Charles J. Lada

Rossi Lectures: The Modern Problem of Galactic Star Formation. Boundary conditions: Embedded Clusters, Stellar Multiplicity and the Stellar IMF

Conceived in the mysterious process that transforms diffuse interstellar matter into massive and dense molecular cloud cores, embedded stellar clusters account for a significant fraction of all star formation currently occurring in the Galaxy. Consequently many of the fundamental properties of the Galactic stellar population are forged in such clusters and such properties form the critical boundary conditions for the development of a theory for star formation. In the first part of this lecture I will discuss the current status of our knowledge of embedded clusters including such issues as their ages and lifetimes, the cluster mass function, and their early dynamical evolution and origin in molecular clouds. In the second part of this lecture I will review the current knowledge concerning stellar multiplicity and the functional form and universality of the stellar IMF. The stellar IMF is one of the most fundamental distributions in astrophysics and the key boundary condition any theory of star formation must satisfy. Detailed knowledge of the functional form of the IMF and how this quantity varies through space and time is necessary to predict the evolution of all stellar systems from star clusters to galaxies. I will discuss how infrared observations of young embedded clusters have enabled the extension of the IMF well into the substellar mass regime, provided insights concerning its universality, and revealed the existence of a characteristic mass for star formation. Finally, I will briefly discuss recent observations of stellar multiplicity that indicate that, contrary to long held opinions, most star systems in the Galaxy consist of single stars.

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