Embedded Clusters in Molecular Clouds

Stellar clusters are born embedded within giant molecular clouds (GMCs) and during their formation and early evolution are often only visible at infrared wavelengths, being heavily obscured by dust. Over the last 15 years advances in infrared detection capabilities have enabled the first systematic studies of embedded clusters in galactic molecular clouds. In this article we review the current state of empirical knowledge concerning these extremely young protocluster systems. From a survey of the literature we compile the first extensive catalog of galactic embedded cluster properties. We use the catalog to construct the mass function and estimate the birthrate for embedded clusters within roughly 2 Kpc of the Sun. We find that the embedded cluster birthrate exceeds that of visible open clusters by an order of magnitude or more indicating a high infant mortality rate for protocluster systems. Less than 4-7% of embedded clusters survive emergence from molecular clouds to become bound clusters of Pleiades age. The vast majority (90%) of stars that form in embedded clusters form in rich clusters of 100 or more members with masses in excess of 50 solar masses. We review the role of embedded clusters in investigating the nature of the IMF which, in one nearby example, has been measured over the entire range of stellar and substellar mass, from OB stars to subsellar objects near the deuterium burning limit. We also review the role embedded clusters play in the investigation of circumstellar disk evolution and the important constraints they provide for understanding the origin of planetary systems. Finally, we discuss current ideas concerning the origin and dynamical evolution of embedded clusters and the implications for the formation of bound open clusters.Comment: To appear in Annual Reviews of Astronomy and Astrophysics v41. Manuscript consists of 73 pages with 14 figures. Due to space limitations 5 figures are included as separate low quality jpeg figures. A complete manuscript with full resolution figures can be downloaded from http://cfa-www.harvard.edu/~clada/preprints.htm