactic stellar clusters. These tight (r ~ 0.14 pc) groupings appear as bright singular objects at the few arcsecond resolution of the Spitzer Space Telescope at 8 and 24 µm 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_☉clumps of molecular material within a larger molecular cloud. The magnitudes of the brightest stars are consistent with mid- to early-B stars anchoring ∼80 M_☉ star clusters. Their evolutionary descendants are likely to be Herbig Ae/Be pre-main-sequence clusters. These ultracompact 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.