The formation of stellar clusters: Time-varying protostellar accretion rates

Identifying the processes that determine strength, duration, and variabilit y of protostellar mass growth is a fundamental ingredient of any theory of star formation. I discuss protostellar mass accretion rates (M) over dot fr om numerical models that follow molecular cloud evolution from turbulent fr agmentation toward the formation of stellar clusters. In a dense cluster en vironment, (M) over dot is strongly time varying and influenced by the mutu al interaction of protostellar cores and their competition for accretion fr om the common cluster gas reservoir. Even for protostars with similar final mass, the accretion histories may differ dramatically. High-mass stars bui ld up in the central parts of dense, cluster-forming cloud regions. They be gin to form in the early phases of cluster evolution and continue to grow a t a high rate until the available gas is exhausted or expelled by feedback. Lower mass stars tend to form at later phases, and (M) over dot declines r apidly after a short initial phase of strong growth. I present a simple fit formula for the time evolution of the average (M) over dot for protostars of different masses in a dense cluster environment.